Brett, Carlton E.¹, Patrick I. McLaughlin¹
(1) University of Cincinnati, Cincinnati, OH

ABSTRACT: Response of Shallow Marine Biotas to Sea Level Fluctuations: Environmental Tracking and the Persistence of Biofacies

Bathymetrically- and sedimentologically-related factors exert major controls on the distribution of shallow marine benthic organisms, resulting in zonation along and across depositional strike. Recurring stratigraphic patterns of fossil distribution are related to sedimentary cycles, suggesting that species commonly respond to long-term environmental changes not by adaptation, but primarily by migrating in tandem with shifting physical parameters (“environmental tracking”). The validity of tracking is indicated by: a) lateral biofacies gradients along successive time planes, b) correlatability of curves generated by quantitative gradient analysis, c) high fidelity recurrence of stenotopic species in very specific litho- and biofacies associations. However, precise tracking of species assemblages rarely occurs. The vertical successions of fossil assemblages through cycles in single stratigraphic sections may appear symmetrical or markedly asymmetrical. “Mirror image” patterns are rare within many siliciclastic dominated environments for two major reasons. First, increased turbidity during relative sea level fall favors low diversity associations of sediment-tolerant, eurytopic taxa. Conversely, the combination of shallow, clear water, and typically firm to hard substrates during sediment-starved early transgressions makes them favorable to diverse stenotopic taxa. Second, despite some marked similarities, no two recurring biofacies are ever identical because species respond independently to a variety of parameters, some of which are not recorded in sediments. Such evidence argues against strong bonding among species or tightly integrated communities. Nonetheless, the ability of organisms to track environmental fluctuations, together with shared environmental preferences of species, may partly explain long term stasis and the recurrence of similar species associations over millions of years.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.