ABSTRACT

Caliche is common in near-surface outcrops of the Cretaceous of central and west Texas. Preferential wedge-like caliche accumulation along shale partings and expansion of shale units close to the outcrop surface leads to non-tectonic anticlines (including tepees), synclines and thrust faults in otherwise flat-lying strata. A variety of deformational features have been observed in several different formations; this paper is primarily concerned with the tepee structures.

Flaggy rocks of the Boquillas Formation (basal Gulfian) crop out extensively in the vicinity of Del Rio, Val Verde County, Texas. Several road cuts 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 are not confined to a specific stratigraphic zone. Tepees are cored by friable, chalky carbonate with abundant 0.5- 1.0 mm (0.02-0.04 in) glaebules, 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 commonly form 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. Caliche may also be introduced along cracks from above and be emplaced along bedding planes, lifting up overlying strata adjacent to the crack.

All indications support a late Cenozoic origin of the occurrences near Del Rio.