AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
2nd- and 3rd-Order Sequence Stratigraphy, Depositional Environments, Provenance, and Diagenesis of the Supermature Eureka Quartzite and Its Equivalents, Western North America
(1) Geology and Geophysics, Texas A&M University, College Station, TX.
The Middle-Late Ordovician Eureka Quartzite and its lateral equivalents of western North America are a diachronous succession of supermature quartz arenite. These units, up to a few hundred meters thick, are unique to this Lower Paleozoic margin that is otherwise formed shallow-water carbonate. These quartz arenites contain little interbedded shale and their depositional environments ranged from eolian to intertidal to shallow subtidal shelf. We are using detrital zircon geochronology to determine provenance of these quartz arenites. So far the majority of the grains 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 not yet known.
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 supermature quartz arenites indicates that there are up to three 3rd-order depositional sequences contained within a longer term 2nd-order depositional sequence. The 3rd-order sequences commonly are shallowing upward successions with distal shelf facies passing upward into proximal shelf facies. Lowstand fans, if they formed on this margin occur to the west and were not studied by our group. Correlation of the 3rd-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 flooding of North America.
The Middle-Late Ordovician quartz arenites of the western North American margin commonly are well-cemented, thus their historical classification as quartzites. The abundance of silica cementation in these units locally obscures all depositional fabrics and appears to be more common near younger volcanics or batholiths. Comparative diagenetic study of these units in western North America should clarify why they are such poor reservoirs whereas coeval Middle-Late Ordovician quartzites of the US Midcontinent (St. Peter Sandstone, Simpson Group Sandstones) are good reservoirs.