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