NUMMEDAL, DAG, Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA; and CHARLES J. STUART, Unocal Science and Technology Division, Brea, CA

ABSTRACT: Scales and Patterns of Stratigraphic Sequences

A depositional sequence is a package of genetically related strata bounded by unconformities and correlative conformities. This broad definition allows for recognition of sequences of a wide range in temporal and spatial scales. Long term sequences (>100 my) are primarily tectonic in origin whereas short term ones (<1 my) probably are controlled by eustasy. Sequences of intermediate duration (several my) may reflect a complex interplay of tectonics, eustasy, climate and sediment supply.

Proterozoic and Phanerozoic strata can be subdivided into five large sequences, each of about 400 my duration. These are bounded by (global?) continental unconformities that result from high continental elevation during periods of global plate assembly ("Pangean states"). The six Phanerozoic North American sequences identified by Sloss, each with a duration of about 100 my, also are bounded by inter-regional unconformities. Whether these reflect periods of eustatic fall or episodes of plate deformation remains controversial. Both types of sequences display great internal variability, and are often incomplete, because time scales of change in basin subsidence mechanisms and broad climatic patterns are shorter than duration of the sequences.

Sequences of intermediate duration (a few my) reflect a multitude of mechanisms including tectonic ones (e. g. thrusting, major growth faulting, intraplate stresses?), and poorly-understood, long-term eustatic trends.

Short term sequences may form in response to astronomically-controlled climate modifications (Milankovitch type). Oceanic water volume responds to changes in continental ice sheets, exchange of ground water, and steric factors (expansion of water by heating). Climatic changes also affect sediment supply, which may drive major shoreline excursions and build sequences, without significant corresponding sea level changes. During "icehouse" states of earth history (e.g. Pennsylvanian, late Cenozoic) glacio-eustasy probably exerted dominant control, but during "greenhouse" states (e.g. Cretaceous) amplitudes of sea level change probably were small, and other factors dominated sequence formation.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.