Petrology, depositional environments and structural development of the Mineta Formation, Teran Basin, Cochise County, Arizona

Abstract

The Oligocene Mineta Formation of the Teran Basin in southeastern Arizona contains approximately 1000 m of interfingering alluvial-fan, fluvial, and lacustrine strata with minor intercalated volcanics. Sedimentation began prior to the middle Oligocene and ended with the eruption of the Oligocene-Miocene Galiuro Volcanics about 28 m.y. ago. Several related depositional facies are present in a cyclic pattern within the Mineta Formation. A complete cycle contains proximal, coarse-grained alluvial-fan conglomerates that fine upward into sandstone and pebble conglomerate of a distal-fan or braided-stream environment. Medium- to fine-grained sandstones are interpreted as sandflat deposits that lie stratigraphically above the distal-fan deposits and below lacustrine sandstone and algal limestone. The cycle culminates with deeper water lacustrine mudstone with intercalated thin-bedded turbidite sandstone. Discrete cycles of bedded gypsum and mudstone are found locally within the lacustrine deposits. These deposits represent a temporary playa-lake phase of the Mineta depositional system. The Mineta Formation filled a half-graben that developed in response to extensional stresses associated with the development of the Rincon/Catalina metamorphic core complex. Paleocurrents flowed southwestward from a source area of predominantly sedimentary and volcanic rocks. Sedimentation was contemporaneous with extensional deformation that produced low-angle normal faults and tilted the section toward the northeast. Maximum extension was directed about N55°E-S55°W, roughly parallel to the direction of paleocurrent flow, parallel to the mean slip direction for low-angle normal faults and perpendicular to the strike of the Mineta section.