ABSTRACT: Tepee Structures in Boquillas Formatlon near Del Rio, Texas, resulting from Sub-Recent Calichification

LOCK, BRIAN E., and SUK-JOO CHOH

Flaggy rocks of the Boquillas Formation (basal Gulfan) outcrop extensively in the vicinity of Del Rio, Val Verde County, Texas. Several road-side outcrops of interbedded thin limestones and laminated shales contain striking tepee structures. The tepees consist of sharply angular anticlines, with brecciated cores, separated from one another by broad, flat synclines. Anticline spacing averages a few tens of meters. They have been interpreted in the literature as Cretaceous tidal flat features.

Several characteristics indicate that the tepees are actually related to caliche formation near the present topographic surface. The tepees always occur near the tops of the road cuts, within a zone of slightly darker color that cuts obliquely across stratification but parallels the topographic surface; they do not occur in a stratigraphic zone. Tepees are cored by friable, chalky carbonate with abundant 0.5 to 1.0 mm pisoliths, 'and the darker color of the zone is caused by surficial lichen, presumed to be geochemically localized by the presence of caliche.

Although tepee structures are commonly found in arid carbonate tidal flat sequences, they are not restricted to this environment. They reflect internal expansion of a shallow, laterally confined rock unit which is free to compensate by upward displacement. The expansion may be caused by growth of evaporites (not present in the Boquillas, which has no indications of being a result of tidal flat sedimentation), or by carbonates, including caliche. All indications support a late Pleistocene or Recent origin of the occurrences near Del Rio.

AAPG Search and Discovery Article #90947©1997 AAPG Southwest Section Meeting, San Angelo, Texas