trilobite Isotelus gigas
Trenton Black River Project
Petrography
Home > Petrography - Constituents
Click button to view Report in:

PETROGRAPHY OF THE TRENTON AND BLACK RIVER GROUP CARBONATE ROCKS IN THE APPALACHIAN BASIN

Introduction | Methods | Constituents | Microfacies and Depositional Environments | Diagenesis | Dolomite Textures, Diagenesis, and Porosity |
References | Text Figures | Appendix I [Skeletal (PDF) - Nonskeletal (PDF)] | Appendix II (PDF) |
Appendix III-Figure Captions | Appendix IV-Figure Captions | Appendix V-Figure Captions |
Table 1 - TBR Core and Outcrop Samples (PDF)

CONSTITUENTS

Skeletal Grains

The skeletal constituents of carbonate rocks reflect the distribution of calcium carbonate-secreting organisms throughout geologic time. Many new carbonate-secreting marine organisms emerged by Middle Ordovician time, some 450 million years ago, and these are well represented in the composition of Trenton and Black River rocks. Fragments of brachiopods, bryozoans, crinoids, corals, trilobites, calcareous algae, and gastropods comprise the principal skeletal grains in the rocks. The distribution of these organisms in the carbonate depositional realm was controlled by environmental factors such as water depth, temperature, salinity, substrate, and turbulence. Thus the correct identification of these skeletal grain types and their depositional texture is critical for correct environmental interpretations.

The original mineralogy of the skeletal grains, i.e., aragonite, low Mg calcite, high Mg calcite, or a mixture of aragonite and calcite, affected the fate of the skeletal grains during diagenesis. Their susceptibility to recrystallization, dissolution, and dolomitization was particularly important to the development of Trenton and Black River carbonate reservoirs in the subsurface.

Skeletal grains are identified on the basis of their size, shape, microstructure, and original mineralogy (Tucker and Wright, 1990; Scholle and Ulmer-Scholle, 2003). Appendix I provides a comprehensive review and photographic guide of the major skeletal grains that occur in the Trenton and Black River rocks of the Appalachian basin.

Non-skeletal Grains

Non-skeletal carbonate grains in the Trenton and Black River rocks include ooids, peloids, grain aggregates, and clasts. Appendix I includes a general review and photographic guide of the non-skeletal grains found in the Trenton and Black River rocks of the Appalachian basin.

Ooids are a type of coated carbonate grain, spherical to sub spherical in shape, consisting of one or more regular concentric lamellae around a nucleus. The nucleus is often a carbonate particle, but can be a non-carbonate clastic particle too. The term ooid is restricted to grains less than 2 mm in diameter, and most ooids range from 0.2 to 0.5 mm in diameter. This is the size of fine- to medium- grained sand in the Wentworth scale. Recent marine ooids exhibit tangential, radial, or random microfabrics. Ancient marine ooids may have relic tangential microstructures or, more commonly, radial microfabrics. Many ancient ooids are micritic or display replacement with neomorphic spar. We only found ooids in portions of the Black River Group. We did not find any ooids in the Trenton Group rocks that we examined, and we could not find any report of Trenton ooids in the literature.

While ooids are locally important, peloids are the most diverse and abundant non-skeletal grains in the limestones and dolostones that we examined, and their origins are diverse and complex. Indeed, many, if not most, of the peloids in the Trenton and Black River carbonates may not be grains at all but cements instead. The original textures of the peloidal limestones appear to have influenced dolomitization processes and subsequent dolomite fabrics and porosity distributions in the Trenton and Black River petroleum reservoirs throughout the Appalachian basin.

Peloids are a type of non-skeletal carbonate particle formed of cryptocrystalline and microcrystalline calcium carbonate, and/or carbonate microspar (Scholle and Ulmer-Scholle, 2003, p. 254). Peloids are spherical, cylindrical, or angular particles composed of aggregated carbonate mud and/or precipitated calcium carbonate. They exhibit no internal structure. There is no defined restriction on the size or origin of peloids, thus the term allows reference to allochems composed of micritic material without implying their specific origin (McKee and Gutschick, 1969).

Peloids are polygenetic, and identifying their precise origin is often difficult in carbonate rocks (see discussions by Macintyre, 1985 and Scholle and Ulmer-Scholle, 2003). Some peloids are fecal in origin (carbonate pellets), while others are grains derived from calcareous algae, micritized allochems, and reworked mud clasts (Tucker and Wright, 1990; see the examples presented in Appendix I). Most peloidal textures in carbonate rocks, however, probably are chemical in origin, i.e., cements in which the peloids appear as clots with a flocculent fabric- the structure grumeleuse of Bathurst (1975, p. 511 - 513 and Figure 350). These clots are the nucleation sites of small crystals of high-magnesium calcite (Tucker and Wright, 1990). The nuclei may be organic, possibly microbial matter (Chafetz, 1986) or simply sub-microcrystalline, radial, acicular calcite crystals that grew around a small number of nuclei (Bosak and others, 2004). In either case, the peloids precipitated in situ as marine cement on or just below the sea floor (Tucker and Wright, 1990; Malone and others, 2001; Bosak and others, 2004). The recent work of Bosak and others (2004) recommends that abiotic mechanisms should be the null-hypothesis for peloid formation.

Peloidal textures are ubiquitous in the Trenton and Black River carbonates throughout the Appalachian basin. They occur in all carbonate rock types and their origins are quite diverse. Appendix I contains numerous examples of peloids that clearly are carbonate grains. In many instances, however, we interpreted peloidal fabrics in Trenton and Black River rocks as cement. We discuss peloidal cement textures in detail below in the section on diagenesis.

Most of the dolomitized carbonate rocks of Black River age in the subsurface of west central New York state and north central Pennsylvania exhibit a precursor peloidal fabric that dominated the limestones there. The petrophysical character of microporosity in these precursor peloidal limestones may have been critical in controlling the migration of dolomitizing fluids through the rocks adjacent to faulted and fractured strata (see Cantrell and Hagerty, 1999). These peloidal textures also are common in dolomitized limestones in western Ohio and central Kentucky.

Matrix

Fine-grained matrix in the Trenton and Black River Formations consists of calcite micrite, microspar, pseudospar, and terrigenous clay minerals. Micrite is composed of small calcite crystals 1 to 4 m in diameter. These crystals formed through the breakdown of coarser carbonate grains, such as calcareous algae, or through inorganic precipitation on the seafloor (Tucker and Wright, 1990; Scholle and Ulmer-Scholle, 2003). Figure 1 shows several examples of micrite matrix in the Trenton and Black River Formations. Microspar consists of calcite crystals 5 to 30 m in diameter. It forms through neomorphic recrystallization of micrite. Pseudospar also is a recrystallization product of finer calcite, but it is even coarser than microspar with crystal diameters of 30 to 50 μm. Clay mineral, predominately illite, mixed-layer illite-chlorite, and smectite, occur in some Trenton Formation samples, particularly in ramp slope carbonates intimately interbedded with terrigenous shales (Figure 2). Some of the matrix material in the Trenton Formation is organic rich (up to 3.74% total organic carbon), and might be important as a petroleum source rock.

Other Components (non-authigenic)

In addition to terrigenous clay minerals, non-carbonate sedimentary components of the Trenton and Black River Formations include detritial quartz silt and very fine sand, bentonite, and rare glauconite. The quartz entered the carbonate environments as air-borne and/or water-borne sediment. Very fine quartz sand and silt dispersed throughout some of the limestones in the Black River Formation (<1% of the rock) might be air-borne, but was more likely reworked from very thin fluvial and paralic siliciclastic accumulations that were deposited on the carbonate ramp during sea level lowstands (Figure 3). The volcaniclastic k-bentonites (Figure 4) are interbedded with the carbonates, and were rapidly deposited below storm wave base by volcanism along the island arc that formed along the southeastern Laurentian continental margin during the Taconic orogeny (Thompson, 1999). Reworked fragments of these k-bentonites occur in some of the limestones. to top