Stratigraphic and structural evolution of the early Diligencia Basin, Orocopia Mountains, Southeastern California
Davisson, Cole M.
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The Diligencia Formation comprises approximately 1600 meters of OligoMiocene continental siliciclastic sedimentary rocks and intercalated basaltic lavas, located in the eastern Orocopia Mountains of southeastern California. These deposits exhibit characteristics which may link them to tectonic processes diagnostic of both the Basin and Range physiographic province of North America, and the Transverse Ranges province of California. These different processes were active, however, during different periods in the history of the formation. The early Diligencia Basin had an asymmetric half-graben geometry, and formed in response to presently north-south directed extension along a listric detachment located along the northern basin margin. Facies associations determined from five measured sections through the lower Diligencia Formation reveal a distinctly asymmetric distribution of facies across the basin. The deposits along the northern margin of the basin are characterized by cobble and boulder conglomerates and very coarse-grained sandstones unconformably overlying marine sedimentary rocks of the Eocene Maniobra Formation. These sedimentary rocks represent a high-gradient alluvial fan system which developed against the steep escarpment of the listric detachment fault. These deposits contrast with those present along the southern basin margin. The sedimentary rocks on the southern margin are characterized by pebble and cobble breccias, pebble conglomerates, very coarse- to fine-grained sandstones and gypsiferous mudstones. These facies define a system of low-gradient alluvial fans/fluvial braidplain that were deposited on the relatively gently sloping hangingwall block of the detachment, and that interfingered with deposits of a playa lake complex that formed in the basin interior. The age of these deposits, basin geometry, and the style of sedimentation are very reminiscent of half-graben basins known from the Colorado River Extensional Corridor of the Basin and Range. Paleomagnetic studies indicate that the block containing the Diligencia Formation may have been rotated more than 90° about a vertical axis since deposition ceased. In addition, regional palinspastic reconstructions indicate that the basin may have been significantly further north and east in the Miocene. These observations suggest that the Diligencia Basin may be genetically related to similar basins in the western Basin and Range, and that Diligencia Formation can be better understood if viewed in this tectonic context. Sometime after deposition had ceased, the rocks of the Diligencia Formation were deformed under an imposed compressional stress field in association with activity along the Clemens Well Fault, a major, dextral wrench fault in the region. The action of the Clemens Well Fault also juxtaposed the Diligencia Basin with distinctive crystalline terranes of the central and western Orocopia Mountains, including the Mesozoic Orocopia Schist. The association of these rock types with structures characteristic of dextral strike-slip deformation have led most workers to tie the Orocopia Mountains to similar complexes in the Transverse Ranges province of southern California. In fact, the post-depositional history of the Diligencia Formation can best be understood only in the context of Transverse Ranges-style tectonic processes. The complicated history of the sedimentary and volcanic rocks of the Diligencia Formation has made it difficult for previous workers to sort out an appropriate tectonic context in which to describe them. Recent advances in our understanding of the sedimentary responses to Tertiary rifting in the western Basin and Range, provide a new opportunity to evaluate the history of these deposits and this work has determined that the Diligencia Formation has had close ties to both the Basin and Range and the Transverse Ranges at different times during its complex history.
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