Facies changes in the Bursum Formation (Pennsylvanian-Permian) related to tectonic activity, Soccorro, Mexico.

Three lithofacies of the Bursum Formation are developed in the hills east of Socorro. The limestone-mudstone lithofacies comprises alternating layers of nodular to bedded marine limestone and non-fissile variegated claystone that represents paleosols. This facies is relatively thin and reflects deposition on stable, relatively high tectonic blocks where sea-level change was the prevalent factor driving sedimentation. The second facies is the laminated lithofacies of thinly layered gray to green shale, siltstone, and fine sandstone. These rocks occur in thick packages and contain fossil land plants and small invertebrates signifying fresh to brackish water. The laminated facies evidently accumulated in tectonic blocks that subsided rapidly and held standing water. The third facies of conglomerate and arkosic sandstone occurs widely at the base of the Bursum and intermittently in the middle and upper part of the formation. Representing deposits of shallow low-sinuosity streams, these beds record distinct pulses of tectonic uplift in the Joyita Hills and at other, unidentified sites.