TAPHONOMIC ANALYSIS OF DEVONIAN RHYTHMIC TRILOBITE BEDS: EVENT SEDIMENTATION AND CYCLIC CEMENTATION

Early to Middle Devonian trilobites from the Ant-atlas region of Morocco are widely known for their exceptional preservation. However, the details of their taphonomy have been little studied. The mid-Emsian Hollardops Member of the Khebchia Formation is a 5-15 meter thick interval comprising ~160 decimeter-scale limestone-shale alterations contains a rich and well-preserved trilobite fauna, including phacopids and the nominal dalmanitid trilobite Hollardops mesocristata. Concretionary limestones occur in both sparsely fossiliferous, dysoxic facies near the base of the Hollardops Member and more abundantly fossiliferous sections near the top, and hence, record a regular, recurring cycle during a long-term regression. These cycles probably record overall durations of 10s of Kyr; concretionary limestone bands probably formed by carbonate cementation during interludes of low sedimentation. Articulated trilobites occur in both the limestones and interbedded shales, but those in the shales are highly compressed and difficult to extract. Limestones yield articulated trilobites, including complete outstretched, incompletely and completely enrolled specimens, as well as molt ensembles, indicating a near absence of transport. Articulated, uncompressed trilobites commonly occur in attitudes perpendicular to bedding, and single blocks show varied orientations, including bed-parallel, inverted, upright, and vertical. This suggests that the trilobites were physically reoriented in single-event mudflows. Enclosing sediments are strongly bioturbated indicating prolonged periods of sediment starvation following mudflow events and prior to early diagenetic cementation, as indicated by thorough bioturbation of matrix sediment that remarkably did not disrupt the entombed trilobite carcasses. Hence, these Moroccan trilobite beds record an apparently periodic signature of carbonate redistribution superimposed upon muds with abundant obrution deposits. Numerous other trilobite beds show similar features indicating a recurring process. In all cases, the excellent preservation reflects both rapid burial and stabilization of sediment by early cementation owing to periods of low sedimentation.