Permian Red Chert-Pebble Conglomerate in Earp Formation, Southeastern Arizona

Abstract

The Earp Formation of southeastern Arizona can be divided into three members on the basis of lithologic character: the lower limestone member, the middle terrigenous member, and the upper interbedded clastic and carbonate member. The red chert-pebble conglomerate is a single, possibly isochronous unit, within the middle terrigenous member and forms an excellent marker bed throughout its area of outcrop. Meandering streams, bringing red chert pebbles south from the eroding Naco Formation in the area of Winkelman and Superior, Arizona, deposited the conglomerate. Fusulinids in the Earp Formation occur almost entirely within the lower limestone member. Studies of these fossils indicate a late Virgilian to possibly late middle Wolfcampian age for the Earp and an early to middle Wolfcampian age for the red chert-pebble conglomerate. The type section of the Earp Formation is thinner and less readily divisible into members than the apparently more complete sections in the Gunnison Hills and Whetstone Mountains. The lower part of the lower limestone member is missing, probably as a result of faulting. The middle terrigenous member is not clearly recognizable and the red chert-pebble conglomerate does not crop out in the type area. The source of clastics in the lower limestone member seems to be in the south and southwest, probably in north-central Mexico. This member undergoes a gradational facies change eastward where, devoid of clastics, the lower limestone member of the Earp Formation is transitional into the upper part of the Horquilla Limestone of southwestern New Mexico.