Wednesday, January 16, 2013

AZGS publishes new geologic map of the Petrified Forest National Park


Each year 600,000 people visit the variegated badlands and fallen forests of petrified wood at Petrified Forest National Park. A new geologic map and accompanying 18-page report, Geologic Map of Petrified Forest National Park, Arizona, display the distribution and nature of fossil-bearing rocks and sediments within the pre-expansion Petrified Forest National Park boundaries [right]. (In 2004, the U.S. Congress expanded the park, but less than 1/3rd of the expanded lands are controlled by the park.)

Park paleontologist and map co-author Bill Parker describes more about the mapping effort in a post on his Chinleana blog.  Bill notes that this is the first published geologic map of the park.





The 1:50,000-scale map, where 1 inch ~ 4160 feet or 0.8 miles, is a joint effort of research teams from the U.S. National Park Service (Jeff Martz and William Parker) and Northern Arizona University (Lisa Skinner, Jason Raucci, Paul Umhoefer and Ron Blakey). The report includes 22 photographs of major rock units, a stratigraphic column, and cross-sections showing how the landscape has evolved over the past 65 million years.

Petrified Forest National Park (PEFO) is located in the Little Colorado River Valley, 25 miles east of Holbrook, Arizona. Widespread exposures of sedimentary rocks – shales, siltstones, and sandstones – of the 225 to 207 million years ago (Ma) Chinle Formation dominate the park landscape.

Rocks of the Chinle Formation represent deposits of river systems originating in what is now western Texas and fed by tributary streams from highlands to the south and north of PEFO. The Chinle Formation preserves a suite of lowland terrestrial environments that includes river channels, floodplains, swamps, and small lakes operating in a strongly seasonal subtropical climate.

Locally, rocks of the Miocene-age Bidahochi Formation (8 to 4 Ma) cap the Petrified Forest and Owl Rock Members of the Chinle Formation. Sediments of the Bidahochi Formation were deposited in ancient Lake Hopi, which persisted from about 16 Ma to 4 Ma. To the north, basaltic rocks of the Hopi Buttes volcanic field, emplaced between 8.4 – 4.5 Ma, create maar volcanoes, diatremes and lava flows.

Erosion over the past several million years has scraped off most of the Bidahochi rocks, limiting outcrops to the northern reaches of the Park.

Fossil Organisms of Petrified National Park. Two-hundred and fifteen million years ago, dinosaurs tracked the swampy lowlands and forests of what is now PEFO. As noted in the Martz et al. report, “sandstones of the Sonsela Member are the primary source of petrified wood within PEFO. Wood traditionally assigned to the conifer genus Araucaryoxylon is most commonly found in the Jasper Forest bed, Rainbow Forest Bed, and Black Forest Bed , and the conifer genera Schilderia and Woodworthia are also known from the Black Forest Bed …”.   The spectacular “forests” found throughout the park, as well as the spectacular multi-colored trees at Jasper Forest, Crystal Forest, and Rainbow Forest lie within the Jasper Forest and Rainbow Forest Beds. 

Among the many vertebrate fossils found in PEFO are bony fish and sharks, therapsids, archosauromorphs and pseudosuchians (crocodylian-line archosaurs) and dinosauromorphs. Invertebrate macrofossils include locally abundant bivalves, gastropods, freshwater crustaceans and trace fossils of a wide variety of insects and other small arthropods.

The map and report are available as free, downloadable PDFs from the Arizona Geological Survey’s Document Repository. Printed copies can be purchased from the Arizona Experience Store, 416 W. Congress, Tucson, AZ 85701; email: store@azgs.az.gov, phone: 520.770.3500.

Background
Petrified Forest National Park covers ~146 square miles in Navajo and Apache Counties. Park headquarters is about 25 miles east of Holbrook, Arizona. Petrified wood logs and dinosaur fossils weathering out of brightly colored shale and variegated, fine-grained sandstones, evinces a subtropical environment of slow-moving, meandering rivers and streams making the Park one of the premiere sites in the world for understanding life during the Triassic Period.

Activities for park visitors include sightseeing, hiking, photography and backpacking. Average elevation is just over 5,000 feet and temperatures vary seasonally from summer highs of 100 °F to below freezing in winter. Park landscape ranges from flat-topped mesas rising from a broad plain to badlands topography.

Citation
Martz, J.W., Parker, W.G., Skinner, L. and Raucci, J.J., Umhoefer,P. and Blakey, R.C., 2012, Geologic Map of Petrified Forest National Park, Arizona. Arizona Geological Survey Contributed Map CR-12-A, 1 map sheet, 1:50,000 map scale, 18 p.

[excerpted in part from the AZGS news release]

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