DIAGENETICALLY-ENHANCED OBRUTION DEPOSITS IN CONCRETIONARY LIMESTONES AND THE PARADOX OF "RHYTHMIC EVENT BEDS"

Certain offshore facies with exceptionally preserved fossils display distinctive motifs of rhythmically bedded, decimeter-scale, concretionary limestone and dark gray shale. Examples are known from Cambrian to Neogene strata, but are especially common in the lower-middle Paleozoic. Such Lagerstätten occur in both sparsely fossiliferous, dysoxic facies and more abundantly fossiliferous sections. Articulated fossils, particularly trilobites in Paleozoic examples, occur in both limestones and interbedded shales, indicating episodic deposition of fine grained sediments, but those in the shales may be highly compressed and difficult to extract. Limestones yield particularly well-preserved fossil material, including in situ lingulid brachiopods and bivalves, cephalopods, and complete arthropods and echinoderms. Articulated, uncompressed fossils may occur in varied orientations relative to bedding. This suggests that some organisms were caught in mudflows and moved slightly. Enclosing sediments are typically strongly bioturbated, indicating periods of sediment starvation following mudflow events but prior to early diagenetic cementation; however, most burrows are small and evidently did not disrupt entombed organism remains. Pyritic coatings and calcite spar fillings in internal spaces of skeletons formed early and rarely preserve remnants of lightly sclerotized tissues. The widespread, facies cross-cutting nature of these beds strongly implicates an allocyclic process superimposed upon episodic burial events. These intervals are particularly well developed in relatively sediment-starved, transgressive intervals and formed selectively in mixed siliciclastic-carbonate settings in which mixed carbonate muds and clays, supplied from relatively low relief platforms, were episodically resuspended in slurries and transported offshore. Carbonate cementation occurred during interludes of low sedimentation; association with pyrite suggests formation of early diagenetic cements within the sulfate reduction zone. The typical rhythmic bedding of these deposits records regular, recurring (Milankovitch band?) cycles with overall durations of 10s of Kyr of carbonate redistribution superimposed upon sediments that include abundant obrution deposits.