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Description
A marine sedimentary section on the southwest corner of Isla Tiburon, Gulf of California, Mexico, consists of interbedded fossiliferous sandstone and conglomerate, conformably overlying fossiliferous marine sedimentary breccia. These marine sedimentary rocks are interbedded with a middle Miocene (12.9 + 0.4 Ma) volcanic debris flow and are unconformably overlain by a late Miocene (11.2 + 1.3 Ma) ignimbrite. They are interpreted as part of a subaqueous fan-delta system within the Miocene Gulf of California. Analysis of the microfossils indicate deposition during the late Miocene to early Pliocene time, conflicting with available K/Ar data. Megafossils indicate that deposition could have occurred as early as the middle Miocene. Integrated study of marine megafossils, benthic and pelagic fossil foraminifera, calcareous nannofossils, and sedimentologic characteristics found within the study area has allowed it to be subdivided into 7 units based on depositional environments and facies changes. The lowermost section, unit 1, consists of a fossiliferous marine breccia with a maximum thickness of 1,700 feet. Unit 2 conformably overlies unit 1, has a maximum thickness of 820 feet, and consists of interbedded conglomerate containing neritic megafossils. Unit 3 conformably overlies unit 2 and consists of 250 feet of interbedded, inversely graded conglomerate and sandstone, both of which contain neritic megafossils. Unit 4 conformably overlies unit 1, is 590 feet thick and consists of three sub-units: unit 4A, the bulk of the unit, which consists of fine to coarse-grained sandstone containing both benthic and pelagic foraminifera and neritic marine megafossils, unit 4B, a volcanic debris flow, and unit 4C, a rhyolite flow. Unit 5 conformably overlies and is interbedded with unit 4 and unit 6 and consists of interbedded conglomerates and sandstones which contain neritic megafossils. Unit 6 conformably overlies and is interbedded with both unit 4 and unit 5, and consists of interbedded conglomerates which contain neritic megafossils. Unit 5 and unit 6 have a combined maximum thickness of 1,954 feet. Unit 7 conformably overlies unit 6, and consists of fine to coarse grained sandstones which contain neritic megafossils. The marine sedimentary section exposed on Isla Tiburon represents neritic marine deposition of the subaqueous portion of a fan-delta system during the Miocene, possibly as early as the middle Miocene. Work in progress on interbedded and overlying volcanic rocks should shed further light on the age of the section.
