Correlation of the Subsurface Structure in the Monroe Gas Field, Northeastern Louisiana, with Overlying Neotectonic Features

Paul A. Washington
Department of Geosciences, University of Louisiana at Monroe, Monroe, LA 

Neotectonic features indicating active north-south shortening abound in northern Louisiana and southern Arkansas. In addition, a prominent northeast-striking set of neotectonic features indicates active northwest-southeast contraction or extension. Ongoing detailed subsurface mapping of the Monroe Gas Field, on the other hand, finds that the principle structures are east-verging thrust faults that stair-step up through the Monroe Gas Rock and overlying strata. Linking these two superimposed, but apparently inconsistent, structure sets presents a significant challenge.

The most prominent northeast-trending neotectonic feature is the C-O-W escarpment, which forms the western margin of the Boeuf sub-basin of the Mississippi alluvial plain. The southern end of this escarpment overlies the Monroe Gas Field and appears to represent the leading edge of major thrust sheets. Deviations in the alignment of the escarpment correlate with lateral ramps and displacement transfer zones in the down dip extensions of these thrusts. The geomorphology of the escarpment indicates that the displacement occurred no earlier than latest Pleistocene and may still be active.

The subsurface structural evidence of the north-south contraction is much less obvious. Apparently, many of the south-verging lateral ramp systems within the east-verging thrust system are also serving as active ramp surfaces for south-verging thrusting. The geomorphology indicates that much of the activity on these structures has happened during the Holocene and that many are still active. Because this displacement is so young, it is probable that the amount of the displacement on many of the fault surfaces has been insufficient to allow them to be distinguished in the subsurface from the lateral features within the east-verging thrust system.

AAPG Search and Discovery Article #90032©2004 GCAGS 54th Annual Convention, San Antonio, Texas, October 10-12, 2004