Lithology
Introduction to the Lithologies of the Trenton Group
In order to understand the basic compositional distinctions within the Trenton limestone, it is necessary to introduce several key concepts related to the discrimination and classification of carbonate rock types (as mentioned previously). The intent here is to briefly consider the range of carbonate rock constituents present in the Trenton, and not to provide a comprehensive understanding of carbonate sedimentology. There are many excellent texts published on the subject of carbonate sedimentology for additional information.
Grain composition
There are two primary categories for the classification of carbonate grains: Non-Skeletal and Skeletal. Non-Skeletal grains can be entirely abiotically formed as in the case of coated grains, grain aggregates and clasts, or both abiotically and biotically formed as in the case of peloids. Within the Trenton limestone, however, the majority of readily observable sedimentary grains are produced directly from the disarticulation and fragmentation of skeletal hard parts and are considered skeletal grains.
Non-skeletal grains
As mentioned above, non-skeletal grains can be produced either abiotically or biotically and can be classified into one of the following four categories: coated grains, as in the case of ooids and pisoids; grain aggregates, as in cemented groups or clusters of carbonate grains; clasts, such as lithified pieces of previously formed carbonate rocks; or peloids, which are any number of sand-sized, microcrystalline and often structureless carbonate grains.
Within the exposures of the Trenton Limestone at Trenton Falls, there are very few examples of coated grains or grain aggregates. However, clasts and peloids do occur in the Trenton and are important contributors, in some intervals, to the overall composition of the limestones.
Skeletal grains
By far the most-dominant carbonate grain type in the Trenton Limestone, skeletal fragments from echinoderms, arthropods such as trilobites and ostracodes, bryozoans, brachiopods, molluscs, sponges etc. make up significant portions of these limestones. The taphonomy or state of preservation of these fossil types helps to constrain the range of depositional processes under which these carbonate grain producing organisms were subjected after their death and leading up to their final burial. Moreover once buried, subtle mineralogical differences in carbonate composition (aragonite versus high- and low-magnesium calcite) determine whether these skeletal grains can be modified by diagenetic processes including dissolution and recrystallization. In the case of the Trenton, there are examples of both of these processes.
Matrix composition
The next major category to consider when discriminating carbonate rocks such as the Trenton Limestone, is matrix classification. This category focuses on the sedimentary material, or lack thereof, found between grains within a rock. Generally sediments are classified as either grain-supported, where granular materials with readily observable size and shape dominate the rock, or as matrix-supported where the material is generally too small to be distinguished as a grain.
Matrix-supported
In this case, matrix materials usually consist of microcrystalline or cryptocrystalline carbonate sediments. These textural descriptors indicate that the crystal size and shape is too small to be readily observable without the use of a petrographic microscope and polished thin-sections. In common practice, carbonate grain sizes of less than 62 micrometers are roughly equivalent to mud, and most carbonate rock matrices are composed of mud-sized particles referred to as micrite in carbonate sedimentology.
As the compositional discrimination of matrices is highly-dependent on diagenetic effects and requires microscopic analysis, matrix classifications of the Trenton limestones is not a focus of the discussion here. One must, however, note the presence or absence of matrix within a rock.
Grain-supported
In the case of grain-supported carbonate rocks, this classification is used to define the dominance of visible sedimentary grains within a limestone. Due to their large concentration relative to cryptocrystalline grains, individual silt to sand-sized particles or larger grains are more likely to be in direct contact with one another. Thus in a grain-supported limestone, there is a general lack of fine-grained micrite and larger carbonate grains compose the majority of the rock framework.
Cements
Although once sedimentary carbonate grains are deposited, their final transition to rock represents an additional process that needs to be at least considered in the classification of most limestones. The cementation of carbonate grains is generally a very complicated process and occurs along several diagenetic pathways depending upon the environment of cementation, on the specific mineralogic composition of the carbonate grains themselves, and the mineralogic composition of fluids that flow through the sedimentary mass. The discussion here is only to briefly present the general description of cement types for classification purposes.
In most modern marine settings, the cementation of carbonate grains occurs on or below the sea-floor. This is accomplished either through the direct precipitation of cements around the margins of grains, or through the micritization of skeletal carbonate grains by microbial diagenesis and borings by other organisms. In either case, the process of cementation relies on environmental conditions such as temperature and salinity, as well as on the concentration of carbonate materials in sea-water. Due to a variety of stability constraints, the type of carbonate cement can vary. The most common carbonate cements in limestones are: high-magnesium calcite (high-Mg calcite), low-magnesium calcite (low-Mg calcite), aragonite, and dolomite. The recognition of such cements relies on mineralogic investigations of crystal morphologies.
Within the Trenton Limestone, due to long periods of mineral diagenesis, the most common carbonate cement is low-Mg calcite. Both aragonite and high-Mg calcite are relatively unstable, and these carbonate materials tend to alter to the more stable form of calcite, which in most of the limestones shows up as either isopachous rim cements or polygonal rim boundary cements. Unfortunately, there is no published petrographic analysis on the limestones from Trenton Falls, so further discussion of cements and cement types is not provided.
Carbonate rock classification
Throughout the last half-century, studies of modern carbonate depositional environments and their grain compositions and size distribution ranges have led to multiple classification schemes for carbonate rocks. Such studies have used modern textural classifications such as grain size, sorting, rounding, and grain composition and cement/matrix proportions to standardize the description of both modern and ancient carbonate rocks. These classification schemes, although slightly different in their nomenclatural development, remain very similar in their overall useage.
The most commonly used classification systems are either based on grainsize analysis or on the classifications of Folk (1962), or Dunham (1962), where both focus on textural characteristics of carbonate rocks. The following discussion will briefly introduce the conceptual usage of both classification schemes. Note that the intention of this discussion is to provide enough background for the reader to understand the lithologic classification of the Trenton Limestones. For a more detailed discussion of these classification systems, the reader is encouraged to review the bibliography section of this webpage and select the pertinent references for more information.
Grain-size classification
As is common in most sedimentary rocks, carbonate rocks can be classified according to the dominant grain size in the rock. Using this classification requires that the grain-size be estimated or measured exactly and then applied to a size-range chart for the establishment of a rock term. A carbonate rock is usually classified in one of three main categories: calcilutite (those rocks where the grain-size is 62 micrometers or smaller); calcarenite (those rocks where the grain-size is between 62 micrometers and 2 millimeters); and calcirudite (those rocks where the grain-size is greater than 2 millimeters).
The main strength of this classification system is to suggest a general association between grain-size, grain-sorting and deposition energy, however this system is most often limited for the description of most limestones.
Folk classification
As mentioned previously, the classification system of Folk (1959, 1962) relies mainly on textural characteristics of limestones and carbonate sediments. Folk, recognizing the three main compositional categories of carbonate rocks: grain composition, matrix composition, and cement composition, suggested that a carbonate rock classification should encompass aspects of all of these characteristics when trying to establish a rock name. The figure to the right shows Folk's nomenclatural system using: grain types (he referred to them as "allochems"); matrix; and cements. In this scheme Folk uses the allochem types (shown in purple) as the prefix, and the dominant matrix or cement composition as the suffix. For example, for a rock that contains a micritic matrix and skeletal fragments, the associated name would be biomicrite.
Folk's nomenclatural system provides a rough framework for the naming of rock types, but he further established that the range of matrix to cement occurs along a continuum from 0% matrix and 100% cement or spar, through 100% matrix and 0% spar. To further differentiate rocks showing different relative proportions of micrite and spar, he constructed the following diagram to relate the relative ratios of each to textural categories. In this later scheme shown below, Folk introduces the percent allochems, and degree of sorting, rounding and abrasion (shown in red) to name specific carbonate rock types. In this classification system, he has added prefix modifiers to establish more refined rock names (shown in blue). For example, if a limestone was shown to be dominantly of micritic matrix with approximately 40 % of the mass of the rock composed of ooids, Folk would have applied the name sparse oomicrite to communicate the rock's composition.
Dunham classification
In an alternative scheme to that of Folk, Dunham (1962) proposed a similar carbonate rock classification system utilizing some of the same principles used by Folk. In Dunham's nomenclature, he looked first at textural considerations of a rock including whether texture was recognizable in the rock. He then looked to see whether sedimentary materials were somehow bound as part of the depositional process. He was interested in separating those rock types where biologic activity had trapped sedimentary materials, as in the case of stromatolites. In his hierarchy, once the basic textural categories were assigned, he then looked at the relative proportion of mud in the sample. If the rock had no mud and was dominated by coarse-grained sediments, he classified these rocks as grainstones. However, if the sample contained any amount of mud it was then considered in percent relative to the number of grains. In this way, he was designated mudstones as having less than 10% grains, wackestones with more than 10% grains but less than the amount required to support the rock, and packstones where the sedimentary grains supported the rock framework but still had appreciable quantities of mud. Dunham's rock names are shown in blue below.
The limestones exposed at Trenton Falls have historically been described as belonging to two major, readily identifiable carbonate rock types: grainstones or crystalline limestones, and fossiliferous fine-grained carbonates with shaly interbeds. These gross lithologic assessments were adequate initially to define substantial textural differences within the Trenton Group. However, these early attempts at distinguishing rock types were more often left to establishing the faunal character of the rocks rather than the specific physical sedimentary characteristics. Beginning early in this century with the work of G. Marshall Kay, the individual lithologies of the Trenton Group were more intensively studied, and due to the sedimentologic or lithologic differences, rock-units were designated. These rock-units are discussed in the sections on stratigraphy.
In order to understand the stratigraphic assessments that have been made in separating the Trenton Limestone into its respective formations, members, submembers etc., the following discussion will present a basic description of each of the major rock types found in the Trenton Falls gorge. The focus is on helping the reader to understand the rock descriptions used in the context of stratigraphic discussions. Note however, the discussion here focuses on groups of lithologies that appear to be commonly associated. Due to their association these groups of lithologies are referred to as facies, especially when sedimentary structures, faunal composition and taphonomy, etc. are added to the basic lithologic descriptors. The emphasis on facies helps the geologist to make assessments of the depositional processes and the environment within which the rocks were formed.
Fine-grained carbonates
There are many limestones within the Trenton Group that are classified as fine-grained carbonates. In many stratigraphic intervals these fine-grained rocks appear on fresh surfaces to be barren of features and are simply referred to as calcilutites, calcmudstones, and biomicrites. Despite the simplicity of names that have been applied to these rocks, when studied in detail these fine-grained rocks demonstrate significant types and ranges of textural properties that enable further differentation. The following images show a few representative fine-grained carbonate rocks which come dominantly from the Rust Formation, but are representative of lithologies found in other stratigraphic intervals.
Thin-bedded calcilutite
The sample below is an example of a thin-bedded calcilutite photographed in alcohol. Notice that the calcilutite here displays a very fine-grained cryptocrystalline matrix texture except for a few minor vertical (Scolithos) and horizontal (Chondrites) type burrows shown by recrystallized calcite cements. There is a single crinoid ossicle, but this limestone is exceptionally barren. In the roll-over, the sample is etched in 10% hydrochloric acid in order to help accentuate compositional variations. In the sample, there is very little evidence for additional mineralogic variation in the sample.
Wavy-bedded, burrow-nodular calcilutite
This sample shown below is an example of another fine-grained carbonate rock. Similar in grainsize to the sample above, this calcilutite has a greater complexity of textures that enable further classification of the rock type. Overall the limestone shown below is nearly 100% micrite, but its bedding type can be classified as very thin (only a few centimeters thick) and wavy in its outline. In addition to the profile character of the bed, this calcilutite has internally been disturbed through the churning action of bioturbators. The rollover image accentuates the burrow outlines, and in some cases there is evidence for pelletal burrow infillings. However, in most of the sample burrow walls are poorly defined, suggesting a high level of mixing or churning by the bioturbators.
Thin-bedded pelmicrite
This following sample, at first glance appears to be a relatively homogenous thin-bedded calcilutite with a few crinoid ossicles at the base and a recrystallized burrow on the far right. After the sample is etched in 10% hydrochloric acid, significant textural features become more noticeable. In this case, the rock is indeed not a calcilutite at all. It is a coarser-grained rock, as the individual grains are readily observable in the etched image. This particular sample is easily classified using Folk's classification scheme, as the dominant grain type is peloids with only a few minor crinoid ossicles. The etched image reveals that this bed shows evidence for normally graded bedding with the coarsest materials at the base and finer-grained materials at the top.
Laminated, thin-bedded pelmicrite
The bed shown below appears on a fresh-cut surface to be a very fine-grained calcilutite, but displays complex textural properties as well as a coarser-grained lithology. The rollover image highlights the development of what can be called a turbidite. In this case, the rock shows evidence for multiple, thinly-laminated horizons. The appearance of vertical burrow escape traces suggest that the sediments were deposited as part of a turbidite flow. The basal few centimeters are composed of a relatively coarse-grained crinoid and trilobite hash showing no lamination and many upward directed burrow tubes. Overlying the more massive basal materials, the bed shows a sharp transition to thinly-laminated beds that are relatively continuous and sub-planar except where interrupted from below by escape traces. The upper few laminae show a transition to finer-grained peloidal micrites with only one minor vertical burrow penetrating to the top of the second uppermost layer. It appears that this and the cap layer represent the settle out of the finer-grained peloidal materials after the flow energies have settled down.
Coarser-grained carbonates: Calcisiltites, wackestones, and packstones
Within the Trenton Group, probably the most dominant limestone types range from fine- to-medium grained carbonate rocks. These rocks typically are classified as wackestones to packstones because, although they have significant percentage of micritic matrix, they also contain a substantial amount of skeletal grains. The following photographs show a variety of wackestone to packstone facies that come from the Rust Formation, but share similar characteristics with other stratigraphic intervals.
Thin-bedded intraclastic calcisiltite
The sample below, appears at first glance to be another fine-grained carbonate, but is actually composed of silt-to-sand-sized grains of a variety of compositions including quartz and calcite. The bed shows evidence for cross-laminations both at the top and at the bottom. The central unit, though rather homogenous, contains a large intraclast that can be seen on the right side of the photograph. In the rollover etched sample image, these features are accentuated, as is the large pyritized burrow fill in the lower right and the chert nodule in the lower left.
Thin-bedded, packed biomicrite/ thin-bedded skeletal wackestone
The following sample would be described in the Dunham Classification as a thin-bedded skeletal wackestone, or using the Folk Classification it would be considered to be a packed biomicrite. Regardless of its name, this rock sample is a classic example of one of the dominant lithologies in the Trenton. Crinoid, trilobite, and bryozoan debris are dominant compositional features, and the rock would be considered matrix-supported, as these sedimentary grains are rarely in contact with one another due to the relatively high proportion of fine-grained micrite.
Thin-bedded, packed biomicrite/ thin-bedded, skeletal packstone
The sample below would be considered a thin-bedded, packed biomicrite using the Folk Classification,and it would be considered a thin-bedded, skeletal packstone according to Dunham's scheme. The large number of skeletal grains are in close contact with each other, and, despite large amounts of micritic matrix, this limestone is grain-supported. Although faintly evident in the first image, the etched overlay photograph shows evidence for multiple beds of packstone. Due to slight differences in the carbonate mineralogy between these layers, they are accentuated, as is the large recrystallized (steinkern?) mass in the center of the image.
Thin-bedded, graded skeletal packstone to fine-grained grainstone
The following rock sample shows a transitional set of graded beds showing a lower packstone type bed with larger skeletal pieces and a fine-grained matrix. This is overlain by a coarser-grained, skeletal grainstone which shows a higher degree of sorting and physical breakage. The dominant allochems are fragments of crinoid, mollusc, bryozoan, and trilobite skeletal elements. This sample shows significant evidence for high depositional energy with reworked cemented cephalopod chambers, upside down Prasopora bryozoans, and a few intraclasts seen on the left. Note also that the region to the right of the cemented cephalopod chambers shows evidence for sparry cements filling void space.
Poorly washed biosparite / thin-bedded, graded skeletal packstone
Although thicker-bedded than many of the above rock samples, the following rock sample represents a slightly more complex lithology. Coarse-grained echinoderm fragments are visible with many grains in contact, while adjacent areas show much finer-grained and even thinly laminated beds. This pattern is accentuated in the rollover image showing the acid-etched version. This particular sample, although partially coarse-grained, shows evidence for bacterial mats (basal lamination and roll-up structures), as well as some bioturbation within the middle to upper portions. This rock type might be classified as a boundstone if there were substantial evidence that the interlayering of finer-grained and coarser-grained materials were bound by algal filaments.
Stromatoporoid, crinoid wackestone
The following sample, although dominantly a wackestone, is particularly interesting because of the presence of the two stromatoporoids seen in this image. They show evidence for multiple stages of growth and toppling. The specimen on the left appears to have toppled over sideways while the specimen in the center has evidently regrown after it also had toppled. The etched version accentuates the internal structure of the stromatoporoids as well as the basal contact with the thin grainstone. Incidentally, this basal grainstone bed shows evidence for early cementation; collectively, with the reworked cephalopod steinkern, this argues for a relatively high-energy depositional event.
Coarser-grained varbonates: Grainstones
Graded, poorly washed skeletal grainstone
The following sample is one example of a coarse-grained graded skeletal carbonate. Clearly seen in the image are at least two main sedimentary layers: a lower coarse-grained skeletal rudstone composed of very coarse fragments of brachiopods, crinoids, trilobites and even some rip-up clasts; and an upper medium-grained rather uniformly bedded grainstone. The grading seen in this rock sample is suggestive of several different depositional processes and may be related to periods of intense high-energy conditions followed by more subdued low-energy conditions as may be consistent with storm followed by normal weather deposition.
Massively-bedded crinoidal and brachiopod grainstone
The final image shown below, limited in distribution to the basal and uppermost Trenton intervals, represents a super coarse-grained skeletal coquina composed dominantly of comminuted crinoid and brachiopod skeletal components with lesser amounts of trilobite, mollusc, and bryozoan debris. From the image below, it is clear that the limestone contains very little micritic mud (almost none), and is rather homogenous from base to top. Although this rock sample does not show evidence for cross-stratification, much of the Steuben and Kings Falls Formations, where these limestones occur, do show evidence for these sedimentary structures. The lack of mud and high degree of disarticulation of fossil components indicate a continuously energetic depositional system.
Squire's Mill Dam Falls, photograph by Carlton Brett