Sedimentation and deformation in a Pliocene-Pleistocene transtensional supradetachment basin, Laguna Salada, north-west Mexico

Abstract

This study examines a thick section of Pliocene–Pleistocene sedimentary rocks exposed in the footwall of an active normal fault (Canon Rojo fault) near its intersection with the dextral-normal Laguna Salada fault in north-western Mexico. These rocks are situated in the upper plate of an inactive strand of the Canada David detachment fault, which is cut on the north-east by the Laguna Salada fault. The stratigraphy is divided into three unconformity-bounded sequences: (1) marine mudstone of the Pliocene Imperial Formation; (2) nonmarine Pliocene–Pleistocene redbeds, consisting of sedimentary breccia, conglomerate, conglomeratic sandstone (all un-named) and fine-grained sandstone and mudstone of the Palm Spring Formation; and (3) uncemented Pleistocene boulder gravel. Coarse deposits of the redbeds sequence were deposited in fault-bounded, high- and low-gradient alluvial fans that passed laterally into a low-energy fluvial plain of the ancestral Colorado River (Palm Spring Formation) which occupied the present-day Laguna Salada. Detailed mapping reveals convergence and lap-out of bedding surfaces in the redbeds sequence onto the west limb of a large anticline cored by Imperial Formation. These geometries, combined with fanning dips and thickening of stratigraphy into the flanking syncline, indicate that the anticline grew during deposition of the redbeds. Fold axes of the growth anticline and smaller related folds trend N to NNE, parallel to the strike of associated normal faults and perpendicular to the extension direction. Based on its orientation, large size and relationship to neighbouring structures, the anticline is interpreted to be a fault-bend fold that grew in response to slip of the upper plate over a bend in the Canada David detachment fault during deposition in a transtensional supradetachment basin. Localized subsidence in the flanking syncline resulted in deposition of >1000 m of alluvial sediments near its intersection with the Laguna Salada fault. Sedimentary detritus is derived exclusively from the north-east (footwall) side of the dextral-normal Laguna Salada fault, indicating that topographic relief was high in the Sierra Cucapa and was subdued or negligible in the footwall of the coeval Canada David detachment. Following deposition of the redbeds and grey gravel units, the northern part of the detachment fault was abandoned and the modern Canon Rojo fault was initiated, producing rapid footwall uplift and erosion of previously buried stratigraphy. Slip rate on the Canon Rojo fault is estimated to be ≈2–4 mm yr−1 since middle Pleistocene time, similar to the late Pleistocene to Holocene rate determined in previous studies.