Eustasy, Tectonics, and Climate: Isolation of Controls on PennsylvanIan (Missourian) Cyclothems from the Sacramento Mountains (Orogrande Basin), New Mexico

William D. Raatz, J. A. (Toni) Simo

Eustasy was the dominant control on Missourian deposition in the Sacramento Mountains despite the presence of active tectonism and major climate changes. These findings suggest that regional correlation of Pennsylvanian cyclothems is possible across different tectonic and climatic settings, allowing comparisons between local relative and Midcontinent eustatic sea level curves. Such comparisons permit quantification of eustatic versus tectonic and climatic influences. Upper Desmoinesian through Missourian strata in the Sacramentos consists of a 120 to 190 meter thick understudied, highly cyclic, mixed siliciclastic/carbonate system deposited on a tectonically active shelf. Both siliciclastic and carbonate cycles are 10 to 20 meters thick, asymmetric, transgressive-regressi e packages bounded by subaerial exposure surfaces. Three factors controlled deposition: eustasy, tectonics, and climate. Eustasy was the major control over formation, frequency, and general thickness of cycles. Eustatic sea level changes created discrete, laterally continuous deposits of deep marine through terrestrial lithologies in close vertical proximity, similar in style to cyclothems found in Midcontinent North America. Tectonic influences affected deposition at two scales. Regional uplift shaped the basin and provided a source for siliciclastic material. Local structures modified the thickness and character of deposits. For example, anticlinal crests created depositional areas of relatively high energy, resulting in anomalous textures and fabrics, and areas of concentrated bioherm l growth. Faults created graben structures that acted as conduits for siliciclastic transport and deposition. Climate affected these deposits through regulation of detrital runoff, and as a feedback in glacio-eustasy. A globally recognized drying event at the Desmoinesian-Missourian boundary may be responsible for the change from lower clastic to upper carbonate depositional systems in the study area.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995