K-Ar geochronology and trace-element geochemistry of 2M1 illite from upper Paleozoic shale of SW Laurentia – Insights into sediment origin and drainage pathways in the Anadarko Basin, USA

The Anadarko Basin of Oklahoma represents a major Paleozoic depocenter that existed along the rifted margins of southwestern Laurentia. In its infancy it accumulated a thick series of Cambrian through Mississippian detritus while further subsidence caused by inversion of the Cambrian Southern Oklahoma Aulacogen resulted in voluminous Pennsylvanian to Permian sediment. This contribution reports new data on K-Ar ages and trace-element geochemistry of detrital illite from middle and upper Pennsylvanian shale used to reconstruct sediment origins at the peak period of subsidence of the Anadarko Basin. X-ray diffraction was used to unveil mineral compositions and abundances of illite polytypes in two size fractions of separated illite (˂1 and 2-1 μm). K-Ar isotopic analyses were completed for both fine fractions, while the laser ablation inductively coupled plasma mass spectrometry was done for the latter. All illite separates consisted of mixtures of authigenic (1Md) and detrital (2M1) illite. The Illite Age Analyses showed that the detrital age of Desmoinesian (Moscovian) shale is the late Ediacaran (584 Ma), while the age of Missourian (Kasimovian) shale is the middle Cambrian (512.5 Ma). Trace-element abundances of all analyzed illite, irrespectively of stratigraphic age, are consistent with those of mica from metamorphic rocks. Based on illite detrital age and geochemistry it was inferred that Desmoinesian (Moscovian) shale represents a mixture of Neoproterozoic and Cambrian detritus sourced locally, whereas Missourian (Kasimovian) shale records a provenance shift toward more distal easterly sources from the Ouachita-(Marathon) foreland. This study has proposed a sediment source transition between the middle and upper Pennsylvanian that likely reflected major changes in the basin paleogeography and progressive development of the east-west (transcontinental) fluvial systems.Financial support for this study was received from the AAPG-Grants-in-Aid program and Geological Society of America. Any findings and conclusions expressed in this contribution are those of the authors and do not necessarily reflect the views of mentioned societies. Further support was obtained from the Geosciences Clay Laboratory of Texas Tech University. We are especially grateful to the core depository of the Oklahoma Geological Survey in Norman, OK, for granting us access to the core samples. We extend our appreciation to Drs. Calvin Barnes and Kevin Werts for their guidance in the GeoAnalytical Laboratory of Texas Tech University. Further on, we are in debt to James Browning and Rufino Lozano for their valuable help with sample preparation for LA-ICP-MS analyses and the acquisition of KAr ages, respectively. Critical comments and constructive reviews by two anonymous reviewers, as well as the editorial handling by Dan Sturmer and Thomas Algeo contribu