# Stratigraphy ## expand\_more ### Introduction to the Stratigraphy of the Trenton Group The strata exposed at Trenton Falls represent one of the earliest, well-studied geology localities in the United States . Early in the 1800's geologists such as Amos Eaton, James Hall, and Lardner Vanuxem were champions of Trenton Falls geology. These workers were humbled by the grand scenery of the West Canada Creek chasm, and even more inspired by the occurrence of thousands of well-preserved fossils on nearly every bedding plane within the exposure. Recognizing the need for further interpretation and diagnosis of the nature of these incredible limestones, later fossil collectors and geologists developed a series of "faunal zones." Still later, they assigned geographic locality names for specific strata as is the practice in today's geological nomenclature system. The Trenton Falls gorge section (from Trenton Falls village, to Prospect, New York) was initially established as the type section for the uppermost Middle Ordovician strata in North America, due to the outstanding rock exposures and well-defined fossil zonations. In today's relative geologic time scale, the Trenton Group is a rock term that is limited to the description of the rocks themselves, and the period of geological time encompassed in their record is known as the Late Mohawkian Epoch of the Late Ordovician Period. The classical sections along West Canada Creek play an important role in understanding the development of eastern Laurentia during a critical period in Earth's history. The following sections discuss the physical details of stratigraphic exposures and introduce geological concepts as applied to the understanding of these rocks. ### Lithostratigraphic Analysis of Trenton Falls expand\_more #### Brief statement regarding the historical lithologic analysis of the Trenton Falls Limestone ![Drs. Carlton E. Brett and Gordon C. Baird Examining West Gorge Wall Photograph by Tom Whiteley](/sites/g/files/omnuum8411/files/styles/hwp_1_1__720x720_scale/public/trenton/files/westgorgewall.jpg?itok=yYkxxrHl) *Drs. Carlton E. Brett and Gordon C. Baird Examining West Gorge Wall, photograph by Tom Whiteley*The first stratigraphic analyses of the present day Trenton Group were performed and documented by Amos Eaton. His classification disregarded fossil zonations, over-simplified facies, and lumped together major distinctive rock types. The lack of a detailed terminology prevented the delineation of distinct rock and facies types within the Trenton gorge. The Trenton Limestones were referred to as the "Metalifferous Limerock", and without diagnosing key differences, Eaton's classification provided little information regarding lithology. The first stratigraphic analyses which delineated vertical differences in lithologies were those of Ebenezer Emmons and Lardner Vanuxem as part of their contributions to the first geological surveys of New York . These authors had begun to use fossils in their stratigraphic synthesis along with lithologic differences within the limestones. As a result of their surveys, the term Trenton Limestone was coined, and recognized for the first time as being composed of multiple lithologies. In Emmons' report (1842; p. 112), he states: > the Trenton, which I may justly remark, is one of the best characterized rocks in the Transition system, both in its fossils and lithological characters. In all localities of this rock, we find it more or less a shaly limestone. It sometimes occurs as a black thick-bedded rock, with argillaceous matter diffused through it; or in thin beds of limestone, alternating with those of a thin shivery shale. In addition to the masses of limestone and shale arranged as here described, there is sometimes another important one in the form of a grey crystalline rock, occupying sometimes a position inferior, and at others superior, to the black shaly limestone. ![Sherman Falls drawing](/sites/g/files/omnuum8411/files/styles/hwp_1_1__720x720_scale/public/trenton/files/vanuxemsfallpict293w.jpg?itok=K1lkCkI8) *Sherman Falls. Drawing by E.C. Taylor (Vanuxem, 1842)*Emmons presented in his analysis one of the most critical lithologic differences within the section, the differentiation of coarse-crystalline limestone and dark shaly fine to medium grained limestone. In outcrop, his "inferior" and "superior" coarse grained lithologies are known today to be represented by the Kings Falls , the Steuben formations. Aside from describing faunal differences between strata, and a few other unique lithologic characters, these early workers lacked a system for classifying carbonate rocks. In contrast, today we have at least two major classification systems for the description of carbonate rocks, as well as a set of physical properties with which to further distinguish rock characters. ![Kay 1937](/sites/g/files/omnuum8411/files/styles/hwp_1_1__960x960_scale/public/trenton/files/kay1937column293w.jpg?itok=lKvEEkvQ) By the turn of last century, Trenton Falls had become world famous for its geology and its paleontology. Early workers such as Vanuxem, White, Prosser and Cumings had delineated several major sedimentologic differences within these strata. Building on these earliest studies, Percy E. Raymond, William J. Miller and Marshall Kay championed the effort to investigate and map the Trenton Limestone and its equivalents elsewhere. By 1937, faunal zonations had been established (see Raymond, 1903), and Marshall Kay had begun to develop a stratigraphic nomenclature for the Trenton Group. He published many articles, abstracts and bulletins on the subject. ### **Comparison of stratigraphic sections: A century apart** Major advances in carbonate sedimentology have been made since the time of Eaton. With the establishment of several classification schemes, geologists look at finer-scales (sub-centimeter) for compositional differences in rocks. Consequently they are able to make more accurate predictions regarding the depositional history of the rock. The following diagram (below) demonstrates this concept very clearly. On the left is a schematic stratigraphic section of the Trenton Falls Group as exposed in the Trenton Falls type locality. It was published by Brett and Baird (2002) in a paper titled "Revised Stratigraphy of the Trenton Group in its type area, central New York State: sedimentology and tectonics of a Middle Ordovician shelf-to-basin succession". The column on the right is just over one hundred years older and was published in 1896, by Prosser and Cumings. Their manuscript, entitled "Sections and Thickness of the Lower Silurian Formations on West Canada Creek and in the Mohawk Valley", represents the first stratigraphic section ever published for Trenton Falls. ![section](/sites/g/files/omnuum8411/files/styles/hwp_1_1__720x720_scale/public/trenton/files/stratcomplabeled400wrightside.jpg?itok=Jf3H5jHL) ![section](/sites/g/files/omnuum8411/files/styles/hwp_1_1__720x720_scale/public/trenton/files/stratcomplabeled400wrightside.jpg?itok=Jf3H5jHL) Although these stratigraphic columns are drawn to slightly different scales, they do illustrate several key points: 1\) "Before Folk and Dunham", Prosser and Cumings designated two main lithologies: a lower dominantly thick-bedded argillaceous limestone from the base of the Narrows through near the base of the railroad bridge (this lithology was their dominant rock type); and an upper crystalline limestone from near the base of the railroad to the top of the gorge section. This assessment is simple compared to the lithology diagnosis of Brett and Baird. In their study, several repeating rock types are found. Although similar in color, the subtle textural, bedding, and faunal compositional differences help to differentiate the facies of each of the stratigraphic units. 2\) "Dilemma of the full cascade": Diagrammatically, the Prosser and Cumings stratigraphic column shows the relative position of each of the four main waterfalls and establishes the total thickness of strata between. The same features are labeled on the Baird and Brett column. Given the scale difference, the relative thickness of strata between each successive set of falls is similar for the Sherman Fall-to-Lower High Falls interval and for the Upper High Falls-to-Mill Dam interval in both diagrams. However, the Lower High Falls-to-Upper High Falls interval is substantially greater in the Prosser and Cumings figure. Given that both diagrams were drawn to scale (albeit different than each other), the anomalous stratigraphic thickness difference within the region of Lower-to-High Falls region is not consistent with the other interval measurements. ![Representation of Prosser and Cumings Figure showing the contact position of their two main facies types.](/sites/g/files/omnuum8411/files/styles/hwp_1_1__360x360_scale/public/trenton/files/upperprosserfig293w.jpg?itok=dbKXjzon) *Representation of Prosser and Cumings figure showing the contact position of their two main facies types.*This "sticking point" is perhaps a puzzling but it is explainable. Since the time when Prosser and Cumings measured their stratigraphic sections, the flow of water through the gorge has been substantially less due to the construction of the power dam. With high discharge rates nearly year round, Prosser and Cumings had to measure sections from disparate sides of the gorge in order to construct a composite, reference column. They traced individual beds across the face of Upper High falls and continued their measurements. Because water flow rates were high, their bed tracing deceived them. ![View north at Upper High Falls with fault trace and breccia](/sites/g/files/omnuum8411/files/styles/hwp_1_1__360x360_scale/public/trenton/files/highfallsfaulttrace.jpg?itok=n-_7G8d5) *View north at Upper High Falls showing fault trace and breccia*Lower water discharge rates flow through the gorge today, and occasionally is completely shut down. Professors Carlton Brett and Gordon Baird made an important observation with respect to the lateral continuity of beds from one side of the falls face to the other. Especially in this falls, the beds cannot be traced laterally. There is a roughly north-south striking high angle normal fault that is hidden in the face of the Upper High Falls waterfall. It is most often concealed with even the slightest amount of run-off. This single observation is most likely the source of the discrepancy between the two stratigraphic columns. Prosser and Cumings' measurements were off because they mistakenly duplicated section. It is more than likely that this same fault extends some distance up and downstream and could potentially have further repercussions for other measurements. #### Modern lithostratigraphy of the Trenton Group Through his intuitive observations and keen eye for subtle facies variations and well-correlated cross-sections, Marshall Kay's studies have paved the way for our modern studies of the Trenton Group. His nomenclatural system, although modified slightly, is the primary stratigraphic column used today. The discussion here on the stratigraphy of the Trenton Group is based on the most recent up-to-date stratigraphic analysis of these rocks. Two recent papers by Brett and Baird (2002), and Baird and Brett (2002), looked at the Trenton Group and its lateral downslope equivalents, the Flat Creek (or Canajoharie Shales) and the Indian Castle Shales (Utica Group). The discussion covers nomenclatural history, lithologic details, distinctive facies components and other key descriptive characteristics that help to delineate the stratigraphic character of each unit. [Modern lithostratigraphic units](/modern-lithostratigraphic-units-trenton-group) ![Steuben](/sites/g/files/omnuum8411/files/styles/hwp_1_1__360x360_scale/public/trenton/files/steuben_label400w.jpg?itok=e8tHDF0K) ![rust formation](/sites/g/files/omnuum8411/files/styles/hwp_1_1__360x360_scale/public/trenton/files/rust_label400w.jpg?itok=NzseyLcD) ![Denley formation](/sites/g/files/omnuum8411/files/styles/hwp_1_1__360x360_scale/public/trenton/files/denley_label400w.jpg?itok=s0m1BPgm) ![Sugar River](/sites/g/files/omnuum8411/files/styles/hwp_1_1__360x360_scale/public/trenton/files/sugarriver_label400w.jpg?itok=MkLB1pnY) ### Modern biostratigraphy of the Trenton Group expand\_more ## Introduction ![View of Upper and Lower High Falls from Above Photograph by Tom Whiteley](/sites/g/files/omnuum8411/files/styles/hwp_1_1__720x720_scale/public/trenton/files/biostratpic.jpg?itok=x6CPKbX4) *View of Upper and Lower High Falls from above, photograph by Tom Whiteley*Biostratigraphy is a sub-discipline of sedimentary geology that relies on the physical zonation of biota, both in time and space, in order to establish the relative stratigraphic position (i.e. older, younger, same age) of sedimentary rocks between different geographic localities. Although the basic rules of biostratigraphic zonation were established in the late 18th to early 19th centuries in Europe (ultimately resulting in the development of the Relative Geologic Time Scale), the implementation of biostratigraphic techniques was in use in the United States during the early to mid-1800's. Some of the first geological surveys to be completed in the United States included those of the New York State Geological Survey. These surveys focused not only on New York's geological resources, but also emphasized the establishment of spatial and temporal relationships of stratigraphic units based on both lithologic and paleontologic composition. By the mid-1800's the New York Surveys had resulted in the development of a relative stratigraphic zonation based primarily on fossil distribution. New York localities are world famous for Cambrian through Devonian strata and fossils, but of particular importance to this website discussion is the contribution of the Ordovician rocks of central New York State to the establishment of a North American focused biochronology. The rocks found in the central New York Mohawk River Region, by definition of their fossil content, are now established as belonging to the Mohawkian Series of the Upper Ordovician Period. The following material focuses on key fossil taxa present in the Trenton Limestone, their distribution or occurrence within the overall succession of Upper Ordovician strata, and their role in the establishment of the Upper Ordovician time scale. #### Biostratigraphy: A few considerations The goal of biostratigraphy is to use fossil occurrences within the rock record to establish correlations between time-equivalent rock strata as determined by the presence of a particular fossil species. Although the concept is generally straightforward, i.e. the presence of a specific fossil species in two geographic localities indicates the rocks containing the fossil specimens were deposited at about the same time, in practice biostratigraphic studies tend to be complex. The complexities of biostratigraphy result from aspects of the biology of the organisms including their environmental range, their evolutionary rates, as well as their tendency for preservation and probability of observation by the biostratigrapher. Ultimately, the most rapidly evolving or short-lived, yet wide-ranging fossil taxa make the best biostratigraphic markers for correlation. If a given taxa is both wide-ranging and evolutionarily short-lived, and if it is robust enough to be preserved in the fossil record, then the taxa is often referred to as an index fossil. An index fossil identified in the rock record would constrain the age of the rock within which it is contained to a very specific interval of time when the organism lived. Fossil taxa used in biochronologic investigations rarely satisfy all aspects of the ideal index fossil. That is, they often violate one or more of the following rules: 1), must have a widespread distribution (fossils tend to be limited to a small region or are found only in a particular depositional environment as opposed to globally); 2), must show rapid evolution (fossils change rapidly in preservable morphology so that distinctive identifiable species are easily recognized); 3), must be present in substantial numbers (so that fossils can be observed by the biostratigrapher); and 4), fossils should be robust mineralogically (so that depositional and diagenetic processes do not remove the fossils from the rock record). Most often the best biostratigraphic markers or index fossils are taxa that live in the open water column either as free-floating plankton or as actively swimming nekton. Such organisms tend to be rapidly evolving, widely distributed and widely deposited. In contrast, benthic organisms which live on or very close to the seabottom, tend to be less widespread, fewer in numbers, and are typically found only in particular environments. Nonetheless, nektonic, planktonic, and benthic forms can be used to establish relative biostratigraphic age zonations. #### Biostratigraphy of the Upper Ordovician Throughout the majority of the Paleozoic Era, including the Ordovician Period, the most important fossil index taxa used by biostratigraphers belong to two main groups: graptolites and conodonts. Both of these, which are now extinct, are believed to have lived in open ocean settings as plankton (graptolites) and nekton (conodonts). These two taxa generally satisfy the qualifications for chronostratigraphic indexing. That is, their occurrence in the geologic record generally show evidence for: widespread distribution, high rates of evolutionary change, abundance in the fossil record, and fairly robust mineralogies. Both graptolites and conodonts are very useful for establishing the relative age of many Paleozoic rocks including those of the Trenton Group. In fact, some of the first biostratigraphic studies of the Trenton Limestones and equivalent Utica Group black shales led to the recognition and establishment of the North American Time Scale for Upper Ordovician time, based partly on graptolite biozonation. The diagram to the below, modified from Holland (2003), represents a compendium of chronostratigraphic data for the Middle to Upper Ordovician of the eastern United States. In this diagram, the distribution of key index taxa including the graptolites and conodonts are shown relative to sequence stratigraphic interpretations, absolute age, and time-rock nomenclature (series and stages). Note that in most cases, the boundaries of graptolite and conodont biozones do not correspond directly with time-rock series and stage boundaries. This is a reflection of the practical difference between using the biostratigraphic distribution of fossils for chronology versus time-rock classification schemes which are based often on the lithologic expression of a rock succession at a given set of geographically constrained outcrop exposures. In addition to biostratigraphic and sequence stratigraphic zonation, also diagramed is the relative scale provided by the composite standard section (CSS). This represents a composite stratigraphic section for all of the Middle to Upper Ordovician rocks of central and eastern North America based on graphic correlations techniques (see Sweet, 1984 for more information). The diagram clearly shows the relative position of key graptolite and conodont species within each representative time sub-unit of the Middle to Upper Ordovician. In the rollover image, the key graptolite and conodont taxa present in the Trenton Limestone are in white. ![Upper Ordovician Chronostratigraphic Time Scale Modified after Holland, (2003) Original data compiled from: Sweet, 1971; Bergström 1971; Sweet 1984; Holland & Patzkowsky 1998](/sites/g/files/omnuum8411/files/styles/hwp_1_1__720x720_scale/public/trenton/files/hpordovicianbiochronologylabeled.gif?itok=h_yVlHv5) *Upper Ordovician Chronostratigraphic Time Scale. Modified after Holland, (2003). Original data compiled from Sweet, 1971; Bergström 1971; Sweet 1984; Holland & Patzkowsky 1998.*#### Biostratigraphy of the Trenton Group Although the diagram above shows all the chronostratigraphic information for the Middle to Upper Ordovician of central to eastern North America, the key interval of importance to the discussion of the Trenton Group here is that of the upper Mohawkian Series (roughly equivalent to the middle Caradocian Series of the European System). The Trenton Limestone represents a single lithostratigraphic group, but it is clearly composed of a variety of carbonate rock types, and lithostratigraphic sub-units deposited during a relatively long period of time (estimated at approximately 4-5 million years). Moreover as clearly shown above, the upper Mohawkian Series is subdivided into three stages (Ro.=Rocklandian, Ki.= Kirkfieldian, and Shermanian) based on biostratigraphic zonation. #### Graptolite and conodont biostratigraphy The roll-over diagram above highlights key taxa useful for biostratigraphic studies of the Upper Mohawkian Series. Within the Trenton Group, the key graptolite taxa used for dating are Diplograptus multidens, Corynoides americanus, Orthograptus ruedemanni, and Climacograptus spiniferus. The key conodont taxa used are Phragmodus undatus, Phragmodus tenuis, Belodina confluens, Amorphognathus tvaerensis, and Amorphognathus superbus (Sweet, 1984; Mitchell and Bergström, 1991; Brett and Baird, 2002). The following two diagrams illustrate the use of graptolite and conodont biostratigraphic zonations from two different studies. ![Biostratigraphic Relationships Between the Cincinnatian and Underlying Mohawkian Series Between New York State and Ohio Modified after Mitchell and Bergström, 1991](/sites/g/files/omnuum8411/files/styles/hwp_1_1__720x720_scale/public/trenton/files/mitchellbergstrom1991.gif?itok=gj7eai3z) *Biostratigraphic Relationships Between the Cincinnatian and Underlying Mohawkian Series Between New York State and Ohio, modified after Mitchell and Bergström, 1991*In the case of the first study, shown in the diagram below, Mitchell and Bergström (1991) establish correlations between distant geographic localities using principles of biostratigraphic zonation. Mitchell and Bergström used the position of graptolite zonal boundaries in the type sections of the Trenton Limestone (upper Mohawkian) and the type sections of the Kope Formation (lower Cincinnatian) to establish direct correlations between these localities. In this figure the position of key graptolite zonal boundaries are shown (i.e. O. ruedemanni, C. spiniferus, and G. pygmaeus) in relation to major lithostratigraphic divisions. In the case of the Trenton, only the middle and upper Trenton are shown and are referred to as the Denmark Ls. and the Cobourg Ls. (Kay, 1943). Using today's lithostratigraphic nomenclature, the Denmark Ls. would refer primarily to the Denley and Lower Rust Formations and the Cobourg would refer to the Upper Rust and Steuben Formations. This is an excellent example showing the use of graptolite biozonation for biostratigraphic correlation, but unfortunately the distribution of graptolites within the carbonate-dominated Trenton Limestone and its equivalents is rather poorly constrained for lower stratigraphic intervals. The lack of preservation of graptolites in the shallow, well-oxygenated depositional settings of much of the Trenton and Lexington limestones (shown above) prevents more accurate biostratigraphic assessments within these strata. Therefore, further discussion of graptolite biozonation is deferred herein. In addition to the graptolite biozones illustrated in the diagram above, Mitchell and Bergström (1991) indicate the approximate position of the Amorphognathus tvaerensis / Amorphognathus superbus conodont biozone boundary. This boundary was later found to correlate with the base of the base of the Poland Formation at Rathbun Brook, with a level in the middle of the C. americanus Zone, and well below the base of the Dolgeville (Mitchell et. al, 1994). The above diagram by Holland (2003) shows that the position of this conodont biozone boundary occurs within the Shermanian Stage and within the M6 sequence. Besides the study by Mitchell and Bergström (1991), earlier studies on conodont biostratigraphy by Sweet (1984) helped to elucidate the relative distribution of conodont forms for rocks across the United States. Sweet plotted the stratigraphic distribution of conodont forms in 61 individual geographic localities from the United States (similar in style to the figure to the right for the Black River to Trenton Groups of New York and Ontario), and created a 477 meter-thick composite stratigraphic section (CSS) using graphic correlation techniques. By plotting the distribution or range of conodont species against a vertical stratigraphic section for each geographic locality, Sweet generated a composite stratigraphic section for all localities combined. He thereby provided the total stratigraphic range of all species found in the Middle to Late Ordovician. Sweet then divided the CSS into 80 individual 6 meter-thick units, and into a series of chronozones based on the first appearance of certain conodont species. As a result of his approach, Sweet (1984) was able to construct a fairly high-resolution chronostratigraphic framework for the Mohawkian and Cincinnatian Series. ![Range and Relative Abundance of Conodonts from the Black River and Trenton Groups of New York State and Ontario, Canada Modified after Sweet, 1984](/sites/g/files/omnuum8411/files/styles/hwp_1_1__720x720_scale/public/trenton/files/sweet1984labeled.gif?itok=gCodQW6t) *Range and Relative Abundance of Conodonts from the Black River and Trenton Groups of New York State and Ontario, Canada, modified after Sweet, 1984*Sweet in 1971 recognized more than 13 individual "faunal units" but only 3 of these (faunal units 8, 9, and 10) apply to the Trenton limestone, and correspond to the P. undatus, P. tenuis, and B. confluens conodont-based chronozones established in the graphic correlation study. The following diagram modified from Sweet's graphic correlation study, highlights four of the conodont-based chronozones and their correlation across the United States from Nevada through eastern New York. Shaded in pink, green, gellow and purple are the chronozones that bracket the Trenton Limestone interval. Although more chronostratigraphic details and correlations are now established that modify Sweet's biostratigraphy slightly, his technique allowed the development of stage level chronostratigraphic correlations for these regions. ![Conodont Biostratigraphic Correlations for the Upper Ordovician of the U.S. Modified after Sweet, 1984](/sites/g/files/omnuum8411/files/styles/hwp_1_1__720x720_scale/public/trenton/files/sweet1984mohawkianbiostrat.gif?itok=_Whsn-aE) *Conodont Biostratigraphic Correlations for the Upper Ordovician of the U.S., modified after Sweet, 1984*A major difference between the Mitchell and Bergström (1991), and Sweet (1984) studies is the use of two different conodont biochronologies. In the case of Mitchell and Bergström (1991), the A. tvaerensis and A. superbus conodont biozonation is established from studies of faunas found in the north Atlantic, while that of Sweet (1984) is focused on faunas from the midcontinent (see figure by Holland above). The different conodont biostratigraphic zonations result from the lack of north Atlantic conodonts in the majority of the midcontinental to eastern United States and vice versa. Moreover, both conodont biostratigraphic zonation schemes are required in order to establish concise inter-regional correlations. Unfortunately, in most cases the two faunas do not overlap in their geographic ranges, resulting in rather poor biostratigraphic constraints between north Atlantic and central and eastern United States conodont faunal provinces. In the case of the type Mohawkian interval in central New York, the conodont faunas are generally allied with those of the north Atlantic as is reflected in the figure by Mitchell and Bergström (1991). However, Sweet's (1984) graphic correlation integrated from midcontinent faunas found in sections from central New York and southern Ontario has helped to establish an overlap between the two conodont provinces (as is shown in the Holland figure above). Much of the Middle to Upper Ordovician series level chronology is based on graptolite and conodont biostratigraphy, but at higher resolutions (i.e. stage level resolution), the recognition of graptolite and conodont biozonation becomes problematic. Often other tools, such as graphic correlation, must be used to provide better control. In practice, most index fossils have some limitations in their application. In the case of graptolites and conodonts, they also have limitations based on their preservation potential (poor graptolite preservation in well-oxygenated, high-energy facies), on their abundance (conodont elements are very small and tend to be very sparse except where concentrated due to sedimentary condensation processes), and their geographic range. Moreover, individual species from both taxonomic groups show relatively low turn-over rates, and have fairly long durations. While graptolites and conodonts are excellent for establishing series level correlations, their use at higher-resolutions including sub-stage level analysis may require the use of additional tools, such as graphic correlation. ![Litho- and Biostratigraphic Terminology for the Trenton Limestone](/sites/g/files/omnuum8411/files/styles/hwp_1_1__960x960_scale/public/trenton/files/trentonstratbiostrat.gif?itok=CjWwiyKX) *Litho- and Biostratigraphic Terminology for the Trenton Limestone*#### Macrofaunal biostratigraphy: Concept of "BioZones" A variety of benthic taxa (brachiopods, bryozoans, molluscs, trilobites, and echinoderms) have been used historically to define individual Trenton sub-units throughout much of New York State and Ontario. As mentioned in discussions in the "Cast of Geologists" section of this website, early emphasis was placed on the description and classification of the numerous fossil taxa from Trenton Falls and equivalent localities. Many species were described, but not until the end of the 19th century had much progress been made toward elucidating a biologic zonation within the Trenton Limestone. From the turn of the century and continuing until the mid 1900's, researchers such as White (1896); Prosser and Cummings (1896); Raymond (1903); and Kay (1937; 1943; 1968) began to investigate and report on the biostratigraphic zonation of the type Trenton Limestone at Trenton Falls. Based on these studies, research primarily by Marshall Kay, helped to develop a much higher series of both litho, and biostratigraphic terms for use in the sub-division and correlation of the Trenton Limestone. There has been much confusion in the literature as to the proper stratigraphic terminology to use (rock terms vs. time-rock terms) especially outside of the New York/Ontario/Quebec region. The modern lithostratigraphic terminology (from Brett and Baird, 2002) is shown in the figure to the right. This figure indicates the relative position of key faunal "zones" relative to lithostratigraphic intervals. In the diagram, it is first apparent that the majority of well-characterized faunal zones are at the base of the Trenton Limestone and include those of the Rocklandian, Kirkfieldian, and Shorehamian (early Shermanian). In contrast, much of the middle Trenton, Denley and Rust Formations, is typified by a variety of taxa that are "typical Trenton" species and are more or less common throughout this part of the Trenton Limestone. In this sense, Kay was unable to further divide this interval. Kay (1968) referred to the entire Middle Trenton using the biostratigraphic-based time-rock term: Shermanian. The figure modified from Kay's1968 paper also illustrates the relative position of time-rock and rock terminologies. Today, the remainder of the Upper Trenton is considered Shermanian in age. Originally, Kay recognized the development of the Rafinesquina deltoidea zone within the uppermost Trenton Steuben Formation as a unique faunal zone and correlated it with outcrops in Ontario. This uppermost Trenton interval containing Rafinesquina deltoidea and an abundance of gastropod species was delineated by the time-rock term Cobourgian. The Cobourgian aged rocks in Ontario were correlated with the upper Rust and Steuben Limestones at Trenton Falls (as shown in Kay's figure below), as Rafinesquina deltoidea first appears in the upper Rust Formation and expands dramatically in the Steuben. Based on the occurrence of these faunal zones, Kay (1937; 1943; 1968) established a number of time-rock terms to emphasize the distinction between biostratigraphic and lithostratigraphic terminologies. In the figure below, Kay illustrates the hierarchical system of series, stage, and sub-stage level classifications and the relative position of lithostratigraphic boundaries. Kay (1968) considered the entire Trenton Limestone as belonging to one series which he termed Trentonian (although it was also used as a rock-term), and further recognized the Wildernessian, and Barneveldian intervals as sub-series time-rock terms. Moreover, Kay identified four time-rock stages (Rocklandian, Kirkfieldian, Shermanian, and Cobourgian), and an additional series of sub-stages within the Shermanian (Shorehamian and Denmarkian). ![Time-Stratigraphic Series, Stages, and Sub-Stages of the Trenton Group Image modified from Kay (1968)](/sites/g/files/omnuum8411/files/styles/hwp_1_1__720x720_scale/public/trenton/files/kay1968_600w.gif?itok=zR_r1CpH) *Time-Stratigraphic Series, Stages, and Sub-Stages of the Trenton Group. Image modified from Kay (1968)*Although the biostratigraphic based stage-level classifications established by Kay work fairly well for correlation in the New York, Ontario, Quebec regions, historically, the application of this complex of time-rock terms outside of the region has been difficult. The development of Kay's high-resolution stratigraphic framework was the result of many years of research and integration of multiple correlation methods including lithostratigraphy, event stratigraphy (K-bentonite correlation), as well as biostratigraphy. Unfortunately, due to facies change, biogeographic provincialism of key taxa, and the variable preservation of correlate K-bentonite horizons, the application of the Upper Mohawkian Time Scale of Kay (1968), has been problematic and is as yet unresolved. #### Macrofaunal biostratigraphy: Coenocorrelation and gradient analysis Concepts of single taxon "biozone" correlation proved to be difficult, especially in the Trenton outside of New York. Some of the most important contributions made by Marshall Kay were toward the delineation of individual lithostratigraphic units in the Trenton and the use of K-bentonites to establish direct time-line correlations between outcrop sections. Moreover in a biostratigraphic sense, the "typical Trenton" faunas although of poor use in biozonation, showed distinctive patterns in abundance. The figure below compiled from data from Professor Ray Gildner (Union College, Schenectady, New York) illustrates the vertical ranges and relative abundance of many taxa of brachiopods, bryozoans, trilobites and molluscs. Although the distribution and relative abundances (indicated by the width of the colored boxes) of individual taxa appear to be random, it is seen that individual taxa can be grouped in assemblages of fossils that often occur together and tend to remain in association. Kay often made the observation that "typical" Trenton taxa were rarely found individually or admixed with taxa from another assemblages. ![Relative Abundance Chart for Key Taxa from the Middle Trenton Interval (Shermanian Age) Image compiled from data by Gildner (2003).](/sites/g/files/omnuum8411/files/styles/hwp_1_1__720x720_scale/public/trenton/files/gildnertrentondata.gif?itok=jLs_nM9F) *Relative Abundance Chart for Key Taxa from the Middle Trenton Interval (Shermanian Age). Image compiled from data by Gildner (2003).*These observations were never fully investigated, but Kay attributed these patterns to minor changes in depositional environment and water depth. Based on modern lithologic descriptions, and the development of small-scale cycles within larger-scale depositional sequences, it is quite reasonable to attribute Kay's small-scale changes in fossil assemblages to a variety of environmental changes impacting the ecology of fossil communities. In the late 1970's, John Cisne and colleagues from Cornell University began to quantitatively document the small-scale variations in fossil assmemblages both in vertical dimension (time) and in the lateral dimension (space). By using changes in fossil assemblages in time and space, Cisne and Rabe (1978) applied concepts of coenocorrelation (community correlation) and gradient analysis to generate correlations, and to explain trends in the vertical and lateral changes in community composition as observed by Kay. Cisne et al. (1982) used the position of K-bentonite (volcanic ash) horizons in localities from Trenton Falls eastward into the the Mohawk Valley to constrain individual stratigraphic intervals. Then by collecting fossil presence/absence and abundance data from individual stratigraphic sampling stations, they applied a variety of statistical techniques to compare their data both vertically between sampling stations and along transect downslope into the Taconic Foreland Basin. ![Gradient Analysis of Fossil Communities from Trenton Falls to Spraker's New York and their relationship to graben structures. from: Cisne et al., 1982](/sites/g/files/omnuum8411/files/styles/hwp_1_1__720x720_scale/public/trenton/files/cisneetal1982fig.gif?itok=Q-BLNWIB) *Gradient analysis of fossil communities from Trenton Falls to Spraker's New York and their relationship to graben structures, from Cisne et al., 1982*The diagram above, from Cisne et al. (1982), illustrates the use of relative abundance of individual taxa from a given stratigraphic interval (which in this case is located within the lower Denley, their 15 m marker) and the calculated reciprocal averaging score statistic. Along this single time-space transect from Trenton Falls (0 km) to Sprakers, New York (~60 km), the relative abundance values for each taxon observed change sequentially. These authors attribute the gradient from west to east to water depth increase from shallow to deep from the Trenton Shelf at Trenton Falls into the Taconic Foreland Basin near Canajoharie and Sprakers. Using this method, Cisne and Rabe (1978) demonstrated the patterns of community change across the transect during several time intervals as shown in the following figure. In this diagram, three time-specific comparisons were made between four main localities including: Trenton Falls, Rathbun Brook, North Creek, and Dolgeville. ![Relative Abundance Chart for Key Taxa from three transect intervals (m3, m15 & m40) from the Middle Trenton Denley Formation (Shermanian Age) between Trenton Falls and Dolgeville, New York Image modified from Cisne and Rabe (1978).](/sites/g/files/omnuum8411/files/styles/hwp_1_1__720x720_scale/public/trenton/files/cisneetalfig1978.gif?itok=uicZYOgJ) *Relative Abundance Chart for Key Taxa from three transect intervals (m3, m15 & m40) from the Middle Trenton Denley Formation (Shermanian Age) between Trenton Falls and Dolgeville, New York. Image modified from Cisne and Rabe (1978).*As observed from the diagram here and shown schematically on Gildner's figure above, most species from the middle Trenton at Trenton Falls, although variable in their relative abundance, are generally not present at all sampling stations. Moreover, when considered laterally across the transects, relative abundances of individual taxa are equally variable and show some patchy distributions. Notice the occurrence of two assemblages that show alternation in their relative abundance: Paucicrura / Sowerbyella assemblage and the Sphenothallus / Isotelus assemblage. In the figure, note the two incursions of the Sphenothallus / Isotelus assemblage, once in the vicinity of standard section marker 8 and again around standard section 20. Although both assemblages are present, the incursion (via increase in relative abundances) of the Sphenothallus / Isotelus assemblage again suggests, as mentioned previously, an environmental/ecological change favoring the change to the later assemblage. It is clearly suggested by Cisne and Rabe's figure above (1978), that the shifting pattern in relative abundances of these two assemblages is the result of water depth changes. The authors demonstrate the decrease in the reciprocal averaging score statistics both along transects at any one time and between transects during different times. In these examples, Cisne and colleagues have shown that the development of faunal assemblages at any given stratigraphic interval can change in time and space. In their estimation, the change in relative abundance of individual taxa is the result of environmental change resulting from sea-level fluctuation and migration of the Taconic Foreland Basin into the Mohawk Valley region. Similar to techniques used by Cisne and his colleagues, Ray Gildner has presented data on his website using the premise that if the presence of a fossil taxon in a given stratigraphic layer reflects the preferred living environment for that species, then the relative abundance of the species should be a measure of the environmental conditions optimum for the growth of the organism. Gildner (2003) used another statistical analysis similar to those used in gradient analysis and coenocorrelation. He has applied ordination methodology to try to establish environmental gradients using relative abundances of a variety of taxa. In his estimation, changes in relative abundance should reflect changes in any number of things: temperature, salinity, turbidity, depth, etc. ![DCA Ordination Scores Image modified from Gildner (2003).](/sites/g/files/omnuum8411/files/styles/hwp_1_1__720x720_scale/public/trenton/files/dcafiguregildner500w.gif?itok=OOrCWLpX) *DCA Ordination Scores. Image modified from Gildner, 2003.* ![DCA Ordination Scores Image modified from Gildner (2003).](/sites/g/files/omnuum8411/files/styles/hwp_1_1__720x720_scale/public/trenton/files/dcafiguregildner500wlabeled.gif?itok=7z3r5x5W) Similar to the reciprocal averaging technique used by Cisne, Gildner used a statistical treatment or correspondence analysis called detrended correspondence analysis (DCA), to explain the variation in relative abundances between samples, between time-slices, and between localities. DCA places each sample along a series of constructed axes in order to establish a gradient explaining the largest variation in the data. In the case of the Mohawk Valley relative abundance data, the greatest axis of variation is illustrated here. The most prominent gradient shows values ranging from low (dominantly to the left and bottom) to high (dominantly the the right and top). Using the ordination score, Gildner has color-coded the values to indicate the range of scores, from low (yellow), to high (red). Based on the DCA ordination scores, the shift in faunal assemblages and relative abundance of each taxa is clearly tied to changes in sea-level. Although Trenton Falls shows the shallowest scores overall, a number of patterns are developed and illustrated in the rollover image above. The most obvious pattern observed using the DCA data is the large-scale emplacement of deep water facies to the east of Trenton Falls. However, upon closer investigation, higher-order patterns are apparent. Even within the Trenton Falls locality, there appears to be a series of deepening and shallowing phases indicated by the faunal relative abundances. Biostratigraphic studies of the Trenton Limestone vary widely and have been developed for a variety of different purposes. Nonetheless, all studies have helped to elucidate important stratigraphic details surrounding the nature of the Trenton Limestone and its depositional history. ### Sequence Stratigraphy of the Trenton Group expand\_more ## Introduction ![View of Upper High Falls Photograph by Tom Whiteley](/sites/g/files/omnuum8411/files/styles/hwp_1_1__360x360_scale/public/trenton/files/brettsfaulthfalls.jpg?itok=O_mqcLyv) *View of Upper High Falls, photograph by Tom Whiteley*For many years it has been recognized that the stratigraphic record contains considerable geologic evidence of both tectonic and global sea-level change. Once geologists began to look in detail at the distribution, relative age, and continuity of various strata units, they began to establish a series of large-scale stratigraphic disconformities that recorded periods of time when the land-surface was exposed and erosion occurred. In 1963, Larry Sloss, having studied the spatio-temporal distribution of rocks on the North American Craton, realized that the deposition of rocks on the craton recorded periods of relative sea-level highs while the development of unconformities and truncation of strata represented sea-level lows. Using this concept, Sloss (1963) delineated a series of "megasequences" which were deposited upon the craton and separated by widespread unconformities. These "megasequences," as illustrated to the right, showed evidence of stratal onlap onto the craton where strata show transgression or sea-level rise. Moreover, these same megasequences show evidence for an upward change to stratal offlap patterns associated with regression or sea-level lowering. Subsequent to sea-level drop, major regions of the craton were left exposed and unconformity ensued. ![Image Modified after Sloss, 1963 ](/sites/g/files/omnuum8411/files/styles/hwp_1_1__360x360_scale/public/trenton/files/slosssfigure293w.gif?itok=aB_E8uhW) ![Image Modified after Sloss, 1963 "Tectonic Cycles of the North American Craton"](/sites/g/files/omnuum8411/files/styles/hwp_1_1__360x360_scale/public/trenton/files/slosssfigurelabeled293w.gif?itok=amZPVsGr) Although he did not realize the full implications of his observations at the time, Sloss's (1963) concept of correlating unconformity-bounded sequences helped to later revolutionize the study of sedimentary geology. Nearly 15 years later, geologists at Exxon studied the spatial and temporal distribution of sedimentary materials through seismic stratigraphic methods and recognized that within Sloss's large "megasequences' were a variety of lenticular shaped sedimentary packages bounded by seismic reflectors. These were also interpreted as regional unconformities, but at a smaller scale. This work, first published by Vail and others (1977), highlighted the importance of these reflectors as important higher-order time-lines representing periods of exposure and erosion, or at the least periods of non-deposition, and called these horizons sequence boundaries. These researchers recognized that when correlated offshore these reflectors expanded into conformable successions having no significant break in sedimentation. In Sloss's figure to the right, the small stepped notches superimposed on each of the megasequences represent the scale of sequences studied by the Exxon team. By delineating individual, unconformity-bounded sedimentary packages by their seismic profiles and comparing the seismic data to the rock-record, Vail and colleagues (1977) were able to create a series of models to explain the development of patterns they had observed. Based on their studies, Vail and others (1977) established the basic sequence stratigraphic model that provided a system of description for the observed depositional geometries and their chronostratigraphic relationships. Their model also predicted how a given sedimentary package might accumulate patterns of sea-level change and subsidence (Vincent, et al., 1998). Although, the sequence framework was initially developed for siliciclastic-dominated basins, the application of sequence stratigraphy to carbonate-dominated or mixed siliciclastic-carbonate depositional systems follows roughly the same conceptual model developed by Vail and colleagues (1977). The following discussion emphasizes the patterns associated with sequence stratigraphic models in the context of the Trenton Group. #### Sequence stratigraphy: The approach As was mentioned above, sequence stratigraphy is a descriptive tool used by stratigraphers to establish or predict the spatial patterns of deposition of a constrained sedimentary succession deposited during a single sea-level rise and fall cycle. Sequence stratigraphy is defined as a stratigraphic method that uses unconformities (sequence boundaries) and their correlative conformities to package sedimentary successions into spatially and temporally constrained sequences (Vail et al., 1991; Vincent et al., 1998; Emery & Myers, 1996). Any unconformity bounded "sequence" can then be divided internally into smaller-scale genetically related units (systems tracts) deposited during individual phases of sea-level change ( i.e. transgression, high-stand, regression, and low-stand). This method is a powerful tool in modern stratigraphic studies because it integrates many aspects of stratigraphy including seismic stratigraphy, lithostratigraphy, cyclostratigraphy, event-stratigraphy, and biostratigraphy into a single stratigraphic framework. The development of a sequence stratigraphic framework for any given depositional basin provides the stratigrapher not only with a temporal framework for studying depositional change, but it also provides a spatial framework. The following discussion introduces the concept of different scales (temporal and spatial) of sequence development observed in the rock record of the Upper Ordovician. An important aspect of sequence stratigraphy is its use in basin analysis through the establishment of very low-resolution (megasequence scale) to very high-resolution depositional spatio-temporal patterns (parasequence scale). The ensuing discussion describes several orders of sequences observed within the Ordovician. The magnitude of such sequences, as discussed by a number of authors including Van Wagnoner (1988), and Vail and colleagues (1991), generally refer to a variety of scales of temporally and spatially constrained depositional sequences. Those deposited during long time scales (i.e. 80-90 million years) and with large sedimentary thicknesses (1000's of meters) (megasequences of Sloss) are generally referred to as 1st-order sequences. Internally within these megasequences a number of 2nd, 3rd, 4th, and 5th-order sequences are developed over shorter time-scales, and range from 100's of meters down to meter-scale thicknesses in the case of higher-order depositional sequences. Low-order sequences are generally considered to be composite sequences and are composed of a hierarchy of smaller-scale, higher-order depositional sequences and parasequences. #### Ordovician tippecanoe megasequence It is clear from the sedimentary record on the North American craton, that Sloss's Tippecanoe megasequence represents one of the highest sea-levels ever recorded in the Phanerozoic. Ranging in duration from the base of the Middle Ordovician to the end of the Lower Devonian, the development of the Tippecanoe megasequence represents one of the longest periods of sea-level highstands at between 80 to 90 million years. The Tippecanoe megasequence was not deposited by a single large-scale, long-term sea-level rise and fall event. Instead, it was punctuated by the development of a series of additional unconformities effective on a variety of shorter and narrow temporal and spatial scales. ![Image Modified after Sloss, 1963 ](/sites/g/files/omnuum8411/files/styles/hwp_1_1__720x720_scale/public/trenton/files/slosssfiguretippecanoe1602w.gif?itok=0L-uPYnZ) ![Image Modified after Sloss, 1963 ](/sites/g/files/omnuum8411/files/styles/hwp_1_1__720x720_scale/public/trenton/files/slosssfiguretippecanoe1602wlabeled.gif?itok=nNcLYEUR) *Tectonic Cycles of the North American Craton, image modified after Sloss, 1963*One of the most important regional unconformities, was produced as a result of tectonism and the end Ordovician Glaciation. This unconformity, referred to as the Cherokee Unconformity, separates the Tippeacanoe Megasequence into an earlier phase (Creek Holostrome; after Wheeler, 1963) and a later phase (Tutelo Holostrome; after Wheeler, 1963), with each approximately 40-45 million years in duration. The Cherokee Unconformity is correlated across most of North America, and coincides with the Ordovician-Silurian boundary. In addition to the major delineation of the Tippecanoe into its lower Creek and upper Tutelo phases (2nd-order? sequences), the diagram above also shows a sea-level curve by Greenlee and Lehmann (1993). The diagram illustrates the high-frequency of short-term, low magnitude sea-level oscillations during both holostrome phases as well as during the overall Tippecanoe megasequence. These high-frequency sea-level changes represent much shorter duration events, that when considered collectively produce the patterns associated with the larger-scale megasequences. For the purposes of this discussion, the development of these high-frequency sea-level changes is directly related to changes seen in the deposition of the Trenton Limestone during the Late Ordovician. As such, the remainder of this discussion will be focused on sea-level changes and sequence stratigraphic interpretations for the upper Mohawkian strata of the eastern U.S. including New York State. #### 3rd-order sequences in the Mohawkian ![Image Modified after Holland and Patzkoskwy; 1996, 1998](/sites/g/files/omnuum8411/files/styles/hwp_1_1__720x720_scale/public/trenton/files/hollandpatzkowksy1996_293w.gif?itok=Du8jFkVe) *Image modified after Holland and Patzkoskwy; 1996, 1998*Beginning in the mid-to-late 1990's, Steve Holland (University of Georgia) and Mark Patzkowsky (Pennsylvania State University) published a series of papers investigating the sequence stratigraphy of Mohawkian and Cincinnatian strata. These studies by Holland and Patzkowsky (1996, 1997, 1998) helped to establish six large-scale third-order sequences (M1-M6) for the Mohawkian aged strata of the Nashville Dome to Cincinnati Arch Region with an additional six sequences (C1-C6) in the overlying Cincinnatian. Overall, the M1-M6 sequences each have a duration between 1 to 2 million years, and reflect deposition in the base of the Tippecanoe Megasequence (Creek Phase) as defined by Sloss (1963) and Wheeler (1963). The figure to the right shows the chronostratigraphic assessment of Holland and Patzkowsky's sequences relative to the Late Ordovician Mohawkian and Cincinnatian Series and their associated stages. Note that the upper portion of the Mohawkian, referred to as the Chatfieldian Stage (sensu Leslie and Bergström, 1996), is dissected into two main sequences, the M5 and M6 sequences for both the Nashville Dome and Cincinnati Arch regions. Given the absolute time-scale estimates provided, Holland & Patzkowsky's sequences equate roughly to third-order sequences of 1 to 2 million years each. Based on the extrapolated absolute age dates, it appears that the duration of the late Mohawkian was approximately 3 to 4 million years during the course of which the Trenton Limestones were deposited. #### Sequence stratigraphy of the Trenton Group Although the sequence stratigraphic frameworks constructed by Holland and Patzkowsky provide an excellent assessment of the stratigraphic record in the southern and central Appalachian Basin, their studies were not directly related to the type Mohawkian strata of New York State. Recent studies by Cornell (2001), Cornell and Brett, 2002, on the lower Trenton Group and the underlying Black River Group, and Brett and colleagues (2004) have investigated the New York succession in order to construct a similar sequence stratigraphic framework. Using a number of different correlation criteria, these workers have constructed a sequence stratigraphic framework based on the studies of Holland and Patzkowsky. The following figure is adapted from Brett et al., 2004, to illustrate the correlation of lithostratigraphic units, and sequences between New York State and the Jessamine Dome of central to northern Kentucky. ![NY](/sites/g/files/omnuum8411/files/styles/hwp_1_1__720x720_scale/public/trenton/files/ny-kysequences.gif?itok=H9ux1dVM) *Comparative sequence stratigraphy of two classic Upper Ordovician successions, Trenton Shelf (New York–Ontario) and Lexington Platform (Kentucky–Ohio): Implications for eustasy and local tectonism in eastern Laurentia. Image modified after Brett et al., 2004*As shown, the overall sequence framework of Mohawkian sequences (M4-M6 of Holland and Patzkowsky) have been identified in the New York and central Kentucky regions. Although the three sequences of Holland and Patzkowsky are recognized, Brett and colleagues (2004) recognized that both of the third-order (M5 and M6) sequences can be further sub-divided into a series of smaller-scale, unconformity-bounded high-order sequences and sequence components. The high-resolution sequence framework of Brett and colleagues (2004), have defined six high-order sequences (fourth-order?), each representing between 400,000 to 500,000 years as extrapolated from the absolute ages established by Holland and Patzkowsky. #### General sequence architecture As mentioned, several orders of cyclicity are recognizable in the Upper Ordovician mixed carbonate-siliciclastic strata of eastern North America. The smallest correlatable cycles are meter-scale shale-limestone successions, interpreted as parasequences (sensu Van Wagoner et al., 1988). Within the context of the Trenton Limestone, Brett and Baird (2002) have explained a number of these meter-scale cycles as presented in the discussion on lithostratigraphy. In subtidal shelf facies these cycles commence with thin-bedded calcisiltites/lutites and shales and pass upward into bioturbated nodular to wavy-bedded wacke- and packstones and finally into amalgamated pack- and grainstones (Brett and Baird, 2002). Larger discontinuity bounded depositional sequences of at least two orders of magnitude are also recognizable within the Trenton Limestone. Decameter-scale sequencesare comparable to larger sequences and include thin analogs of transgressive and highstand systems tracts. These sequences are typically 5 to 15 m thick, and are thought to record depositional cycles of a few hundred thousand years, comparable to fourth- order cycles of Vail et al. (1991). Larger scale sequences have thicknesses of tens of meters and inferred durations of between 1 to 2 million years, falling within the envelope of third-order sequences (Vail et al., 1991). Sequences recognized by Brett and colleagues (2004) represent subdivisions of the third-order composite sequences (M5 and M6) as recognized by Holland and Patzkowsky (1996, 1998). **Sequence Boundaries** Large, composite, and smaller scale sequence boundaries are marked in most study sections by a sharp contact at which significant erosion or at least a facies dislocation occurs. These surfaces may actually be erosion/transgression (E/T) surfaces, wherein the sequence boundary and transgressive ravinement surfaces are merged to form one surface. Such surfaces are laterally extensive and on a regional scale can be seen to overstep underlying truncated beds. **Lowstand deposits (LST's)** Lowstand deposits (LST) are generally lacking in shallow shelf Black River and Trenton facies, but may be present in basinal facies as bundles of allodapic carbonates intercalated with dark shales (e.g. Dolgeville Formation in the Mohawk Valley of New York; see Baird and Brett, 2002). **Transgressive systems tracts (TST's)** Thin, glauconitic lag beds followed by retrograding successions of clean, fenestral micrites to fossiliferous wackestones occur in the transgressive systems tracts (TSTs) of shallow water areas, especially in the Black River and basal Trenton sequences. Intervals interpreted as TSTs within the Trenton's shallowest subtidal facies are often composed of widespread pelmatozoan-brachiopod pack to grainstones. The general lack of siliciclastics in these intervals suggests that during TSTs the siliciclastics are sequestered in nearshore regions during rising sea level and are not transported offshore. Many of these skeletal limestones have been interpreted as the caps of shallowing upward parasequences (small-scale cycles). However, pack- and grainstones of the TST typically show more pristine, intact preservation of fossils, and poorer sorting. Shallowing upward intervals display highly abraded, cross-bedded calcarenites are more typical of RST (Regressive Systems Tracts). While TSTs may appear massive and homogeneous, careful examination shows evidence for increased condensation and deepening upward within the carbonates. Within the Trenton, TSTs often show development of mineralized hardgrounds and upward increasing appearance of reworked nodules, and phosphatic, glauconitic and/or chamositic grains. Meter-scale parasequences within the TST's exhibit subtle retrogradational patterns, and are usually very consistent in thickness and overlie surfaces of regionally angular discordance. In some cases TST grainstones may be slightly diachronous and formed as transgressive blankets of skeletal debris during initial sea level rise. **Maximum flooding surfaces (MFS)** Dramatic evidence for flooding appears at the top of TST grainstones. In these cases the abrupt shift to condensed, finer-grained deeper water facies lies in dramatic contact with coarser skeletal limestones of the underlying TST. In most cases these contacts occur at hardgrounds and show evidence for corrosion with pyritic or phosphatic coatings. Some previous workers (Titus and Cameron, 1976) have interpreted these surfaces as subaerial unconformities, but they show evidence for deepening and a high degree of condensation. These contacts are interpreted by Brett and colleagues (2004) as surfaces of maximum sediment starvation (SMS; sensu Baum and Vail, 1988) associated with maximal rates of sea level rise. The corrosion surface on top of the limestone may represent a considerable amount of time with no sedimentation, as there is evidence for a strong facies shift from relatively shallow water carbonate facies to offshore, shaly sediments. This degree of water depth change would require many thousands of years, for which there is no sedimentary record. Typically, within Trenton sequences the SMS is overlain by a thin interval of calcareous shale and fine-grained, argillaceous limestones commonly with hardgrounds and corrosion surfaces. These thin intervals are considered to mark a condensed interval, with at least some evidence for condensation and mineralization marked by thin concentrations of comminuted fossil fragments, phosphatic or pyritic debris, and organic matter enrichment. These intervals typically show very high gamma ray values and are used to infer maximum flooding zones and the position of deepest water conditions associated sediment reduction. **Early highstand systems tracts (HST's)** Early highstands in proximal sections are characterized by a few meters of thin, wavy bedded pack- to grainstones with shaly partings. In more down-ramp sections they consist of wackestones, fine grained calcisiltites, calcilutites, and shales, or simply dark, organic-rich shales. In the Trenton, HST parasequences show bed thickening and coarsening-upward trends typical of aggradational to progradational parasequence stacking patterns. These intervals show a high proportion of mud and silt, indicating a renewed influx of terrigenous sediments during times of high to slightly falling sea level. **Late highstand systems tracts : Regressive systems tract** A notable feature of many Trenton sequences is the development of a sharp facies dislocation in the later highstand. This sharp and locally irregular and often channelized erosional contact within late highstand is considered to represent a forced regression surface (FRS). In this case, rather than full subaerial exposure it is suggested by Plint and Nummedal (2000) that these horizons represent submarine erosion associated with a rapid drop in sea level. Often this surface is sharply overlain by coarser skeletal wacke- to grainstone beds, in some cases with abundant siliciclastic silt, that exhibit a shallowing-upward (progradational) pattern. This interval, between the FRS and the overlying sequence boundary is referred to by Brett and colleagues (2004) as the Regressive Systems Tract (RST). The RST often shows a sharp base and a general shallowing-upward pattern which is then truncated often at another obscure contact. The RST interval is overlain sharply by coarser skeletal packstones and grainstones of the next sequence TST. #### Sequences of the Trenton Group The diagram to the right shows a composite stratigraphic column for the entire Trenton Group from the Trenton Falls to Black River Valley region of New York State. Important stratigraphic marker beds discussed in other sections of this website are labeled to guide the reader. The roll-over image shows the sequence stratigraphic interpretations based on the assessments of Brett and colleagues (2004). The right hand side shows the relative pattern of sea-level rise (overall deepening-upward) as indicated by blue triangles, and sea-level fall (overall shallowing-upward) as indicated by yellow triangles. The image indicates the relative position of key sequence stratigraphic intervals and surfaces including: transgressive systems tracts (TST's), highstand systems tracts (HST's), regressive systems tracts (RST's), sequence boundaries (SB), maximum flooding surfaces (MFS), and forced regression surfaces (FRS). In the diagram the sequence components are color coded with TST's shaded in light green, HST's shaded in pale yellow, and RST's labeled in pink. The following discussion presents a summary of descriptions for each individual sequence illustrated in the diagrams below. ![column](/sites/g/files/omnuum8411/files/styles/hwp_1_1__960x960_scale/public/trenton/files/trentonstrat_labeledcolumn.gif?itok=5xv8D_f4) ![sequences](/sites/g/files/omnuum8411/files/styles/hwp_1_1__960x960_scale/public/trenton/files/trentonstrat_sequences.gif?itok=WpdH19yt) **M5A sequence: Watertown, Selby & Napanee formations** The basal Trenton sequence begins with the Watertown Limestone which was previously assigned to the upper Black River Group (sensu Young, 1943; Walker, 1973; Fisher, 1977; Cameron and Mangion, 1977), but the presence of a substantial erosion surface at the base of the Watertown regionally truncates underlying beds of the upper Lowville Formation and is interpreted as a subaerial sequence boundary. This sequence boundary is most dramatically developed in the central Mohawk Valley of New York where a channeled erosion surface at the base of the Watertown Formation at Inghams Mills, truncates more than 3 m of Lowville Formation. At this locality there is evidence for karstic cave development at the surface that later collapsed into a sinkhole. Northward into the Black River Valley the sequence boundary is more subtle, as less material is removed below it. Nonetheless, it can be recognized through the superposition of the Watertown on the shaly mudcracked stromatolitic micrites of the Upper Lowville Formation (Weaver Road beds of Cornell, 2001). The TST of the M5A sequence is marked by massive, locally cherty skeletal wacke- to packstones of the Watertown Formation and overlying Selby Formation. The contact between the Selby and the underlying Watertown is fairly sharp and shows evidence for condensation and sediment starvation especially in the southern Black River to western Mohawk Valley regions. From base to top, the Selby continues to show evidence of deepening and as such the contact with the underlying Watertown probably represents a sediment starvation surface at the base of the upper TST condensed interval. In the central Mohawk Valley, the shaly calcisiltites of the Napanee Formation, overlie the Selby-Watertown interval with unconformity (Cameron and Mangion, 1977). This sharp corrosion surface was formerly interpreted as a sequence boundary separating the Black River and Trenton Groups. Cornell (2000) has reinterpreted this contact as a submarine corrosion surface, as it displays evidence for dissolution, pyritization, and records a very high gamma-ray signature. Thus, this surface is now considered to be the maximum flooding surface of the M5A sequence. The highstand interval of the M5A sequence, is represented by the Napanee Formation of New York -Ontario. This interval is dominated by rhythmically interbedded calcilutite/calcisiltite and dark shale facies, and commonly shows a thick, amalgamated, middle bed of dalmanellid-rich grainstone that may represent a minor condensed interval. Based on recent biostratigraphy of Melchin et al. (1994), the Napanee also appears to correlate with shaly, thin-bedded facies of the middle Bobcaygeon Formation in Ontario (Armstrong, 1997). Acritarch and chitinozoan assemblages from the Napanee also indicate a deeper shelf environment (Melchin et al., 1994). In contrast to previous interpretations the Napanee is no longer interpreted as representing a lagoonal depositional environment (Titus and Cameron, 1976). Instead it is inferred that the Napanee and its faunas record a quiet-water offshore shelf depositional environment, that shows an aggradational to slightly progradational shallowing upward pattern characteristic of a HST. Near the very top of the Napanee, in several localities in the southern Black River to eastern Mohawk River Valleys (where the Napanee is still present), there is evidence of a sharp discontinuity surface and development of a few coarser-grained skeletal packestones. These are sharply overlain by a thick succession of very coarse-grained calcarenites of the Kings Falls Formation. This thin interval shows substantial shallowing and is inferred to represent a very thin RST. **M5B sequence: Kings Falls & Lower Sugar River formations** On the Trenton shelf, the Rocklandian rhythmite facies (M5A HST) is succeeded rather abruptly by skeletal grainstone facies of the Kings Falls (upper Bobcaygeon or Kirkfield in Ontario). This sharp facies dislocation marks the M5B sequence boundary. and the surface shows substantial erosion in a number of localities in central New York, where the basal Kings Falls contains clasts of Black River lithologies, as well as Grenville basement rocks. In several localities in the Middleville area, the entire Napanee Formation is truncated by the basal M5B sequence boundary. The accentuation of this sequence boundary in some localities is directly related to local tectonically uplifted highs developed during the later part of the M5A sequence and in the lowstand of the subsequent M5B sequence. In central to northern New York, the Kings Falls formation shows evidence for rapid upward transgression with very coarse grained facies grading upward into more condensed shaly nodular brachiopod, and echinoderm packestones and wackestones. The Kings Falls represents both the early and later condensed portions of the TST. In Ontario, the upper portion of this unit is famous for the development of the "Kirkfieldian Echinoderm Faunas." The top of the M5B TST is slightly more obscure due to the lack of outcrop exposures. However, in the southern Black River Valley region, the top of the Kings Falls shows evidence for condensation and phosphatization of skeletal grains before transitioning into shaly nodular wacke- to packstones with abundant Prasopora seen in much of central New York. The maximum flooding surface of the M5B sequence is placed at the contact of the Kings Falls and Sugar River. In outcrop exposures of the type Kings Falls Formation this contact is just below a distinctive irregularly-bedded disturbed zone within the overlying Sugar River HST. The Sugar River Formation is composed of wavy bedded pack- to fine-grained grainstone facies particularly noted for beds containing the domal bryozoan Prasopora simulatrix, and the trilobite Cryptolithus tesselatus. This lithofacies represents deposition in substantially deeper water setting than much of the underlying Kings Falls and is interpreted as the HST of this sequence. Throughout much of the western Mohawk Valley to the northern New York region, the Lower Sugar River Formation is generally aggradational in character, but shows some minor evidence for progradational cycle development. The highstand facies of the Sugar River and correlative units in eastern New York show an abrupt upward change to dark Flat Creek Shales (Mitchell et al., 1994; Joy et al., 2000). In the eastern localities the contact with the Flat Creek shows evidence of extreme starvation, phosphatic-pyritic staining and is inferred to be the result of tectonically enhanced deepening associated with the migration of the Taconic Foreland Basin into eastern New York. Although much of the lower to middle Sugar River is poorly constrained in outcrop, the transition into the later HST is associated with increased bioturbation near the contact with the Rathbun Member of the Sugar River. This interval, although dominantly fine-grained, is distinctive in the lack of substantial shaly interbeds found both below and above. In the Middleville, New York region this interval shows great abundances of Prasopora along a single bedding plane and represent an epibole horizon. **M5C sequence: Upper Sugar River (Rathbun Mbr) to Lower Denley (Poland Member)** Contrary to lower sequences, abrupt lateral changes are seen in sequence M5C beginning in the Rathbun Member of the upper Sugar River Formation. The lateral differentation of facies continues throughout the remainder of the Denley Formation and higher sequences on the Trenton Shelf. Although the basal sequence boundary of the M5C sequence is subtle in most regions of the Trenton Shelf, the dramatic change to crinoidal grainstones in the West Canada Creek Valley stand in sharp contrast to the underlying shaly-nodular fine-grained carbonates of the middle Sugar River. As these coarser grained carbonates are traced both east and west of Middleville, the facies transition laterally into finer-grained carbonates that are difficult to separate from the remainder of the Sugar River. It appears that the localized development of the Rathbun grainstones is related to tectonic warping of the Trenton Shelf and development of the "Middleville Arch." However, the expression of these grainstones shows an upward deepening, upward-condensing pattern and is interpreted as a TST succession. The MFS interval of the M5C sequence is fairly well-established. Within the base of the Poland member, a rather distinctive set of amalgamated condensed beds is well-developed. These beds, referred to above as the Glendale submember, continue the upward-deepening pattern shown in the underlying Rathbun, and become substantially more condensed (the late TST interval) with the development of the City Brook Trocholites Bed. The upper surface of this bed is very distinctive and shows phosphatic and pyritic staining. This contact marks the MFS of this sequence. The remainder of the Poland Member (above the basal Glendale sub-member) shows an upward coarsening series of small-scale cycles, which although thin, demonstrate a progradational pattern. This is also supported by faunal assemblage DCA scores as calculated by Gildner (2003) and shown in the section on biostratigraphy in this website. Within the upper part of the Poland a series of amalgamated packstone and fine-grainstones represent a change to rapid shallowing and is thus interpreted as the transition out of the lower HST and into the RST interval. Again as in the underlying sequence, the sequence boundary is generally subtle and recognizable through the identification of the overlying condensed zone of the overlying sequence. **M6A sequence: Upper Denley Russia member to Lower Rust Formation (Lower Mill Dam Member)** Brett and colleagues (2004) recognize the transition out of the upper Poland Member of the Denley into the Russia Member as representing a very condensed, basal transgressive systems tract. Although very fine-grained and calcilutitic, the development of very pure carbonates with very little clastic component represents a period of siliciclastic sediment starvation. In this case, the capping beds of the Poland are afossiliferous and their cap is accentuated by the deposition of the twin Kuyahoora K-bentonites. Unlike the equivalent sequence in Kentucky, which was identified by Holland and Patzkowsky (1996, 1998) by a sharp karstic contact within the Perryville Member and recognized as a major sequence boundary, the same sequence boundary in New York is very subtle and is developed in substantially deeper water facies. The remainder of the TST interval shows several upward-deepening, retrogradational cycles. Near the top of the TST of this M6A sequence condensation occurrs again with the development of several amalgamated condensed beds in the condensed late TST. These two condensed beds informally referred to as the "overhanging ledge bed" and the "Castle Road bed" most likely represent the condensed zone below the maximum flooding surface. As in Kentucky, the change from the TST, into the overlying HST is well-defined by the dramatic development of thin-bedded calcilutites and shales of the middle to upper Russia Member, thus representing a widespread deepening. This facies is widespread and analogous to that seen in the basal Trenton M5A sequence in the Napanee Formation. The occurrence of K-bentonites, widespread soft sediment deformation or "seismites" within the later part of the HST (in the Upper High Falls submember of the Russia), and dramatically increased siliciclastic input on the Trenton Shelf signals intensified tectonism during this time. Based on correlations of the Denley Formation downramp into siliciclastic dominated deposits, much of this tectonic development resulted in substantially steepened ramps during this time and is closely timed with the Amorphognathus tvaerensis / A. superbus conodont zonal boundary discussed in the biostratigraphy section of this website. The upper part of the M6A sequence is very well developed. The later part of the HST (going into the RST) is represented in Trenton Falls, by the transition out of the Upper High Falls submember of the Russia, into the basal Rust Formation. The development of two unique cycles represented by the Taylor Fork Bed and the "Lower Disturbed Zone," signals a dramatic change to coarser grained limestones and shallower depositional conditions. This interval shows evidence for progradation and most likely represents the RST interval. **M6B sequence: Rust formation (Mill Dam & Spillway Members)** The sequence boundary of sequence M6B is fairly sharp near the base of the Mill Dam Member of the Rust Formation. Although evidence for erosion is minimal, the sequence boundary is established at the upper contact of the "Lower Disturbed Zone" where the shaly nodular to brecciated fabrics of the disturbed zone transition to more massively-bedded coarse grained packstones and grainstones of the Upper Mill Dam. Within the Mill Dam the coarse-grained brachiopod and echinoderm facies show only minor argillaceous interbeds and cycles are poorly constrained. However, the taphonomic signature of large Rafinesquina brachiopods as well as other fossil fragments show an upward change from abraded and fragmented textures to substantially more complete and pristine fossil specimens. This change in taphonomic signature, along with an increase in mineral staining of bedding caps suggests that this interval is indeed deepening and condensing-upward. As such, the Mill Dam Member is thus interpreted as a TST interval. The upper surface of the Mill Dam TST in some localities in the Mohawk Valley is marked by pyrite and phosphate impregnated hardgrounds showing evidence for corrosion. Thus the contact with the overlying Spillway Member is interpreted as a maximum flooding surface. Overlying the MFS, as mentioned above, is the Spillway Member. On one scale of observation, the Spillway Member represents a single shallowing upward parasequence, it is composed of several smaller-scale cycles that become amalgamated upwards through the loss of shaly interbeds. As is common in most HST deposits, cycles show a general upward shallowing, progradational stacking pattern. In the case of the Spillway Member, the late HST although very thin, shows very dramatic evidence for channelization and slumping associated with rapid sea-level fall. Within the succession at Trenton Falls, the base of the spillway for the hydrodam is composed of the beds of the "upper disturbed zone" and demonstrate the dramatically folded and convoluted strata at the top of the Spillway Member. Thus defined, the "Upper Disturbed Zone" represents the regressive systems tract of the M6B sequence. **M6C sequence: Rust formation (Prospect Quarry Member)** The M6C succession in the Trenton Falls region begins with a very thin zone of grainstones at the base of the Prospect Quarry member sharply overlying and infilling structural depressions formed on the top of the Spillway deformed interval. Both the top and bottom contacts of the Spillway deformed interval represent depositional discontinuities, which in the case of the upper contact is interpreted as a sequence boundary. As mentioned, the thin grainstone interval deposited ontop of the underlying Spillway member shows dramatic evidence for upward condensation, and is capped by a pebbly phosphatic intraclast bed that is is sharply overlain by thin-bedded, shaly calcilutite carbonates of the Rust Quarry submember. This thin coarse-grained limestone package to shaly interbedded calcilutite interval is interpreted as the TST, MFS, early HST of the M6C sequence. Of particular importance is the unique preservation pattern and depositional history of the early HST of this sequence. As described by Brett and colleagues (1998), the basal portion of the Prospect Quarry Member is represented by the Walcott-Rust Quarry interval, from which many well-preserved species of trilobites, echinoderms and other faunas have been collected. The exquisite preservation of these fossils indicates relatively deep water settings and are in stark contrast to underlying and overlying grainstone facies. The HST interval of this sequence is fairly thin but shows, as with lower sequences, an upward shallowing pattern out of the fine-grained shaly lutite cycles of the Rust Quarry beds into bioturbated wackestone to packstones of the upper Prospect Quarry member. The top of the Prospect Quarry Member of the Rust Formation is marked by the change upward into regressive fine-grained grainstones that show some evidence of deformation, especially in the equivalent sequences in Kentucky. **C1 sequence: Steuben to Hillier formations** In contrast to underlying sequences, dramatic changes in sedimentation and basin configuration appear to have occurred during deposition of the C1 sequence in New York. In the Trenton Falls to northern New York region a widespread interval of crinoidal grainstone, referred to as the Steuben Limestone signals relatively shallow, but transgressive conditions, and sharply overlies the underlying Rust Formation. With the development of cross-stratified crinoidal sand shoals, especially in the Trenton Falls region, the Steuben formation most likely represents the TST component of the C1 sequence. The uppermost Stueben passes both upward and basinward into a succession of fine-grained turbiditic calcarenites-calcisiltites and interbedded black shales. The upper contact of the Steuben, where conformable, shows an abrupt transition to a back-stepping succession of shales and argillaceous packstones and then into a major shale-rich succession of the Indian Castle Shale of New York (Baird and Brett, 2002). Given these observations, the upper contact of the Steuben is interpreted as the MFS, with the overlying Hillier Formation representing the HST deposits of the sequence. As the Steuben represents the culmination of the Trenton, further discussion of sequence architectures is deferred. ![View of Upper High Falls Photograph by Carlton E. Brett](/sites/g/files/omnuum8411/files/trenton/files/upperhighfallsfromwest.jpg) View of Upper High Falls, photograph by Carlton E. Brett