Microbialite Bioherms & Their Cementation

The Ledger Project
Carol de Wet

Figure 1

The Middle Cambrian Ledger Formation is pervasively dolomitized, destroying primary fabrics, throughout much of its geographic extent. In York Co., Pennsylvania, however, part of the Ledger Formation retains its original calcite mineralogy, revealing an exceptionally well-preserved shelf margin system characterized by microbial fabrics and fibrous submarine cavity-filling cements. The lithologies and fabris in the limestone include dense microbialite with fenestrae, cavities containing Renalcis and stromatolites, and intraclast beds. These are interpreted as shallow subtitdal, high energy deposits.

Figure 2

Centimeter-to-meter scale primary cavities are lined with multiple generations of fibrous submarine cement. Petrography of the cements. Evidence for precursor high magnesium calcite mineralogy of the cements consists of abundant dolomite mircroinclusions, relict high Mg, and low Sr trace element values. Interestingly, the herringbone calcite cements are consistently depleted in oxygen and carbon isotopic value relative to the radiaxial calcite cements.

Figure 3: Location of the study site at J.E. Baker Quarry, York Co., PA



Figure 5: Overview of the Ledger field site with karst pinnacles overlain by Triassic red silt. Person for scale denoted by white arrow.

New Aspects:

Microbial Shelf Margin Reef (high energy system; important role of calcifying cyanobacteria in shelf margin stabilization)

Figure 9

Herringbone Calcite Cement (significant as a marker bed elsewhere in field area, impt. as a recently described calcite morphology)

Table 1: Summary of trace element analysis results (ppm) by ICP in each cement layer of stromatactis cavity.

Table 2: Summary of isotope analysis (%PDB) in each cement of stromatiactis.

Firm Middle Cambrian age (constrains stratigraphic succession)

Origin of large submarine cavities (new model for creating submarine voids)

Submarine cement stratigraphy (correlation of submarine, syndepositional cements)

Figure 10

Preliminary documentation of fossil algal contributors to the microbial complex.