Petrology of Krebs Subgroup (Pennsylvanian, Desmoinesian) of Western Missouri

Abstract

The Krebs subgroup of Pennsylvanian age is a rock association of varied lithologic character that crops out in several counties of southwestern Missouri. The Krebs was divided into 10 rock types on the basis of lithologic differences observed in the field. Stratigraphic measurements were made of each rock type. Sandstone Type A (medium- to coarse-grained sandstone: submature to mature subarkose) constitutes 63 per cent of the rock sections measured. Other rock types present in the Krebs are orthoquartzitic (chert) conglomerate, fine- to very fine-grained subgraywackes (sandstone Types C1 and C2), supermature calcitic subarkose (sandstone Type B), black shale, gray shale, coal, underclay, and limestone (silty brachiopod biomicrite). Textural and miner logical analyses of each rock type indicate that the field division into rock types was generally valid. The Krebs subgroup is made up of six formations and in some places as many as three or four of the designated rock types occur within a single formation. Cross-bedding studies indicate that the Type A sandstones were deposited by currents with a modal southeasterly flow direction (mean azimuth = S 34° E). Cross-bedded units average 2.5 feet in thickness and cross-beds have a modal inclination of 22°. The provenance of these sediments is thought to be a combination of distant and local sources. The distant source was located northwest of the study area and was part of the Canadian shield, and the local source was the Nemaha uplift. Seven depositional environments--beach, lagoon, tidal channel, low tidal flat, high tidal flat, swamp, and shallow offshore marine--are represented by Krebs subgroup sediments. The Warner formation, composed of channel and sheet deposits, is of particular interest in that its grain-size characteristics and structures closely resemble those of the Recent Wadden Sea.