INVERTED TRILOBITES: KEY TO COMPLEX PRESERVATION OF AN ORGANICALLY TEXTURED SURFACE IN OFFSHORE SILICICLASTIC MUDSTONE AND CARBONATE FACIES: KOPE FORMATION (UPPER ORDOVICIAN), KENTON COUNTY, KENTUCKY, USA

摘要

Unusual preservation of small primaspid trilobites may provide a key to interpretation of an enigmatic, apparently organically textured surface or sedimentary structure in the subtidal Upper Ordovician (Katian) Kope Formation of northern Kentucky. A thin, elliptical lens of silty mudstone in the lower Kope Formation (Pioneer Valley submember, Southgate Member, C-1 Sequence) has several unique features mainly present on its basal surface, as a hyporelief, but also to a limited extent on the upper surface. Toward one end, the basal surface has sinuous fluting that terminates at the other end in a complex of conjoined, flattened, ellipsoidal or spatulate lobes that is partly overlain by an outlier of the primary lens. The basal surface has small-scale corrugations (3 to 4 mm wide) and polygonal reticulation variably developed. Articulated exoskeletons of small, spinose primaspid trilobites [Primaspis crosotus (Locke)] occur mostly with their ventral surfaces applied directly to the textured surfaces (i.e., dorsal side down). On some lobes the reticulate texturing wraps around from the basal to the upper surface and is covered with minute rounded pustules (similar to 0.3 mm). These enigmatic features are interpreted either as a recumbent microbial mat or an erect three-dimensional organism that was smothered and infilled by influx of silty sediment, preserved ultimately as a mold in hyporelief with trilobites in life position. Evidence for similar organic structures has not been previously reported from the Kope Formation. The taphonomic processes responsible for their preservation carry paleoenvironmental implications for possible microbial or macroalgal biotas within photic depths, and complex mediation of episodic depositional processes by organic biofilms or tissues.