Comparison of Two Enigmatic Contacts: Palliser-Exshaw, Devonian, Southwestern Canada, and Cupido-La Pena (Cretaceous), Northeastern Mexico: ABSTRACT

Abstract

In both the Palliser-Exshaw and Cupido-La Pena sequences, uncommonly sharp contacts separate carbonate bank deposits from overlying dark shales. Three environments discernible in each sequence may be attributed to gradual deepening of water during detrital influx. Figure At Potrero de la Mula, Coahuila, the uppermost Cupido consists of poorly sorted, oolith lime grainstones (environment 1). Abundant filled scolecoid burrows 1 to 2 mm in diameter extend 8 cm down into the Cupido from the iron-stained upper surface on which occur gastropods, pelecypods, and unabraded, hemispherical scleractinian colonies (environment 2). Dark shales of the La Pena Formation (environment 3) rest on this surface. Environment 1 was an active shoal with a shifting substratum which may have been stabilized as a result of deepening water (environment 2) permitting occupancy by corals and small burrowers. Bypassing prevented sedimentary accumulation except for End_Page 604------------------------------ shells and burrow fillings. Shales of the La Pena Formation accumulated as still deeper water (environment 3) caused bypassing to cease. At the type section of the Exshaw Formation on Jura Creek, Alberta, beds of the uppermost Palliser Formation accumulated on a shallow-marine carbonate bank (environment 1). These are overlain abruptly by a 2 to 7 cm sandy bed bearing collophane, bone fragments, and abundant pyrite. This bed may have accumulated in turbid, deeper water (environment 2) during bypassing of finer grained sediment. Further deepening of water resulted in decreased currents, and the black shale of the Exshaw Formation accumulated (environment 3). End_of_Article - Last_Page 605------------