Trenton Black River Project
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|Figure 1. Carbonate matrix in the Trenton and Black River Formations.|
Skeletal wackestone (Dunham, 1962) or a packed biomicrite (Folk, 1962) from the Black River Formation, Union Furnace, Huntingdon County, PA. The matrix consists of microcrystalline calcite, or micrite, which is presumed to have formed through the breakdown of coarser carbonate grains (Scholle and Ulmer-Scholle, 2003). Note that most of the skeletal material has undergone neomorphic recrystallization or solution followed by cavity filling with calcite cement.
| Figure 1B.
Dark organic-rich matrix in a skeletal wackestone or packed biomicrite from the Trenton Formation at Union Furnace, PA. The carbonate matrix material consists of microspar (crystals 5 - 20 m in size) presumably recrystallized from micrite. The dark, clotted organic matrix matter is kerogen.
|Figure 2. Trenton Formation at Union Furnace, PA.|
Limestone tempesites or turbidites interbedded with siliciclastic shales in the Trenton Formation at Union Furnace, PA.
|Figure 3. Very fine-grained quartz arenite from a very thin sandstone bed in the Black River Formation, Union Furnace, Huntington County, PA.|
Low magnification view (crossed polars) showing well sorted subrounded grains of quartz and minor feldspar cemented by calcite microspar and some anhydrite.
Higher magnification view of sandstone shown in Figure 3A showing calcite and anhydrite cements and feldspar grains.
|Figure 4. K-bentonites in the Trenton and Black River Formations.|
| Figure 4A.
Deicke k-bentonite exposed at Union Furnace, PA.
| Figure 4B.
Millbrig k-bentonite recovered in core of the Trenton Formation cut 500 ft. west of the Union Furnace outcrop.
| Figure 4C.
Millbrig k-bentonite at 1550 ft. in the Chevron 1A Prudential core, Marion County, OH.
|Figure 5. Completely micritized ooids in the Black River Formation.|
Completely micritized ooids in the Black River Formation.
|Figure 6. Micrite envelope on echinoid grain and peloidal cement.|
| Figure 6A.
The large echinoid fragment in the left-center of the photomicrograph exhibits a bored rim and well-developed micrite envelope.
| Figure 6B.
The same photomicrograph in cross-polarized light reveals that the micrite envelope actually consists of calcite cement with a peloidal texture. Note that this peloidal cement grades into coarser peloidal cement that fills some of the pore spaces adjacent to the grain in the southeast and northwest quadrants. Identical peloidal cement fills the zooecia of bryozoan grains.
| Figure 6C.
High magnification view of the peloidal cement that fills both the bored edge of the echinoid grain and the void space immediately below it. The rest of the void space is filled with calcite spar. Montgomery #4 well, Mercer County, PA. Trenton Formation, 8495 ft.
|Figure 7. Peloidal cement fabric in the Black River Formation.|
|Figure 7A. |
Core sample of apparent burrowed and bioturbated wackestone (Dunham, 1962) or sparse biomicrite (Folk, 1962). Gray #1 well, Steuben County, NY, 7823 ft.).
|Figure 7B. |
Thin section photomicrograph of the same sample. Peloids comprise 75% of the limestone. Based on this information we can modify the Folk (1962) name for this rock to a sparse biopelmicrite. Using the Dunham (1962) classification we could rename the rock a peloidal/mixed-fossil grainstone, which is erroneous because the peloids are not grains but are cement.
||Figures 7C and 7D.
Progressively higher magnification views of the same sample. Note that the peloids consist of 1) a dark nucleus of micron-size calcite (opaque clots) surrounded by 2) rims of euhedral microspar.
Another thin section view of peloidal cement in the same sample. Here peloidal cement fills the zooecia of a bryozoan fragment as well as lithifies the matrix.
Peloidal cement fills a void formed earlier through dissolution of part of a bryozoan fragment.
Peloidal cement that once filled all of the zooecia of a bryozoan grain now mimics the original fossil structure.
|Figure 8. Hardgrounds and peloidal cements.|
Stacked, amalgamated hardground in the Trenton Formation exposed southeast of Lexington in central Kentucky. The finger points to the surface of a smooth and rolling hardground, but those beneath exhibit hummocky, undercut, pebbly, and reworked morphologies (see Brett and Brookfield, 1984, Wilson and Palmer, 1992, and Laughrey and others, 2003).
Plan view of bryozoans encrusting the top of a hardground, Trenton Formation, central Kentucky.
Thin section photomicrograph of the hardground surface that the geologist's finger points to in A. Note the peloidal cement texture of the limestone. Also note the authigenic pyrite at and just above the hardground surface.
A planar and undercut hardground in the Black River Formation exposed at Union Furnace, Pennsylvania.
||Figures 8E and 8F.
Thin section photomicrographs of the hardground shown in D. The clotted fabric characteristic of peloidal cements is evident in both photomicrographs.
|Figure 9. High magnification view of chalcedony replacing nonplanar dolomite that mimically replaces a crinoid grain (see Figure A3-8 in Appendix III for additional views of this sample).|
View under crossed polars showing the typical radiating habit of chalcedony.
Same view, but with the gypsum plate inserted into the microscope. The birefringence colors in the northeast and southwest quadrants increased, and the birefringence colors in the northwest and southeast quadrants decreased, indicating that the chalcedony is length-slow.
|Figure 10. Macropores and mesopores in the Black River Formation|
Macropores and mesopores in the Black River Formation, Whiteman #1 well, Chemung County, NY, 9529 ft. Porosity consists of isolated small vugs (not fabric selective), minor intercrystalline pores (fabric selective), and fractures (not fabric selective). The rock consists of nonplanar-a dolomite, and nonplanar (saddle) dolomite cement partially fills the vugs. The vugs formed through dissolution of both calcite and dolomite. Note that dolomitization mostly obliterated porosity in this rock.
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