Unique Signature of Carbonate Strata and the Development of Depositional Sequence and Systems Tract Models for Ramps, Rimmed Shelves, and Detached Platforms

HANDFORD, C. ROBERTSON, and ROBERT G. LOUCKS, ARCO Oil and Gas Company, Plano, TX

Eustasy, tectonic subsidence, and sedimentation rates are the most important variables controlling the development of depositional sequences in both siliciclastic and carbonate depositional provinces. However, both sediment types have unrelated depositional origins and they respond differently to relative changes of sea level. For example, carbonate environments generate sediment and create sea-floor topography (rimmed shelf margins and steep slopes), and carbonate rocks weather into karst topography during lowstands. These differences are manifested by unusual and perhaps unique stratal patterns, especially in rimmed shelves with depositional versus bypass margins and ramps characterized by fringing versus barrier shoals. Furthermore, rimmed shelves may exhibit marineward onlap of sh lf strata (carbonate, siliclastic, or evaporite) against carbonate shelf margin buildups and carbonate clinoforms that prograde landward from rimmed margins and downlap deep shelf strata. When combined

with environmental factors (morphology, continental linkage, marine setting, latitude, magnitude of shelf, climatic factors, sediment type, and age), significant variations in systems-tract geometries and stratal patterns are likely to result.

Factors listed above must be integrated with established facies models in order to arrive at sequence and systems tracts models that adequately address carbonate facies. As should be the case with all models, however, they are not meant to serve as rigid templates within which all carbonate sequences must fit. They are meant, instead, to function as working hypotheses to help geologists visualize how and why carbonate strata were laid down and fit together as they do.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)