SURPRISES IN THE CULEBRA GRABEN, A MAJOR SUB-BASIN WITHIN THE SAN LUIS BASIN OF THE RIO GRANDE RIFT

San Luis basin is the largest structural depression in the late Cenozoic northern Rio Grande rift. Culebra graben is a major sub-basin in the east-central part of San Luis basin. New discoveries and confirmation of prior unpublished work, both made while mapping for the Colo. Geological Survey, call for modifications to existing tectonic, depositional, and hydrologic models of the graben.

Middle Miocene syn-rift sedimentary deposits and intercalated 12 Ma volcanic rocks crop out on and east of the Culebra Range crest, well beyond the generally accepted eastern margin of the rift. The rocks are preserved in a downdropped block that cuts obliquely across the range and extends eastward nearly to thrust faults on the west side of the Laramide Raton basin. A north-trending fault within the downdropped block lowers syn-rift rocks about 600 m from the range crest into a triangular-shaped structural depression at Devils Park east of the range crest. Paleocurrent and provenance studies of syn-rift sediments in Devils Park indicate they were deposited by southwest-flowing streams. The triangular-shaped fault-bounded block at Devils Park formed after 12 Ma, is downdropped in the opposite direction from the rift, and caused the range crest to migrate about 5 km west since the middle Miocene.

San Pedro Mesa horst is a small north-trending uplifted block within the larger Culebra graben. Historically the horst was considered a block of permeable syn-rift sediments capped by early Pliocene Servilleta basalt. However, a small island of Proterozoic and Paleozoic(?) rocks surrounded by basalt exists on the mesa, which could require modifications to the tectonic history of the horst and alter its hydrogeologic properties.

Mesita volcano, a 1 Ma basaltic cinder cone west of San Pedro Mesa horst, rises slightly above the Costilla Plains. Widespread sedimentation on Costilla Plains was previously thought to essentially cease about when the volcano formed. However, remnants of fluvial gravels locally overlie the cinder cone. Within the past 1 m.y., the cinder cone was at least partly buried by sediment and later exhumed by erosion. Existence of similar unconformities within stacked sequences of syn-rift strata can complicate stratigraphic studies and attempts to unravel the tectonic history of individual blocks within the rift.