Distribution of Dolomite in Lower Ordovician El Paso Formation

Russell E. Clemons

The El Paso Formation contains four members (in ascending order): Hitt Canyon, Jose, McKelligon, and Padre. The thickness of the El Paso decreases from a maximum of about 430 m in the southern Franklin and Hueco Mountains of west Texas to a pinch-out in the Oscura and San Mateo Mountains of New Mexico, 200 km to the north. Forty sections between Willcox, Arizona, and Van Horn, Texas, were examined and sampled for petrographic study.

No correlation was found between stratigraphic members and occurrence of dolomite. Lower Hitt Canyon beds are typically dolomite but are limestone in 25% of the studied sections. The El Paso Formation is mostly or completely dolomite at Bishop Cap near Van Horn, and in the Sacramento, San Andres, Oscura, and Pedregosa mountains. The formation is predominantly limestone in the San Diego, Caballo, and Mud Springs mountains, 60 km west of the Bishop Cap-San Andres-Oscura mountains trend. Other locales have varying intermediate limestone: dolomite ratios with no clear geographic relationships. Locally, single beds of dolomite are interbedded in limestone sections. More typically, dolomite or limestone prevails for tens of meters of section.

Evidence for a supratidal environment appears only locally. Hydrothermal dolomite is common in some sections. The subtidal dolomitization model proposed in 1973 by L. D. Harris for Ordovician carbonate rocks in the eastern United States best accounts for the El Paso dolomites. However, evidence of evaporite minerals was found only in the Victorio Mountains section.

AAPG Search and Discovery Article #91034©1988 AAPG Southwest Section, El Paso, Texas, 21-23 February 1988.