Paleoclimatic Setting: Modern Tools and Ancient Rocks
In order to understand the basic paleoclimatic setting and environmental conditions under which the Trenton limestones were deposited, it is necessary to consider the dynamics of global air circulation and oceanographic circulation patterns and their impact on sedimentary environments. Both processes impact limestone production in modern settings and their study can help provide some insights for application to the rock record and the conditions under which the Trenton limestone was deposited.
The following sections introduce climatic and oceanographic circulation patterns and potential sedimentary indicators of environmental conditions. These observations will then be applied to the depositional environments under which the Trenton carbonate sediments were deposited.
Modern Oceanographic Circulation: Warm versus Cool-Water Carbonates
In modern depositional settings, carbonate sediments are produced in a variety of climatic regions across the globe. The production of carbonate materials, via the growth of calcium carbonate-producing organisms, occurs in most bodies of water on earth, both fresh and marine. Usually the amount of carbonate grains deposited is far exceeded by siliciclastic sediments derived from weathering of cratonic rocks. As a result, only in a few environments, where land derived sediments are not being deposited, can carbonate grains accumulate in sufficient quantities to produce a carbonate-dominated deposit.
The diagram below, modified from Nelson (1988), shows the distribution of carbonate-dominated shallow shelf depositional environments today. Nelson makes the distinction among three major groups of carbonate deposits: those from relatively warm water regions, generally >28° C minimum winter temperature; those from relatively cool waters, where the maximum water temperatures rarely exceed 28° C even during the summer; and those found under polar conditions. In the diagram, the location of shallow shelf carbonate deposits are color coded with respect to the water temperatures under which they are deposited.
The locations of cool-water carbonates are depicted using green highlights in the figure. Although this particular carbonate type is more prevalent in temperate waters (those above 30° North and South latitude) there are some occurrences of cool water limestones along the western coast of Africa.
Conversly, warm-water carbonate systems are much more latitudinally constrained thatn are the cool-water systems. Except for minor cabonate-dominated settings in the Mediterranean Sea, all warm-water carbonate depositional settings, as shown in red, are located withtin 30° north to 30° south of the equator. Moreover, their predominance along the western shores of ocean basins (the western Atlantic in the Caribbean, the the western Pacific in the Indonesia northern Australia region, and in the western Indian Ocean along the west coast of Madagascar) is the result of westward transport of warmed tropical surface waters. In these unique positions, the seawater temperatures remain fairly constant during the year and rarely go below 25 to 28° C.
Modern Atmospheric Circulation Patterns: Driver of Seasonality
Earth receives the major portion of its surface thermal energy from the sun. Patterns of global circulation of air and water result from unequal heating of regions of the earth, and are complex and difficult to predict. Modern studies of meteorology, climatology, and oceanography are helping to investigate this complexity.
Because of the Earth's tilt and orbit around the sun, the amount of energy delivered to the Earth's surface varies on a yearly basis with the perpendicular angle of incidence oscillating between the southern hemisphere and the northern hemisphere. This oscillation produces climatic fluctuations that may change seasonally on large scales and impact major atmospheric circulation cells on the Earth's surface. As the largest amount of energy strikes the equatorial regions, it is not surprising that these regions are typically very hot and humid. Here, surface heating generates an ascending, low-pressure cell that circles the equator. Generally as the warmed air mass ascends into the upper atmosphere, its water vapor cools, condenses and falls back to the surface. During this process the air mass is moved away from the equator and descends once again to the Earth's surface near the 30 degree north and south latitudes. These regions typically develop regional high-pressure cells and are commonly very arid.
The basic principles of modern atmospheric circulation dynamics can be applied to Ordovician paleogeography. The following diagram was constructed to show the potential atmospheric and surface circulation patterns during the Ordovician. In both the northern and southern hemispheres, prevailing surface air circulation patterns are driven primarily by the rotation of the Earth, known as the Coriolis effect. The prevailing wind directions are labeled in the diagram for each latitudinal belt.
The images above show components of vertical air circulation patterns relative to the development of surface convergence zones. In the diagram these ascending and descending circulation cells are drawn and labeled as Hadley and Ferrel Cells. The upward curved arrows for the Hadley Cells indicate the position of the equatorial tropical low-pressure belt where relatively warm humid air masses ascend. Downward arrows depict areas where the air masses cool and then descend at or near the 30 degree latitudes. Likewise, the same arrows are drawn to show the atmospheric circulation of the temperate latitude Ferrel Cells. In this case, low pressure zones near the 60 degree latitudes facilitate the upward movement of air masses that travel either poleward in the Polar (Hadley-Type) Cells, or equatorward in the Ferrel Cells. The descending air masses, both at the 30 degree latitudes and at the poles, help to establish high pressure zones which are typically very windy and arid.
Although the basic pattern of atmospheric circulation holds on an average yearly basis, the oscillation of light energy incidence between the northern hemisphere and the southern hemisphere can change the dynamics of the atmospheric circulation in both hemispheres. The result is the shift in sub-tropical convergence and divergence zones that ultimately causes seasons.
Implications for Trenton Climatic Setting
Within the context of global Ordovician circulation patterns, southeastern Laurentia was situated in the vicinity of the southern sub-tropical high-pressure belt. Given this latitudinal position the eastern Laurentian epicontinental sea and the Trenton shelf would have been located in a region that was impacted by seasonal migrations of sub-tropical high-pressure zones. With relatively warm, dry summers (due to the emplacement of high pressure cells), and relatively stormy or monsoonal winters (due to the migration of low pressure cells northward), seawater in the southeastern seas on the Laurentia craton most likely experienced seasonal alternations in salinity due to variations in increased runoff and evaporation. Oceanic upwelling and temperature inversions also occurred, especially during the later Mohawkian when global sea-level was high.
Although these assumptions are based on a general model, the following discussions on sedimentology and paleoceanography will provide additional insights on the subject.
Modern Sedimentologic Indicators of Environmental Ranges
Carbonate grains are dominantly produced through the biotic growth and precipitation of skeletal hard parts, although some abiotic precipitation of calcium carbonate also occurs. Given that carbonates can form in a variety of environmental conditions, it becomes necessary to investigate the dynamics involved in the type and quantity of carbonate grains produced. Carbonates can be grouped, at least in theory, into two major environmental classifications: warm-water and cool-water.
The main controlling factor in this system of classification is seawater temperature. As most carbonate grains are produced biotically within a specified range of salinity, temperature, oxygen, and nutrient conditions, seawater temperature plays a direct role in the production and distribution of carbonate sediments. Thus the distribution of organisms in modern as well as ancient marine settings is an indicator of seawater temperatures as well as a gauge of salinity, oxygen, and nutrient conditions.
Salinity, and Temperature controls on the distribution of carbonate producing organisms
As part of an early attempt to quantify the environmental parameters under which major faunal assemblages are distributed, Lees (1975) introduced the following figure to document the relative temperature and salinity tolerances of major groups of carbonate producing organisms. In the figure he shows the approximate range of salinity and temperature conditions in the modern oceans, and the red or blue colors represent the field range of these conditions. The regions shaded in white represent salinity and temperature conditions not present in modern carbonate environments. The figure at the bottom left plots minimum salinity values (in parts per thousand) against maximum temperature (in ° C), while the figure in the bottom right plots maximum salinity versus minimum temperature.
A critical point to be taken from these figures is the environmental range (in salinity and temperature) for several major sediment producing biotic assemblages. Foramol sediments (colored in blue) represent cool-water carbonates dominated by forams and mollusks. The Chloralgal and Chlorozoan dominated assemblages (colored in red) dominate warmer water conditions. The Chloralgal assemblage consists dominantly of photosynthetic organisms such as green and red algae; Chlorozoan sediments are comprised of organisms which contain photosynthetic symbionts such as marine hermatypic corals and tropical marine algae.
Nutrient controls on the distribution of carbonate producing organisms
In addition to the pioneering studies by Lees and Buller (1972) and Lees (1975) on modern sediments, more recent studies have further investigated the relationships between temperature, salinity and nutrient supply on the production of carbonate materials by organisms. Fundamentally these studies have shown that with an increase in seawater temperature, not only does the production rate of carbonate materials increase, but so does the need for nutrient supply to fuel the growth of heterotrophic invertebrates. Thus nutrient supply as well as salinity and temperature becomes a limiting factor in the growth, and hence distribution, of carbonate producing organisms.
In place of the "chlorozoan" assemblage, James has reclassified this group as the "Photozoan Association", which he feels "better reflects shallow, warm-water, benthic calcareous communities and their resultant sediments". He further distinguished 5 sub-association facies types that can be recognized from sediments (James, 1997b). These "Photozoan" or warm-water associations were named to emphasize the major biotic components including living and fossil organisms such as hermatypic corals, rudistid bivalves, fusilinid forams, as well as green and red calcareous algae.
James introduced the term "Heterozoan Association" to replace (or include) the original Foramol Association of Buller, and to expand its use to more accurately define the variety of cooler-water carbonate deposits. In this category, James also introduced 5 sub-association facies types to distinguish the dominant heterotrophic biota composing the sediment. Particularly important in the context of the study of the Trenton Group, is the inclusion of two facies types, which James named bryomol (bryozoans and mollusks) and brynoderm (bryozoans and echinoderms) facies. He suggested that these were important facies types in the recognition of cool-water carbonates in the rock record. In addition to the dominance of bryozoan, echinoderm, and mollusk constituents, James suggested that trilobites would also be included in this association.
Through this classification system, James has uniquely separated out the biota on the basis of their trophic guild (heterotrophs versus phototrophs or mixotrophs) through their reliance on nutrients, light intensity, and temperature constraints. He has also provided a more detailed hierarchy for classifying the sedimentary record associated with each major environmental grouping. The sub-association facies types can help to establish additional environmental parameters such as salinity variations, turbidity (amount of clastic detritus), as well as hydrodynamic conditions (amount of wave or current energy).
Salinity, and Temperature controls on the distribution of abiotic carbonate production
Lees and Buller (1972) and Lees (1975) also investigated the environmental ranges under which most abiotic precipitation of carbonate occurs. In a diagram similar to the one above, and shown below, Lees has diagrammed the environmental conditions (maximum temperature versus minimum salinity and minimum temperature versus maximum salinity) under which oolith (any of a variety of rounded, layered carbonate grains) and pellet aggregates (flocculated carbonate particles) are found.
There are two main environmental implications documented by these two graphs. First, very little abiotic precipitation of ooids or pellets occurs under low temperature and salinity conditions (i.e. those conditions under which cool-water carbonates tend to form). And second, with respect to the type of sedimentary grain assemblage produced (i.e. whether ooid/aggregate or pellets) the main controlling factor appears to be salinity. Under lower salinity conditions the preferred sedimentary grains, given the necessary warm-water conditions, are pellets, and under higher salinities are ooids. In fact the average, normal marine tropical seawater conditions range between 28 to 30° C and have salinities in the vicinity of 35 ppt (parts per thousand). Under these conditions, abiotic precipitation of carbonate is possible and with slight variations in salinity (i.e. seasonal changes), both pellets and ooid/aggregate grains are produced.
The work of Lees and Buller helped to distinguish the temperature and salinity range requirements for the production of micritic sediments, and their studies on the abiotic carbonates have been integrated into the "Photozoan Association" nomenclature of James (1997b). In this nomenclatural system, James indicates that the tropical or warm-water carbonate setting is dominated by the micritic component and that the cool-water or heterozoan associations "contain little or no lime mud". Thus, he includes the peloidal/oolitic facies as components within the photozoan association.
Paleoclimatic Implications from the Trenton Group Limestones
Paleogeographic setting of the Trenton Limestones
Given the position of eastern Laurentia during the Late Ordovician, (although there are some disagreements about the specific latitudinal position of eastern Laurentia during this time), most sources agree that the southeastern seaboard of Laurentia was located between 20 to 30° south of the equator at the time of deposition. Thus paleogeographically, the Trenton shelf, as well as much of the Laurentian craton, was situated within the sub-tropical to tropical belt.
In the diagram shown (below right), the approximate position of the Trenton shelf is in the subtropical meridian belt. Applying our modern understanding of these depositional environments, the Trenton Platform and associated coeval epeiric sea regions in Laurentia were likely located in a position that promoted the production of warm-water carbonates. Provided that the Taconic Arc Terrains off the southeastern coast of Laurentia) did not completely restrict westerly flowing currents, its position on the western side of the Iapetus Ocean further substantiates the potential for warm-water carbonate deposition on the Trenton Shelf.
Eastern Laurentia was therefore situated for the deposition of carbonates in tropical to sub-tropical environments. Sedimentologically, there is evidence to support this claim, at least during the early to mid-Mohawkian time. However, recent papers by Brookfield (1988), Holland & Patzkowsky (1993), and Pope & Read (199?) present some sedimentologic evidence that by mid-Mohawkian time, warm-water deposition was modified through invasion of cooler-water masses into the eastern Laurentian epeiric sea.
Sedimentary signatures of climate from the Trenton Limestones
Brookfield (1988) investigated the Trenton limestone equivalents (the Bobcaygeon, Verulam and Lindsay formations) from southern Ontario (up-dip from Trenton Falls). Brookfield suggested that the lack of ooids, very low sedimentation rates, high strontium isotopic values, as well as the dominance oftrilobites, brachiopods, bryozoans, and echinoderms and relative lack of calcareous dasycladacean algae and corals in these mixed carbonate lithologies, indicate cool-water deposition. Although there are a number of sedimentologic characters that favor the alternative (i.e. large amount of micrite and peloidal carbonates, numerous hard-ground and early cementation features etc), Brookfield 's assessments are generally supported by other researchers.
Coordinated patterns of brachiopod extinction, phosphatization, and siliciclastic influx
In some of their earliest analyses of Upper Ordovician strata from the Nashville Dome region and time equivalents of the Trenton limestones, Steve Holland and Mark Patzkowsky began to document the regional mass extinction of many brachiopod genera from the central and eastern United States. They also documented the sedimentologic changes they observed in order to attempt to explain the extinctions of brachiopods and other taxa.
Holland and Patzkowsky made some of the same observations as Brookfield (1988), and also observed that in addition to faunal constituents, limestones of the Nashville Dome and Cincinnati Arch showed several key shifts in nutrient content. By mid-Mohawkian time (just after the onset of Trenton deposition) the limestones of central Kentucky and western Tennessee showed increased levels of phosphate as well as terrigenous clastics. In their diagram, shown below, Holland and Patzkowsky documented these changes in sedimentation as occurring simultaneously at the base of the Chatfieldian Stage (near the base of the Rocklandian Stage). They suggested that this event was "characterized in eastern North America by coordinated changes in faunal assemblages, carbonate lithologies, siliciclastic influx and phosphatization. [They] ascribed these changes to a regional paleoceanographic event brought on by thrust-driven flexural subsidence and sea-level fluctuations that lowered water temperature and increased turbidity and nutrient input."
Environmental significance of micrites and hardgrounds
Holland and Patzkowsky's analyses were based on geographic localities over 600 miles or more to the southwest of Trenton Falls, but some of their observations are also true for the Trenton shelf. There appear to be some of the same biotic outages in the Trenton region, and there is definitely an increase in siliciclastic influx. Despite these similarities in biotic events, the limestones of the type Trenton Group do not share all the sedimentologic characteristics which would be considered typical of the heterozoan or cool-water carbonate model. In fact, there are many characteristics that would be considered by James (1997) to be typical of the photozoan or warm-water associations and are contradictory to the observations of Brookfield (1988).
One of the most important sedimentologic characteristics typical of most of the Trenton limestone is the predominance of micrite and micritic envelopes on many shelled organisms (see image at right). The presence or dominance of fine-grained, cryptocrystalline micrite in these limestones is a feature shared with modern warm-water carbonate depositional environments. Within the range of carbonate lithologies of the Trenton, the fine-grained micritic matrix is perhaps the most dominant sedimentologic component of all Trenton limestones (with the exception of the very coarse grainstones of the Steuben). Moreover, in thin-section, many of the micrite-dominated mudstones, wackestones and packstones appear to be composed of sometimes normally graded to homogeneous peloids. This latter fact suggests that deposition, at least in the model of Lees (1975), occurred under warm-water, normal to slightly brackish (salinity <35 ppm) marine conditions.
In addition to the predominance of micrite, the Trenton limestone often has well-developed hardground surfaces that show evidence of early syn-sedimentary lithification on the sea floor. Such hardground surfaces, and the early cementation of sedimentary particles at or just below the sediment-water interface, is a well-established component of modern tropical carbonate shelf environments (Wilson, 19??). In these areas, the pore-waters are often supersaturated with respect to carbonate and readily form carbonate cements very near or at the sea-bottom. Such hardgrounds are often important for the colonization of hard-substrate communities and the predominance of such features in the Trenton and its equivalents elsewhere (Kentucky, Ontario etc.) suggest that at least intermittently, environmental conditions (warm-tropical) were right for the development of these features (Brett and Brookfield, 1984; Brookfield and Brett, 1988; McLaughlin et al. 2000?).
Paleontologic signatures of climate from the Trenton Limestone
There are at least two other lines of evidence from the paleontologic record suggesting that both warm and cool-water carbonate depositional conditions may have played a role in the deposition of the Trenton limestone. Two related studies of the orthid brachiopod Paucicrura rogata (Sardeson), one geochemical and one anatomical, suggest that the depth of deposition may have been a determining factor in the temperature under which these carbonates were deposited.
Punctal density in Brachiopods and temperature profiles
First, based on well-established chronostratigraphic correlations of the Trenton Group of central New York, studies by John Cisne and his students (Ackerly et al., 1993) helped to delineate a relationship between the number of punctae (small perforations in the brachiopod shell) and the estimated water depth from which they were collected. In this work, these researchers found a positive correlation between the number of punctae (per square millimeter) and the inferred water depth conditions in which they were living. They found that the number of punctae in Paucicrura rogata increased from 350 to almost 450 punctae per square millimeter from the shallowest facies into the deepest facies within a single correlated chronostratigraphic interval. This relationship seems peculiar since it is known from studies of modern brachiopods that the number of punctae (or punctal density) increases with an increase in water temperature. If Paucicrura rogata grew according to the same parameters found in some five different genera and 18 different species of modern brachiopods, then this suggests that there would have been a temperature inversion in the Trenton basin. That is, as water depth increased, the water temperature also increased.
Oxygen sotopic analysis of Brachiopods: Constructing temperature and salinity profiles
The second study on Paucicrura rogata was based on oxygen isotopic analysis of these brachiopods from a variety of water depths from shallow shelf to deep basin. In their study, Railsback and Anderson (1989), attempted to estimate temperature-salinity profiles for the same interval of rocks analyzed by Cisne and others (see above). Railsback and colleagues were able to measure stable oxygen isotopic values (see image at right) from the well-preserved calcite shells of Paucicrura rogata . The distribution of values (delta 18 O values relative to the PDB standard) ranged from approximately -5 to -8, and generally decreased (i.e. became more depleted) downslope into deeper waters (samples A through F). However, the deepest water settings (samples G and H) showed a return to more enriched delta 18 O values.
Railsback and Anderson found that if the data were modeled assuming modern oceanic isotopic compositions, the observed pattern shown above right, could not be produced. Instead, they suggested that d 18 O values of the Ordovician ocean was depleted (relative to the modern ocean) and had lower temperature conditions at the surface and increasing temperatures at depth. Using these assumptions they postulated a stable seawater column that was reasonable and fit their data.
Understanding that calcite precipitation from seawater requires a temperature-dependent fractionation of oxygen isotopes, (i.e. heavy oxygen, d 18 O, is preferentially concentrated in the precipitation of carbonate at relatively low temperatures), Railsback and colleagues were able to plot the range of measured
brachiopod isotopic compositions relative to temperature (shown in orange). Assuming modern parameters for density variance (versus known salinity values) they also constructed the diagram as shown in the roll-over image to the right. In the figure, the dashed green lines show the range of seawater density values plotted against the oxygen isotopic values measured from the brachiopods. Using the approximate depth range of each of the samples (and a reasonable seawater density value for their depth), they could then plot their sample values relative to both temperature and salinity. The isotopic values and approximate depth ranges, transformed in their model into a temperature-salinity profile, allowed them to draw some conclusions regarding the temperature and salinity conditions under which the brachiopods were living, and hence the paleoenvironmental conditions under which the Trenton limestones were deposited.
In the figures above, two important aspects of the graph are highlighted. The first shows the relatively low salinity values (26 to 29 ppt) under which the shallow-water samples: A, B, C, and D, were deposited, and the relatively high salinity values (36 to 38 ppt) under which the deep-water samples: F, G, and H were deposited. The second image highlights the temperature range under which these samples were deposited. The shallowest samples A, B, C, and D, appear to have been deposited in relatively cool-water conditions (13 to 20 degrees Celsius), while the deeper samples show relatively warm-water conditions (28 to 38 degrees Celsius). Railsback and colleagues suggest then that the Upper Mohawkian seas contained warm, saline deep waters with slightly brackish, cool-water surface conditions.
Although both studies on the brachiopod Paucicrura suggest that the Trenton shelf may have been cooler at the surface and significantly warmer with depth, there is some controversy that dysoxic conditions within the foreland basin may have contributed to reduced growth rates of the brachiopods. This would complicate the signal by increasing the packing density of punctae per unit area without any temperature variation. Moreover, the temperature and salinity conditions as suggested by Railsback and Anderson (1989) may be unreasonable considering the sedimentologic data presented previously.
Paleoceanography of the Trenton Limestone
It is not surprising that the paleoceanography of the Trenton limestones is puzzling, given the somewhat controversial and disparate conclusions from paleo-environmental studies. The following discussion presents inferred paleoceanographic circulation patterns for the Ordovician oceans (based on the global assessments of Wilde, 1991), and the implications of more regional paleoceanographic conditions under which the deposition of the Trenton limestones most likely occurred. In addition, sedimentologic and stratigraphic evidence is presented to further investigate the potential circulation patterns by which the Trenton shelf may have been influenced.
Paleoceanography of Middle to Early Upper Ordovician oceans
In a volume dedicated to the "Advances in Ordovician Geology" published in 1991, geologist P. Wilde presented the case for paleoceanographic circulation patterns of the Ordovician seas. The paper entitled "Oceanography in the Ordovician", focused on "fundamental oceanographic circulation principles and their implications for the biogeography of the Ordovician Period as a function of paleogeographic reconstruction of the major continental blocks" (Wilde, 1991; p. 284). Wilde presented a series of models suggesting the inferred paleoceanographic circulation patterns for the Middle Ordovician. These models are shown in the upper images below, and although they are based on some preliminary paleogeographic reconstructions, they are designed to show the potential circulation patterns of the global oceans during the Caradocian Series of the Ordovician.
The images below show the inferred differences between summer and winter seasons and the expansion and contraction of cool temperate waters along the sub-tropical-to-temperate subtropical convergence zone. The lower image was constructed using more recent paleogeographic reconstructions from Blakey, 2003 with paleocurrent information plotted in the rollover image.
In the upper diagrams, Wilde (1991) has labeled key paleoceanographic circulation components that may have been active during this time. Several of the key features include: SE and NE (south equatorial and north equatorial currents respectively), ST (south tropical current), M (monsoonal counter current), SSP (south subpolar currents), and H (oceanic high-pressure zones). Because most of the northern hemisphere was continent free at this time, much of the northern ocean remained relatively cool and relatively unmodified between winter and summer seasons. Significant seasonality and monsoonal conditions would have developed in the southern hemisphere due to the large continental masses concentrated there. In the southern hemisphere, winter would have been dominated by relatively dry conditions with the development of three separate high-pressure zones. Conversely during the summer seasons, increased sea-surface temperatures and high insolation values would have contributed to reversal of several high pressure cells with the creation of at least one monsoonal cell.
Paleoceanography of Middle to Early Upper Ordovician of Eastern Laurentia
Investigation of the impact of global Upper Ordovician circulation patterns on the development of the Trenton Shelf requires determining the most plausible paleoceanographic conditions for eastern Laurentia. Using the paleogeographic diagrams of Blakey (2003) and the principles discussed by Wilde (1991), the author has inferred the regional paleoceanographic conditions which may have been in place during the deposition of the Upper Trenton Group.
The following paleogeographic diagrams show the central cratonic mass of Laurentia and the major circum-Laurentian currents. In the region just north of the equator, westward-flowing tropical equatorial currents would have directed warm waters away from the Laurentian craton. To the south and east of Laurentia, in the Iapetan Ocean, warm westward-flowing equatorial currents would have flowed toward the eastern shelf of Laurentia and descended along the shelf into temperate latitudes. A return flow current would have moved cooler sub-polar waters northeastward along the western side of the Baltic craton and delivered cooler waters into the sub-tropical portion of the eastern Iapetus. In addition to the east Iapetan sub-polar current, another sub-polar gyre would have delivered cool waters to the southwestern margin of Laurentia in the area of thethe present-day Gulf Coast region. This particular gyre would have been important in the circulation patterns of the eastern Laurentian epicontinental sea. As proposed by Holland & Patzkowsky (1993), Pope and Read (199?) and more recently by Kolata et al. (2001), and Ettensohn (2001), the development of the Sebree trough during mid Mohawkian time along the Reelfoot Rift Zone would have aided in the transport of the cool, nutrient-rich, south Laurentian sub-polar current into more sub-tropical regions. It is inferred that a return flow circulation of warm, saline tropical waters may have been in place through the Champlain trough (the Taconic Foreland Basin).
Paleoceanographic setting of the Trenton shelf
The activation of the Sebree Trough during Mid-Mohawkian time may have substantially influenced the depositional environments of the eastern Laurentian epicontinental sea. Despite the fact that the Sebree Trough funneled cool, nutrient-rich waters into the Nashville Dome to Cincinnati Arch regions and impacted deposition in those areas, it is likely that the south Laurentian sub-polar current became substantially warmer and perhaps ascended or upwelled to the surface in the region of the Pennsylvania Embayment and south Trenton Shelf. The temperature-salinity profiles constructed by Railsback and Anderson (1989) would suggest that the cool, less-saline surface waters of the Trenton Shelf may have originated with the upwelling of the south Laurentian sub-polar current. The warm, saline waters at depth may have been produced in the shallower regions of the Trenton, Galena and Ontarian platforms in more tropical waters and due to their increased density sank below the less dense waters funneled through the Sebree.
An alternative explanation, in the absence of Sebree trough upwelling, is that the cooler, less-saline surface waters may have formed on a seasonal basis as fresh-to-brackish, quasi-estuarine, surface lenses resulting from increased monsoonal runoff from the craton or the newly uplifted Taconics. In order for this later scenario to contribute significant amounts of fresh water runoff to the Trenton Shelf region, fairly substantial regions of land proximal to the Trenton shelf would have had to been periodically producing large volumes of runoff. Moreover, with increased runoff, there would have been a substantial increase in the sediment influx into the region. This would have shut down carbonate production. As this latter scenario is not borne out in the sedimentology of the Trenton limestones, nor is there any substantial evidence supporting the presence of a large land-mass nearby, especially during times of elevated sea-level, it is likely that this hypothesis does not account for Railsback and Anderson's temperature-salinity inversions.
Sedimentologic record of paleoceanographic currents in the Trenton limestone
The paleontology and sedimentary record of the Trenton limestone suggests a variety of environments under which these carbonates were deposited. These limestones preserve many sedimentary structures and distinctive fossil orientation features that help to decipher paleocurrent directions. In addition to direct current lineations via grove and gutter casts, tool marks, and aligned fossil specimens, there are other sedimentary features that help to decipher the paleoceanographic circulation patterns of the Trenton shelf. These sedimentary features include: inclined cross-bedding, current and oscillatory ripples, as well as larger-scale features such as channel structures and slump deposits. Collectively, these distinctive features help to decipher paleocurrent flow regimes under which the rocks were deposited.
The following discussion presents some of the paleoflow data that have been published from the Trenton limestone. Although most of the paleocurrent indicators are not just from Trenton Falls, most of them are from exposures nearby in the upper Black River Valley or the western Mohawk River Valley. The diagrams below are arranged in ascending stratigraphic order; the oldest strata are shown first, with successively younger stratal intervals towards the bottom of the page.
The two images below show raw paleocurrent data plotted as rose diagrams to indicate both modern compass bearings and quantity of observations for each measurement. The character and quantity of observations are provided in the upper right corner with the inner and outer circles showing 20 and 40% of the observations respectively. The rollover image shows a double-ended arrow with the inferred current lineation (if no one direction is dominant) or a unidirectional arrow if the data show one dominant paleocurrent direction.
The following 3 diagrams are from Chenoweth (1952), and represent paleocurrent data as collected primarily from the lower Trenton limestones from the lower Black River and West Canada Creek Valleys.
To the left, is a diagram depicting the orientations of 44 rippled beds from the Kings Falls Formation, which generally show a bimodal pattern suggesting an oscillatory current oriented NE-SW.
The image to the right is for the Sugar River Limestone (Shoreham of Chenoweth, 1952). Like the Kings Falls, the image shows a bimodal pattern in nautiloid shell orientation, thus suggesting that oscillatory currents were involved in the final deposition of these particular deposits. The current lineation suggests a NNE by SSW or roughly N-S current orientation.
As with the Kings Falls, Chenoweth has diagramed compass orientations from 22 rippled horizons primarily from the lower Denley (Denmark of Chenoweth). In this rose diagram, oscillatory currents are suggested although the directions of currents show a significant change relative to the currents associated with the both the Kings Falls and Sugar River. In this case, current orientation is nearly E-W.
In the diagram to the right, paleocurrent data converted from map diagrams of Cisne and others (1982) have been consolidated here by the author for the regions west of the Little Falls fault zone. This region includes the Trenton Falls type section. Measurements are made from turbidite paleocurrent lineations from the Upper Denley and its lateral equivalents. In this case the observations show unidirectional currents that are directed nearly W to E. Cisne and his colleagues believed that these data represent current direction from the shallower shelf down-dip to deeper portions of the basin.
The diagram to the left has been constructed from data presented in Jacobi and Mitchell (2002). This rose diagram has been constructed using data from two localities in nearly the same region as the measurements from Cisne and others but were taken from the Dolgeville Formation. In stark contrast to the measurements from Cisne and others, the measurements of cephalopod orientations and the dip of cross-bedding suggest a dominant NE to SW unimodal current direction. These data are also supported by the studies of Ruedemann (1897) on graptolite orientation from both the Dolgeville and Utica Shale.
Although the paleocurrent data considered collectively show no specific dominant paleocurrent direction, the data do show a dynamic pattern of current change for the Trenton Shelf during the deposition of the Trenton limestone. The mechanisms for this change are poorly constrained,but the data do allow us to evaluate the timing, and potentially, in the context of paleocirculation models presented above, the relationship of these current indicators to paleoceanographic patterns for the region.
When one focuses on data from the lower Black River valley (western portion of the Trenton Shelf), paleocurrent lineations for the Kings Falls and Sugar River formations generally show a northeast-southwest preferred direction. Although these data suggest bi-directional currents, additional data presented by Chenoweth (1952), show the preferred orientation of high-spired gastropods including Hormotoma and Subulites. This data set shows a more substantial unidirectional current with the orientation of the apical end indicating a NE directed current.
Considering that the Sugar River Formation appears from sedimentologic evidence to have been deposited under relatively low-energy, deep shelf conditions and well-below normal wave base, these data still provide some evidence of at least periodic high-energy events. It appears that, although bidirectional currents were occasionally involved in the formation of ripple marks in these limestones, they were oriented along roughly the same trend as unidirectional current indicators.
It also appears that at other times, unidirectional northeasterly-directed currents impacted the deposition of these rocks. This pattern of deposition probably was true for the underlying Kings Falls as well, although it is dominantly a shallower water deposit (probably very near normal wave base conditions). In light of stratigraphic evidence from the Middleville, New York area, it appears that the region immediately to the southeast of Trenton Falls had until this time been characterized by intermittent exposure and truncation of underlying units. The presence of this paleo topographic high (i.e. Middleville Arch) undoubtedly influenced the northeasterly directed currents.
The sedimentologic record of paleocurrent flow directions becomes somewhat modified during the deposition of the late Sugar River and early Denley formations. The data presented by Chenoweth (1952), show the development of more E-W directed oscillatory currents as indicated by ripple orientations. In this case, it appears that the NE-SW trends of pre-Denley times shifted to more E-W orientation. This observation suggests that the presence of the Middleville arch was no longer impeding current flow to the east. This change in current direction is coincident with the timing of synsedimentary block faulting in the eastern Mohawk Valley region. In the Trenton Falls area, the same strata used in determining the paleoflow directions for the basal Denley also show some of the earliest deformed intervals in the Trenton. Thus it appears that the onset of block faulting, foreland-basin subsidence and a shift in paleocurrent flow direction all occur at nearly the same time.
During deposition of the Denley Formation, data of both Chenoweth (1952), and Cisne and others (1982), suggest that the dominant down-slope, down-dip paleoflow direction was roughly west to east into the newly subsiding foreland basin. In contrast, the more easterly equivalents of the upper Trenton limestone show yet another paleocurrent flow direction. In some of the first studies on paleocurrent data, Rudolf Ruedemann (1897), used graptolite lineations to suggest that paleoflow directions from the deep-water, shale-rich environments of the Taconic Basin were to the southeast. More recent studies by Jacobi and Mitchell (2002), show the same general trend, but with added complexity.
In summary, from early to Trenton time, paleocurrent flow indicators for the Trenton Shelf region show SW to NE paleoflow direction; that changed to a more W to E flow by middle to early late Shermanian. By the end of the Shermanian, paleoflow had reversed direction, relative to early Trenton paleocurrent indicators. These paleocurrent data suggest then that the topographic expression of the Trenton Shelf, and its sub-components, were especially important in the paleoceanographic circulation of the shelf. In light of both the geochemical and the paleoceanographic circulation reconstruction shown above, it is entirely plausible that these changes were related to larger-scale changes in the circulation of eastern Laurentia.