2nd- AND 3rd-ORDER SEQUENCE STRATIGRAPHY, DEPOSITIONAL ENVIRONMENTS, AND PROVENANCE OF MIDDLE-LATE ORDOVICIAN SUPERMATURE QUARTZ ARENITES, NORTH AMERICA

Middle-Late Ordovician quartz arenites of North America (eg., Mt Wilson Quartzite, Kinnikinic Quartzite, Eureka Quartzite, Cable Canyon Sandstone, Simpson Group sandstone units, St. Peter Sandstone) are a diachronous group of quartz arenite units deposited around the Transcontinental Arch. These units, up to a few hundred meters thick, record unique Lower Paleozoic supermature siliciclastic deposition within a predominantly shallow-water carbonate successions. The Middle-Late Ordovician quartz arenites commonly are well-cemented, thus their historical classification as quartzites. The abundance of silica cementation in these units locally obscures depositional fabrics and appears to be more common near younger volcanics or batholiths. These Middle-Late Ordovician quartz arenites were deposited in eolian to intertidal to shallow subtidal shelf environments. We are using detrital zircon geochronology to determine their provenance. So far the majority of the grains in the western U.S. have dominant modal peaks at 1.8-1.9 Ga, 2.1 Ga, and 2.5-2.8 Ga. The 1.8-1.9 Ga zircons indicate a Trans-Hudson Orogeny source, the 2.5-2.8 Ga zircons were derived from numerous Archean basement provinces, some of them proximal to these sediments, others from more distal sources. The 2.1 Ga detrital zircons are enigmatic, as they are ubiquitous, but their source is unclear.

Generally the quartz arenites are older to the south and young toward the north, likely reflecting variations in long-term tectonic subsidence during flooding of North America. Sequence stratigraphic analysis of these quartz arenites indicates that there are up to three 3^rd-order depositional sequences within a longer term 2^nd-order depositional sequence. The 3^rd-order sequences commonly are shallowing upward successions with distal shelf facies passing upward into proximal shelf facies. Correlation of the 3^rd-order sequences may provide a better understanding of the lateral variation and diachronous nature of depositional processes along this passive margin. Correlation of the detrital zircon signatures within the sequence stratigraphic framework will constrain provenance changes during the extensive Late Ordovician flooding of North America.