Skip to Main Content
Skip Nav Destination


The Great American Bank, deposited along the Cambro-Ordovician coast of Laurentia, consists of over 3000 m of carbonate deposits. Middle Cambrian Ledger Formation dolomitized ooid shoals and partially dolomitized microbial reefs, exposed in the Magnesita Refractories quarry (York, Pennsylvania), formed on the shelf margin [de Wet, Dickson, Wood, Gaswirth, Frey, 1999, “A new type of shelf margin deposit: rigid microbial sheets and unconsolidated grainstones riddled with meter-scale cavities,” Sedimentary Geology vol. 128, pp. 13-21; de Wet, Frey, Gaswirth, Mora, Rahnis, Bruno, 2004, “Origin of meter-scale submarine cavities and herringbone calcite cement in a Cambrian microbial reef, Ledger Formation (USA),” Journal of Sedimentary Research vol. 74, pp. 914-923; de Wet, Hopkins, Rahnis, Murphy, Dvortetsky, 2012, “High energy shelf margin carbonate facies: microbial sheet reefs, ooid shoals, and intraclast grainstones: Ledger Fm. (Middle Cambrian), Pennsylvania.” In Derby, Fritz, Longacre, Morgan, Sternbach (Editors), The Great American Carbonate Bank: The Geology and Economic Resources of the Cambrian–Ordovician Sauk Megasequence of Laurentia, Memoir 98: American Association of Petroleum Geologists, Tulsa, Oklahoma, p. 245a–251a (extended abstract) and p. 421–450]. This depositional setting meant that Ledger strata were well positioned to be bathed in updip migrating burial fluids during shallow burial. Shallow subsurface fluids precipitated dolomite with different types of textural preservation: fabric retentive (mimetic) and fabric obscuring. Ooid shoals were pervasively dolomitized due to high primary porosity and permeability, but, because adjacent microbial reefs were syndepositionally cemented, they formed local aquitards that funneled porefluids into shoal and grainstone channel deposits. Local to regional faulting, associated with Paleozoic burial, created additional permeable conduits for dolomitizing fluids to infiltrate reef strata. Both fabric retentive and fabric obscuring dolomite types have overlapping geochemical and δ18O and δ13C signatures, interpreted as representative of a single porefluid origin. The similarity in isotopic and geochemical results between the calcite reef rocks and dolomites suggests that the porewaters were primarily buried marine water, mixed with Mg2+ enriched porewater associated with diagenetic stabilization of high-Mg calcite to low-Mg calcite. The δ18O values provide evidence that diagenetic porewaters re-equilibrated with higher temperature burial fluids, but δ13C values and trace elements reflect at least a partial Cambrian seawater signature. Primary fabric preservation is interpreted as a function of the rate of dolomitization rather than different porefluid compositions. Baroque (saddle) dolomite cements precipitated from higher temperature fluids associated with deeper burial. Mesozoic uplift and regional faulting accompanied Pangean rifting, producing a karst Cambrian-Triassic unconformity. Ledger Formation deposits are patchily dedolomitized, forming coarse calcite lenses, typically containing red Triassic sediment. Petrographic and geochemical data (trace elements and stable isotopes) show that the diagenetic fluids responsible for dedolomitization were primarily low temperature meteoric waters associated with karstification.

You do not currently have access to this chapter.

Figures & Tables




Citing Books via

Related Articles
Close Modal

or Create an Account

Close Modal
Close Modal