High-energy Shelf-margin Carbonate Facies: Microbial Sheet Reefs, Endolites, and Intraclast Grainstone—Ledger Formation (Middle Cambrian), Pennsylvania

Cambrian–Ordovician shelf-margin deposits of the great American carbonate bank (eastern North America) experienced significant regional dolomitization and/or metamorphism, but the Middle Cambrian Ledger Formation in south-central Pennsylvania contains a shelf-margin facies complex that includes exceptionally well-preserved microbialite sheet reefs riddled with centimeter- to meter-scale submarine cavities. The reefs and associated sands, composed of reef-related allochems, interfinger with ooid shoals, forming a high-energy shelf-margin facies association located near the seaward margin of the Middle Cambrian Laurentian platform. The Ledger Formation's ooid shoal complex, exposed in the Magnesita Refractories quarry in York County, Pennsylvania, is pervasively dolomitized. Forthcoming research documents multiple stages of dolomitization and dedolomitization in the ooid dolostone; therefore, the ooid dolostone is not discussed here. In contrast to the ooid dolostone, most of the Ledger reef facies remains limestone. This has facilitated detailed interpretation of the reef depositional and diagenetic history, including new information presented here. Previous publications describe the Ledger reef geologic setting, mechanisms for generating the cavities, and petrographic and geochemical analyses of radiaxial fibrous and herringbone calcite fibrous submarine cements within the cavities. This chapter provides new information on the microbial reef sheet facies, describes a previously undocumented type of cryptic microbial morphology (endolite), and interprets a 1-m (3.3-ft)-thick intraclastic grainstone bed. Modern reefs in high-energy settings adapt by building robust coral frameworks that can withstand normal current activity and wave action. In the Middle Cambrian, coral framebuilders were absent, so to exploit high-energy ecological niches, organosedimentary constructers, primarily cyanobacteria (algae and bacteria), had to develop a similarly robust morphology. We propose that low-growing, thick, cohesive microbial sheets, such as documented here from the Ledger Formation, provided minimal wave resistance and, therefore, outcompeted stromatolites and thrombolites to form subtidal wave-resistant structures in such high-energy settings. Similar to modern reefs, these microbial sheets contain cavities across a range of scales from millimeter-size fenestrae to meter-size stromatactis-type voids capable of sheltering and supporting delicate shrubs of Epiphyton-like dendrites and cryptic endolites, as detailed later in this chapter. Microbial processes dominated all ecological niches, forming the substrate, colonizing cryptic spaces, and coating and encrusting other microbes. The reef microbialite consists of weakly bedded sheets composed of shrubs and stubby strands of calcified Epiphyton- and Angulocellularia-like elements. Centimeter-scale domal stromatolites, thrombolites, oncolites, dendrolites, and oval multiple-layered organosedimentary cryptic structures, termed “endolites,” form lenses and distinctive structures. Petrographically, the microbialite is expressed as clots, stringers, arborescent garlands, and dendritic shrubs. Stromatactislike and fenestral cavities within the microbialite formed primarily through processes of gas and water escape, although syndepositional slumping and channel undercutting produced other types of cavities and void spaces. Grainstone, composed of microbial clasts and fragments, accumulated as cross-bedded intrareef channel sands. Large stromatactislike cavities were stabilized with multiple generations of microdolomite-bearing calcite radiaxial fibrous and herringbone calcite cements and intercalated internal sediment. Cement morphology, internal sediment associations, stable isotopes, and trace element geochemistry suggest that the cements precipitated from marine fluids as magnesium calcite and subsequently stabilized to calcite during diagenesis. The Ledger microbial assemblage closely resembles living cryptic, mat, and domal cyanobacterial forms reported from the Tikehau Atoll, French Polynesia. Detailed descriptions of the cyanobacteria involved in creating the modern structures provide useful analogies for enigmatic Middle Cambrian fossil morphologies.