Description
Salton basin, a transtensional rift basin, lies along the Pacific-North American plate boundary. Neogene/Holocene sediments filling Salton basin were brought in by the Colorado River and smaller rivers draining crystalline basement rocks of the Peninsular Ranges. Limited sampling and a lack of internally standardized point-count methodologies produced data that cannot be used to rigorously compare the compositions of Neogene sand with the compositions of probable source material. This study is one of several studies that comprise the Salton Basin Project, which was designed to document the petrology and provenance of Holocene and Neogene sand and sandstone filling Salton basin. A total of 134 Neogene/Holocene samples have been analyzed in the Salton Basin Project using the Gazzi-Dickinson point-count method. Data collected provide the first internally standardized data base for comparing the compositions of olocene and Neogene sand and sandstone in Salton basin. Recent work on the compositions of Holocene sand derived from the Colorado River and mountains adjoining Salton basin provide a framework for studies on the provenance of older Neogene sediments. Sand/sandstone in the Pliocene-Pleistocene Borrego Formation have compositions suggesting mixing of ancestral Colorado River sediment with sand derived from local plutonic/metamorphic rocks of the Peninsular Ranges. Data from the Palm Spring and Imperial Formations (this study) accurately record derivation of material from uplifted plutonic/metamorphic basement rocks, Tertiary volcanic rocks, and the ancestral Colorado River. Mixing of end-member source material is suggested in areas where deltaic material inferred to be derived from the Colorado River drainage basin was deposited in areas with plutonic/metamorphic basement and/or volcanic rocks in close proximity. Colorado River-like compositions in fossiliferous sandstones of the Imperial Formation indicate the arrival in Salton basin of sand derived from the ancestral Colorado River by the early Pliocene. The basin interior was dominated by deltaic sedimentation of the ancestral Colorado River during the time frame represented by both the Imperial and Palm Spring Formations. Changes in the relative role of locally derived plutonic/metamorphic detritus versus Colorado River sediments likely occurred as a result of basin displacement along faults of the San Andreas system which increased the distance between the entrance point of the Colorado River into Salton basin and the final depositional site of deltaic material. The similarity in compositional mixing fields for Holocene sand and sand/sandstone of the Imperial Formation strongly suggests that the compositions of sand being supplied to Salton basin have changed very little since Miocene times. Results of this study support paleotectonic reconstruction models proposing that sediment of the ancestral Colorado River delta has been progressively displaced northwestward along faults of the San Andreas system. Provenance of Neogene clastic detritus is generally accurately portrayed on conventional QFL/QmFLt/LmLvLs provenance-discrimination diagrams. However, simplistic use of the QFL diagram may lead to erroneous interpretations. In order to present a more complete provenance picture, particularly when mixing of various source rocks is suspected, use of the QFL diagram in conjunction with the QmFLt and LmLvLs diagrams is advocated. The recycled-orogenic field on the QFL provenance-discrimination diagram should be redefined to include detritus likely derived from sources with complex tectonic histories like the Colorado River plateau and the Salton basin area.
