Diagenetically-enhanced trilobite obrution deposits in concretionary limestones: The paradox of “rhythmic events beds”

Abstract

Certain offshore facies with extraordinarily well preserved fossils, particularly trilobites, display distinctive motifs of rhythmically bedded, decimeter-scale, concretionary limestone and dark gray shale. Examples are known from Cambrian to Neogene age, but are especially common in the lower-middle Paleozoic. The Hollardops trilobite beds of the Lower Devonian (Emsian) Khebchia Formation, Morocco, provide the exemplar of this phenomenon. Comparative examples include trilobite beds from the Upper Ordovician Katian of Oklahoma, New York State, and Ontario, Canada, and the Lower to Middle Devonian (Lochkovian–Givetian) of Tennessee, Oklahoma, and New York State. These Lagerstätten are observed to crosscut facies belts, occuring in both sparsely fossiliferous, dysoxic facies and more abundantly fossiliferous sections. Articulated fossils occur in both limestones and interbedded shales, indicating episodic deposition of fine grained sediments in both instances, but those in the shales are highly compressed and difficult to extract. Conversely, limestones yield particularly well preserved fossil material, including in situ lingulid brachiopods, and bivalves, cephalopods and complete outstretched, and enrolled trilobites. Articulated, uncompressed fossils may occur in varied orientations relative to bedding. This suggests that the organisms were locally entrained in mudflows, although occurrence of exoskeletal molt ensembles in some beds suggests that in certain events organisms could be preserved in situ. Enclosing sediments are typically strongly bioturbated indicating prolonged 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 and calcite spar fillings in internal spaces of fossils formed early and rarely preserve remnants of lightly sclerotized tissues. Carbonate cementation occurred during interludes of low sedimentation; association with pyrite suggests formation within the zone of sulfate reduction.These intervals are particularly well developed in relatively sediment-starved, transgressive intervals and constitute a type of time-specific facies; these rhythmic obrution beds are especially characteristic of the Late Ordovician, and late Early Devonian (Emsian), where they occur on at least four paleocontinents. These beds formed selectively in warm, in semi-arid climates in which minor siliciclastics supplied from a relatively low relief hinterland were episodically re-suspended in slurries and transported offshore; they were favored in systems poised between deposition of siliciclastic and carbonate muds. The commonly rhythmic bedding of these deposits records a regular, recurring cyclic motif with overall durations of 10s of Kyr of carbonate redistribution superimposed upon muds that include abundant obrution deposits.