YOU GOT CHERT IN MY CARBONATE! YOU GOT CARBONATE IN MY CHERT! INTERPRETING THE WEIRD SEDIMENTARY STRUCTURES ASSOCIATED WITH SHALLOW-WATER, MIXED CHERT-CARBONATE, MARINE SYSTEMS OF THE PERMIAN

The deposition of marine mixed chert and carbonate sedimentary systems both in the modern and the Cenozoic are typically interpreted as forming in deep basinal settings. As siliceous plankton gradually perish, they sink to the seafloor forming realtively thin planar beds. However, these modern environments do not serve as a sufficient depositional model to explain the structures which occur in shallow-water nodular cherts common during the Paleozoic. For example, spiculitic cherts (derived from siliceous sponge spicules) of the late Permian Gerster Limestone form as part of massive 1-meter scale bioclastic carbonate beds. In many instances, these cherts lack any semblance of planar deposition and form mixed beds which exhibit great heterogeneity along strike. Although chert nodules form many different size and shaped nodules within the beds of the Gerster, it does not appear to occur randomly or simply along bedding planes. Observed cherts form chaotically ordered deposits morphologically similar to load cast typically observed between beds of siliciclastic sand and mud. These gravity-driven mobilizations of sediment correspond to density differences due to overloading of sediment or liquidization of lower layers. We propose a model of chert formation in which biogenic siliceous beds overload micritic carbonate beds forming pinches, swells, and dewatering structures, which later diagenetically alter to chert.