Abstract

The Marfa Basin, encompassing approximately 6,000 square miles of Presidio and Brewster counties in west Texas, is a foreland basin that formed in the late Paleozoic in response to the encroaching Ouachita-Marathon thrust belt. The basin is one of several, including the Arkoma, Fort Worth, and Val Verde basins, that developed along the southern margin of the North American craton during convergence of North America and Afro-South America in Pennsylvanian to Permian time. We present a model for the formation of the Marfa Basin in which basin subsidence is effected by compression from plate convergence and by loading due to the emplacement of the Marathon fold-thrust complex.

A model of foreland basin evolution by thrust loading as applied to the Idaho-Wyoming thrust belt by Jordan (1981) can be applied with some modification to the Marfa foreland basin. Pre-existing northwest-trending faults in the Marfa region were reactivated by the Marathon thrust belt as the latter advanced onto the continental margin toward the craton. Subsidence due to compression and thrust loading first formed the Tesnus basin, a Pennsylvanian basin now buried beneath the Marathon overthrust. In the later stage of thrust sheet emplacement, the depocenter split into two prongs and the Marfa and Val Verde basins collected thick sections of Wolfcamp sediments.

Pre-existing northwest trends, which result from a Precambrian rifting event and the late Precambrian to Cambrian development of the Delaware aulacogen, controlled the location of subsidence in front of the thrust sheet. The fragmented craton was composed of northwest-trending high and low areas including the Diablo Platform and the Delaware Basin. These fragments behaved much like piano keys, subsiding first in a central region to form the Tesnus basin and later in adjacent regions forming the Marfa and Val Verde basins.

The model is supported with data from 63 well logs that indicate the position of the depocenters through time and that suggest the differential elevation of crustal slices controlling the formation and location of the three Pennsylvanian-Permian foreland basins.