ABSTRACT: Outcrop Recognition of Sequence Boundaries in Carbonate Rocks

Evan K. Franseen, Mark T. Harris, Toni Simo

Carbonate sedimentary sequences have long been subdivided into packages based on erosion surfaces, nondepositional surfaces, exposure surfaces, sharp facies boundaries, or faunal discontinuities. Within the framework of sequence stratigraphy, synchronous surfaces that formed during global sea level falls are the most useful surfaces of correlating platform and basinal sections. Recognition of these surfaces combines their facies relations, stratal geometry, and regional extent.

Recent studies suggest various criteria for recognizing sequence boundaries, illustrated by examples from Permian, Triassic, Cretaceous, and Miocene carbonate sections. In platform sections, sequence boundaries typically are marked by the thinnest platform cycles, decreased sedimentation rates due to decreased accommodation, and increased subaerial exposure and erosion. Sequence boundaries in outer shelf and slope locations are distinguished by increased submarine erosion and redeposition. In distal slope and basinal positions, sequence boundaries may be recorded only as subtle changes in sedimentation patterns or biotic changes.

Despite the increasingly widespread use of sequence stratigraphic concepts, controversies remain concerning the synchroneity and regional significance of sequence boundaries. Specific problems arise in some outcrops because of the subtle nature of sequence boundaries, the presence of numerous stratigraphic breaks, and the amalgamation of surfaces. Proper recognition of sequence boundaries requires an integrated approach that incorporates studies of facies relationships and diagenetic patterns on a local and regional scale. Definition of sequence boundary surfaces provides a framework for smaller scale depositional packaging, thereby improving regional correlations, basin analysis, interpretations of platform profiles, and comparisons of three-dimensional platform geometries.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990