INTRODUCTION:

The Trenton Limestone is an adequate term to be applied when discussing the very broad context of the Upper Ordovician limestones of New York. The term, on the other hand, is quite inadequate when discussing the actual rocks encompassed within the several hundred feet of strata that compose the Trenton limestone. Despite the fact that the carbonate rocks that make up the Trenton Group are mineralogically quite similar, there are key differences in the physical characteristics of the rocks that enable the discriminating observer to differentiate many types of limestone.

Relying on three main components common to all carbonate rocks: grain(s) type(s), matrix composition, and cement(s), generations of geologists have been able to divide the Trenton Limestone into several formations, members of formations, submembers of members, and even beds of members. In this way, it is possible to communicate the more subtle differences in composition of these limestones, and to gain insights into the depositional histories of the rocks.

This section introduces the reader to a discussion of key lithologies (limestone rock types), and facies (assemblages of commonly occurring rock types) within the Trenton Limestone. The goal is to provide the reader with a background understanding of the rock types present in the Trenton Falls area, and it is intended to complement discussions on sedimentary processes, depositional environments, as well as lithostratigraphy. >>Back to Top

UNDERSTANDING CARBONATE SEDIMENTOLOGY

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. >>Back to Top

• 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. >>Back to Top

• 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. >>Back to Top

• 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 desciption 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. >>Back to Top

• 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.

Modified after Folk, (1962)

• 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. >>Back to Top

Modified after Dunham, (1962)

LITHOLOGIES OF THE TRENTON GROUP

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. >>Back to Top

• FINE-GRAINED CARBONATES

• 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. >>Back to Top

 

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.