LITTLE CREEK STRUCTURE, NORTHWESTERN LA SALLE PARISH, LOUISIANA

Little Creek is unique among geologic structures in Louisiana and possibly in the U.S. The surface feature, a collapse structure 4.3 km (2.7 mi) across in Cenozoic strata, lies above and crosscuts at depth a broader domal structure marked by areally restricted unconformities in Upper Cretaceous strata. Drilling shows the collapse structure extends to a minimal depth of nearly 8,000 ft (~2,440 m). H. N. Fisk originally mapped the feature in the late 1930s, at the surface on his geologic map of La Salle Parish and in the subsurface at a regional scale using oil and gas well data. Since his work, little information about the Little Creek structure has appeared in the public domain, though it appears to have garnered attention from some geologists at least intermittently. One of these, J. E. Rogers, interpreted the feature’s surface unit as Carnahan Bayou Formation, Fleming Group rather than the Catahoula Formation of Fisk’s original mapping, based on correlation of water well logs. This interpretation greatly increases the displacement attributable to the structure at the surface and in the shallow subsurface. Displacement measured on the contact between the Midway and Wilcox Groups at greater depth within the collapse structure is as much as 3,500 ft (~1,070 m).

Little Creek’s singular suite of characteristics and apparent lack of relation to surrounding regional structure have led to disparate conceptions of its origin. Fisk proposed no hypothesis of origin of the structure. Unpublished hypotheses formulated during the 1960s and 1970s include (1) salt withdrawal marking the location of a former salt diapir near the southeastern edge of the north Louisiana salt basin (J. E. Rogers), (2) long-term response to a meteor impact that occurred during deposition of the Upper Cretaceous chalk (M. D. Butler), and (3) response to emplacement of a deep post–Jurassic igneous diapir (D. H. Wilson). Nearly eight decades after its discovery, the structure remains an anomaly, and its distinctive aspects continue to challenge straightforward interpretation.