SEQUENCE STRATIGRAPHIC CONTROL OVER TAPHONOMICALLY COMPLEX PELMATOZOAN ECHINODERM LAGERSTÄTTEN IN PALEOZOIC EPEIRIC BASINS

Eustatic fluctuations and consequent effects on sedimentation patterns exert tremendous control over the stratigraphic distribution of echinoderm Lagerstätten. As a result, the sequence stratigraphic paradigm represents a powerful conceptual framework for genetically interpreting deposits containing exceptionally preserved echinoderms and predicting their occurrence within the stratigraphic record. Although sediments containing abundant articulated pelmatozoan echinoderms can occur within packages representing any phase of base-level change (i.e., within any systems tract), the interplay between background sedimentation rate, character and frequency of episodic rapid burial events, and physio-chemical environmental parameters during the late transgressive (TST) to early highstand systems tracts (HST) makes possible taphonomic complexity unmatched by Lagerstätten in other portions of the sequence. In both siliciclastic and mixed carbonate-siliciclastic systems, the relative decrease in rate of transgression characteristic of the latest TST and gradual transition to slow normal regression characteristic of the early HST ensures opening of a narrow window during which detrital sediment is sparingly, but increasingly, delivered to formerly sediment-starved environments. Deposition during this interval is primarily via episodic rapid-burial events, which smothered benthic fauna. However, the low background sedimentation in such settings resulted in either 1) stacking of multiple sedimentologically subtle obrution layers with little physical record of intervening “normal” conditions (commonly enhanced by the effects of compaction), or 2) reworking of earlier event-buried populations into amalgamated assemblages (commonly aided by early diagenetic cementation of thecae). This unique combination of stratigraphic condensation, repeated rapid burial, and exhumation/minor degradation prior to final burial generated thin Lagerstätten where pelmatozoans are largely articulated, but rarely perfectly complete, and assemblages display considerable taphonomic variability even at the intraclade level. This contrasts with the taphonomic signatures of most Lagerstätten in other stratigraphic settings, which tend to preserve single-event “snapshots” of the seafloor.