Petrology and Stratigraphy of Earp Formation, Pima and Cochise Counties, Arizona: ABSTRACT

Abstract

Stratigraphic and petrologic studies show that the Earp Formation of Pennsylvanian and Permian ages in southeastern Arizona and southwestern New Mexico consists of fluvial clastic rocks that interfinger with supratidal, intertidal, and subtidal marine carbonate rocks. The clastics encroached initially from the northwest into the shallow Earp sea during Missourian time and continued migrating eastward during Virgilian and Wolfcampian time. Clastic-ratio and isopach maps, a convex-upward geometry shown by a thinning of the overlying Colina Limestone, and primary sedimentary structures indicate that the clastic-dominated part of the Earp Formation was deposited as a delta. In the carbonate lithologies, 3 environments of deposition are recognizable: supratidal, intertidal, and subtidal. The supratidal environment is characterized by pelmicrites filled with sparry calcite surrounded by a micrite envelope; desiccated, interlaminated carbonates of algal origin (biomicrites); the absence of fossils; evidence of dedolomitization; and bird's-eye structure (dismicrite). The intertidal environment is composed of pelmicrites, biomicrites, micrites, and biosparites. Fusulinid biomicrites and biomicrites that have delicate tests preserved are characteristic of the subtidal environment. End_of_Article - Last_Page 349------------