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

DIAGENESIS

The term diagenesis refers to all of the processes that affect sediment from just after deposition up to the lowest grade of metamorphism, the greenschist facies (Pettijohn and others, 1987). It is the sum of physical, chemical, and biochemical changes occurring in a sedimentary deposit after its initial accumulation, excluding metamorphism (Friedman and Saunders, 1978). Diagenesis in the rocks of the Trenton and Black River Groups included seven major processes: 1) microbial micritization; 2) cementation; 3) neomorphism; 4) replacement; 5) physical and chemical compaction; 6) dissolution; and 7) dolomitization (see, Tucker and Wright, 1990, p.314). Dolomitization is the most important diagenetic aspect of the Trenton and Black River petroleum reservoirs and is discussed separately in the next section of this report.

Note: The term diagenesis is used differently by organic geochemists, and we employ their usage in the petroleum geochemistry reports of this research. In petroleum geochemistry, diagenesis is the process involving the biological, physical, and chemical alteration of the organic debris in sediments without a pronounced effect from rising temperature (Hunt, 1996). It covers the range of temperature up to about 50°C (122°F). Readers should bear these different usages in mind when reading this report.

Microbial Micritization

Partial to complete microbial micritization of skeletal grains, ooids, and probable pellets occurs in most carbonate rock types in the Trenton and Black River Groups throughout the Appalachian basin. Microbial micritization is most evident in bioclastic grainstones and packstones where micrite envelops developed on skeletal grains, and in some mixed oolitic/peloidal grainstones and packstones where the peloids show compelling evidence of carbonate grain degradation and replacement with micrite. In both cases, the micritization process may have been mediated by endolithic algae, fungi, or bacteria and associated biochemical or physiochemical processes.

The micrite envelops shown in Appendix II (Micritization Examples #1 - 4) formed around many of the bioclasts in mixed-fossil grainstones. These envelopes are essentially identical to those shown and discussed by Milliman (1974), Bathurst (1975), Tucker and Wright (1990), and Scholle and Ulmer-Scholle (2003). Bathurst (1975, p.381 - 389) suggested such envelopes formed through the filling of altered grains rather than precipitation of a new rind around the grain: algae, fungi, or bacteria bore into the grain, die, and the subsequent alteration of the organic material provides a chemical environment conducive to calcium carbonate precipitation, thus filling the voids.

Intense activity by endolithic microbes can lead to complete micritization of carbonate grains. This was relatively common during deposition of both Trenton and Black River carbonates. ( Figure 5) shows wholly micritized ooids in the Black River Formation in the subsurface of western New York.

The micrite envelope surrounding the echinoid fragment in Figure 6 originated in the same way as those shown Appendix II, i.e., through marine cementation within spaces created by endolithic boring organisms. The micrite cement shown in Figure 6, however, displays a distinctive clotted or peloidal texture Figure ( 6b and 6c). Higher magnification ( Figure 6c) reveals that the "clots" of microcrystalline calcite in the micrite rind consist of unimodal, decimicron-size spherical clusters composed of even smaller (micron- and sub micron-size) calcite crystals. Each clot or peloid consists of a brownish, cloudy nucleus and a rim of clear, well-developed euhedral crystals. These clots grade into adjacent patches of coarser, centimicron-size microcrystalline matrix with an identical peloidal fabric or into coarser calcite spar cement. These micrite envelopes and contiguous patches of clotted microcrystalline calcite may be bacterially induced calcite precipitates, or strictly abiotic cement (Lighty, 1985; Macintyre, 1985; Chafetz, 1986; Tucker and Wright, 1990; Scholle and Ulmer-Scholle, 2003; Bosak and others, 2004).

Cementation

The precipitation of calcium carbonate cements in the Trenton and Black River limestones was a major diagenetic process during and shortly after deposition on the sea floor in Ordovician time. Burial cementation also affected the rocks. The principal calcite cements in the rocks are 1) peloidal calcite, 2) prismatic fibrous to bladed calcite rinds on allochems, 3) meniscus calcite cement, 4) syntaxial calcite overgrowths, and 5) calcite spar. Dolomite cements are important, but we discuss these later in the section on dolomite textures. Several late-stage non-carbonate cements are associated with dolomite cements and we discuss these in the section on dolomitization.

Peloidal Cements

While microbial micrite envelopes around allochems and micrite replacement of grains are common in the Trenton and Black River carbonates of the Appalachian basin, most of the microcrystalline calcite, microspar, and pseudospar in these rocks occur as matrix in packstones, wackestones, and mudstones. As discussed above, much of this matrix originally formed in situ as carbonate mud derived from the breakdown of organisms, and it now exists in the rocks as neomorphic, recrystallized calcite. A great deal of the carbonate matrix in all of the limestone types, however, also formed in situ as peloidal cement. The clotted fabric of peloidal cement is ubiquitous in the Trenton and Black River limestones, and can usually be discerned even through most dolomite fabrics.

Figure 7 shows core and thin section photographs of Black River Formation limestone recovered in the Gray #1 well core, Steuben County, NY. Figure 7a shows the macroscopic appearance of this limestone - a seeming medium gray, bioturbated and burrowed sparse biomicrite (Folk, 1962) or wackestone (Dunham, 1962). There are problems with using the Dunham (1962) classification for this sample (see discussion below) so we prefer the Folk (1962) name. Dissolution structures (solution seams) indicate moderate chemical compaction.

Thin section examination of this sample reveals that skeletal grains comprise about 20 percent of the limestone, and include crinoids, bryozoans, brachiopods, trilobites, alga, and gastropods ( Figure 7b). Most allochems were altered by neomorphism, specifically recrystallization and degrading recrystallization. Authigenic pyrite, quartz, feldspar, and anhydrite along with dolomite make up about 5.5 percent of the rock. About 75 percent of this limestone, however, consists of decimicron-size peloids that could be interpreted as either framework grains or cement (Scholle and Ulmer-Scholle, 2003; Figure 7b). If framework grains, then the peloids might have originated as 1) algal material, 2) detritial sediment, 3) pellets, or 4) a replacement of other framework grains (Tucker and Wright, 1990, p.321 and references therein). If the peloids are cement then they are in situ precipitates.

Careful examination of the peloids at higher magnifications (Figure 7c and 7d) reveals that they most probably are cement and not carbonate grains. Individual peloids are 50 to 100 μm in diameter and consist of 1) a dark brown nucleus composed of micron-size calcite surrounded by 2) a rim of euhedral, dentate to blocky microspar. The average crystal size of the later is ~25μm. The highest magnification view ( Figure 7d) shows the nuclei consist of clots of submicron-size opaque material; this material might be organic, possibly microbial matter (Chafetz, 1986) or simply submicrocrystalline, radial, acicular calcite crystals that grew around a small number of nuclei (Bosak and others, 2004). If the nuclei are organic, the peloids likely originated as in situ precipitates around clumps of bacteria and the microspar likely is a neomorphic recrystallization product of earlier micrite matrix (Lighty, 1985; Macintyre, 1985; Chafetz, 1986; Tucker and Wright, 1990). If the nuclei are inorganic, the opaque appearance of the peloids probably is a consequence of the small crystal grain size relative to the thickness of the thin section (Bosak and others, 2004). In this case, the peloidal nuclei are strictly abiotic in origin and formed as calcite cement from suspension and geopetal settling (Bosak and others, 2004).

The peloidal fabric in this limestone occurs as: 1) dominant groundmass or "matrix" (Figure 7a, 7b, 7c, 7d); 2) as an internal cement within the zooecia of bryozoan skeletal grains and the lumens of crinoid fragments ( Figure 7e); 3) as cement filling fabric-selective pores, i.e., intraparticle voids ( Figure 7f); and 4) as mimic replacement of bryozoan grains ( Figure 7g).

All of the characteristics of the peloids - their uniform crystal size, restricted size range, consistent texture, monomineralogy, opaque nuclei, and euhedral outer rims- suggest that they are in situ precipitates which formed through cementation on or just below the sea floor (Tucker and Wright, 1990; Malone and others, 2001; Bosak and others, 2004. This peloidal fabric characterizes most of the fine-grained rocks in the Trenton and Black River carbonates that we examined.

The most compelling evidence for interpreting these peloidal textures as marine cement is the fact that this fabric is ubiquitous in Trenton and Black River Group hardgrounds throughout the basin (Figure 8). Hardgrounds are synsedimentarily lithified carbonate seafloors that become hardened in situ by the precipitation of carbonate cement in the primary pore space (Wilson and Palmer, 1992, p.3). They are sedimentary horizons in marine carbonates that exhibit evidence of exposure on the sea floor as lithified rock. Detailed discussions of hardgrounds in the Trenton and Black River rocks of the Appalachian basin can be found in Brett and Brookfield (1984) and Laughrey and others (2003). All of the fine-grained or finely crystalline hardground lithologies that we examined petrographically have a peloidal fabric.

A note on classification: The Dunham (1962) classification of the sample shown in Figure 7 as a wackestone based on hand sample description or a mixed-fossil/peloidal grainstone based on thin section description is wrong because the fine calcite crystal size and peloidal fabric do not reflect the limestone's depositional texture. The Folk (1962) classification sparse biopelmicrite provides a better name for the rock, although one might argue for sparse biopelsparite instead (see Scholle and Ulmer-Scholle, 2003, p.266 - 271). We prefer the former Folk (1962) name because the peloidal nuclei volumetrically dominate the rock and their micron-size calcite crystals can be properly called micrite (see Scholle and Ulmer-Scholle, 2003, p. 266). An alternate name, from the classification scheme of Wright (1992) is cementstone.

Other Calcite Cements

In addition to the peloidal calcite cements just discussed, calcite cement also occurs as prismatic fibrous to bladed rinds, meniscus-type cement, syntaxial overgrowths on allochems, poikilotopic calcite spar, and void-filling spar. We interpret these various calcite cements as products of both marine and burial diagenesis.

Prismatic fibrous to bladed rinds of calcite are common on all allochems we observed in the Trenton and Black River limestones. These crystals may grow directly on the allochems, or atop micrite envelopes. Good examples are shown Appendix II (Prismatic fringe examples 1 - 3). These morphologies are typical of modern high-Mg calcite and aragonite cements (Scholle and Ulmer-Scholle, 2003), but probably precipitated as calcite in the Ordovician sea (Lowenstein and others, 2001).

We observed unique meniscus-type cement (Hillgartner and others, 2001) in oolitic grainstones of the Black River Formation at Union Furnace, Pennsylvania (Appendix I, nonskeletal grains, ooid example #5). Meniscus cements usually are cited as evidence for meteoric diagenesis in the vadose zone (James and Choquette, 1984; Scholle and Ulmer-Scholle, 2003), but Hillgartner and others (2001) demonstrated a marine diagenetic origin for microbially-induced meniscus cements in carbonate sands of the Bahamas and Mesozoic platform carbonates of the Swiss and French Jura Mountains. These authors cautioned that an interpretation of early vadose diagenesis should not be based on meniscus cement alone. They suggested the term meniscus-type cement for these unique marine cements that form in association with the calcification of microbial filaments and the trapping of percolating micrite in subtidal settings. The meniscus-type cements in the Black River oolitic grainstones occur along with grapestone, oolitic intraclasts, micritized grains, bladed calcite rinds, and hardgrounds all suggesting sea floor lithification.

Syntaxial calcite overgrowths are common on echinoid fragments in the Trenton and Black River rocks of the Appalachian basin (Appendix II, Syntaxial Overgrowths Examples #1 and #2). Such cements are reported from meteoric, marine, and burial diagenetic environments (Scholle and Ulmer-Scholle, 2003), ands require careful geochemical and petrographic arguments in order to be diagnostic (Tucker and Wright, 1990). We interpret the syntaxial overgrowths in the Trenton and Black River Formations as products of marine and/or burial diagenesis because of their association with other marine cements in the rocks and the lack of any evidence for exposure to the meteoric environment (i.e., lack of grain dissolution and no evidence whatsoever of karst processes). Poikilotopic spar is likewise common (Appendix II, Poikilotopic spar Examples 1#1 and #2) and we interpret it to be of marine and/or burial diagenetic origin too.

Drusy mosaics of calcite spar fill most primary pore space in the Trenton and Black River Formations in the Appalachian basin (Appendix II, Equant Calcite Spar Examples #1 and #2, Drusy Spar Examples #1 and #2). The precipitation of this calcite spar followed chemical compaction of the limestones (Appendix II, Compaction Examples #5 and #6) indicating that these are clearly burial diagenetic environment cements (Wright and Tucker, 1990, p.352). Additional observations supporting this conclusion include broken and collapsed micrite envelopes within the spar and fractured grains cemented by the spar.

Neomorphism

A number of neomorphic fabrics occur in the Trenton and Black River rocks of the Appalachian Basin (Appendix II). Microspar and pseudospar commonly replace micrite in muddy carbonate rocks. Most of this is aggrading neomorphism (Appendix II, Neomorphism examples #1 - #3). Microspar and pseudospar also commonly replace allochems (see Appendix I, Coral Example #2 and #3).

Replacement

Numerous noncarbonate minerals replace both limestone and dolomite in the Trenton and Black River Formations. These include chert, chalcedony and quartz, feldspar, iron sulfides and oxides, sphalerite, fluorite, phosphate, sulfates, and chlorides. Some examples follow.

Chert and chalcedony replace both limestone and dolomite in the Trenton and Black River rocks throughout the Appalachian basin. Appendix II (Silicification Examples 31 - #4) and Figure 9 show examples of this replacement. Chalcedony that replaces hydrothermal dolomite is length-slow (Figure 9), possibly implying that the replacement took place in a sulfate-rich aqueous environment (Folk and Pittman, 1971; Scholle and Ulmer-Scholle, 2003).

Compaction (including pressure solution)

Compaction fabrics in the Trenton and Black River Formations are widespread throughout the basin. They include mechanical compaction features such as plastic deformation of soft grains and brittle fractures in grains, and chemical compaction features such as cocavo-convex and sutured contacts between grains, dissolution seams, and stylolites. Examples of these are provided in Appendix II.

Dissolution

There is very little evidence for dissolution features in the limestones of the Trenton and Black River Formations in the basin. This is consistent with what is known of the Ordovician carbonate systems on a global scale (Markello and others, 2005), and with recent work on the local scale by Harris (2005) and Smith and others (2005). Limestone dissolution adequate for creating commercial reservoirs is restricted to processes associated with dolomitization adjacent to fractures. These are discussed below. to top