ABSTRACT: The Application of Palynology to Stratigraphic Interpretation in the Marathon Orogenic Belt, Texas

R. P. Wright, B. A. Savage, T. M. Laroche, J. E. Barrick

The Marathon orogenic belt contains exploration targets in Paleozoic sediments deposited in continental slope to ocean basin settings. Subsequent deformation into complexly folded and thrust-dominated structures has hampered seismic stratigraphy as an exploration tool in the region. Palynology provides a method of correlation in subsurface mapping and of predicting possible reservoir occurrence. Fossils used include marine organic-walled microphytoplankton and chitinozoans for Devonian and older strata, and spores for the younger Carboniferous. The fossils occur in different lithologies which represent contrasting depositional settings: from starved basin chert/clay-mudstones of the Devonian age Caballos Novaculite to rapid fill siliciclastic flysch of the Tesnus Formatio .

Devonian microphytoplankton present in the Caballos Novaculite are consistent in their floral composition throughout the orogenic belt. Their presence locates and substantiates repeated thrust packages of Caballos. The subjacent Ordovician Maravillas Formation, which is structurally coupled to the Caballos as a competent unit, and the Woods Hollow Shale below the Maravillas contain diagnostic microphytoplankton and chitinozoans. These fossils are the key in distinguishing Ordovician strata from the younger Carboniferous flysch, a task not easily done based on lithologic and log characteristics alone. The Carboniferous Tesnus Formation, like the Ordovician Woods Hollow Shale, is a structurally incompetent unit and its subsurface presence is substantiated by spores. Recycling of older P leozoic fossils within Carboniferous flysch is pervasive and must be considered when making geologic interpretations.

Palynological results, when integrated with other forms of subsurface information, provide an accurate evaluation of the stratigraphic relationships in the Marathon belt.

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