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The Petrology of Mesoproterozoic Unkar Group Shales and Detrital Zircon Geochronology of Interbedded Sandstones, Grand Canyon: Grenvillian Influence on Sedimentation of Inboard Rodinia*

By

John D. Bloch 1, J. Michael Timmons 2, Laura J. Crossey 1, George E. Gehrels 3, and Karl E. Karlstrom 1

 

Search and Discovery Article #50010 (2005)

Posted August 20, (2005)

 

*Oral presentation at AAPG Annual Convention, Calgary, Alberta, June 19-22, 2005.

 

Click to view presentation in PDF format.

 

1University of New Mexico, Albuquerque, NM ([email protected])

2New Mexico Bureau of Geology and Mineral Resources, Socorro, NM

3University of Arizona, Tucson, AZ

 

Abstract 

Unkar Group (1.25-1.10 Ga) shale geochemistry and petrography coupled with detrital zircon geochronology of interbedded sandstones, identify specific source terranes for Unkar Group sediments. The dominant sediment source for most of the Unkar group is the southern Grenville Orogenic Front (GOF). The Hakatai Formation is composed dominantly of GOF volcanic detritus with some material derived from the Southern Granite Rhyolite Terrane (SGRT) and the Yavapai - Mazatzal crustal provinces (YMP). Unconformably overlying the Hakatai is the Shinumo Sandstone, which is composed primarily of locally-derived YMP and SGRT detritus with a minor GOF component. The uppermost Dox Formation, which conformably overlies the Shinumo, is largely derived from the GOF. These data indicate that a protracted period of orogenesis along the southern GOF resulted in uplift of the interior Rodinian platform which evolved from an epicontinental sea or marine embayment to a foreland with well-developed fluvial drainage.

 

Variable provenance of the Unkar Group, as revealed by detrital zircon data and mineralogy, occurs despite relatively homogeneous bulk chemical trends. It is concluded that Unkar Group shales are a well-mixed recipe of variable ingredients that subtley reveal the distinct nature of the components. This is attributed to a number of factors that include; rapid rates of uplift and erosion along the GOF, the similarity in composition of a large number and volume of crustal domains (YMP and SGRT), a significant component of intraformational recycling, and the distal location of the Unkar Basin, ~800 km, from the dominant sediment source.

 

 

Location map

 

 

Stratigraphic column

 

Rodinian paleogeography (~1.10 Ga)

 

 

Conclusions 

  • Zircon geochronology of sandstones links Unkar sediment to all tectonic provinces of SW Laurentia.

  • Bulk chemical trends indicate moderate weathering for most Unkar shales.

  • Major, trace, and REE element abundances are consistent with sediment derivation from a mixture of granitic to mafic sources. Granite is the dominant component.