Triassic Forests - A tour of the Colorado Plateau Part 2

In Part 1 of our tour of the Colorado Plateau, I talked about the 190 to 180 million year old Navajo Sandstone and the spectacular landscapes it produces. When the Navajo Sandstone was deposited, the southwest USA was a massive desert. Part 2 of our tour takes us further back in time to the Late Triassic (205 to 227 million years ago) when the southwest USA looked completely different (1,2). It was during this time that the huge trees exposed in Petrified Forest National Park grew and flourished, and the park is definitely worth visiting!

Petrified Forest National Park is east of Flagstaff and borders the Navajo Nation Reservation Land. Three sites on the Colorado Plateau with spectacular views of the Navajo Sandstone are also shown in green and check out the post of them as well. All are a must see!

About 218 million years ago, Arizona and much of the southwest USA was a low-lying area dominated by swamps, rivers, and floodplains and surrounded by mountains (1). The area probably looked a bit like the Lower Mainland and eastern Vancouver Island with rivers draining to a nearby ocean and large trees dominating the landscape. The Abbotsford area is a pretty good analog for what it might have looked like but instead of farmland around Abbotsford imagine the area covered by swamps and forests of large trees.

Examples of what Arizona would have looked like 220 million years ago. Top left: Meandering rivers, oxbow lakes and ponds on the floodplain in Alaska, USA (source: 3). Top right: Swampy area in Strathcona Provincial Park. Bottom left: Burnaby Lake, Burnaby, B.C. Bottom right: Burrard Inlet, Port Moody, B.C. In the Triassic it probably would have been a small dinosaur rather than a Great Blue Heron!

Trees would occasionally fall into river channels and be covered by silt and mud (4). Being buried in mud prevents decay, and slowly over time silica in groundwater replaced the organic material in the wood producing perfect quartz casts of the trees (4). Quartz is normally clear or white, and the rainbow colours that make the petrified forest famous come from iron and manganese oxides preserved in the quartz. Iron oxides give the trees red, yellow, orange and purple colours, while manganese oxides give it black and grey colours (4).

Top and middle left: silicified logs showing their characteristic rainbow colour. Bottom: The trees in Petrified Forest National Park are huge!

It is hard to resist picking up all the small pieces of silicified wood lying around, but don't give in to the temptation as the fines are hefty and come with jail time! You can buy pieces of petrified wood in literally every gas station or visitor centre near the park. The best of place for pieces, however, is Jim Gray's Petrified Wood Company in Holbrook, Arizona. They apparently own 3000 acres of land next to the park and sell stumps, logs, polished slabs, and plenty more. It is a pretty amazing place and give yourself at least an hour to wander round!

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Info Sources:

1) Woody, D. T., 2006, Revised stratigraphy of the Lower Chinle Formation (Upper Triassic) of the Petrified Forest National Park, Arizona, in Parker, W. G., Ash, S. R., and Irmis, R. B., eds., A Century of Research at Petrified Forest National Park, Volume Bulletin No. 62, Museum of Northern Arizona, p. 17-45.

2) Petrified Forest National Park | nps.gov

3) Travis | Flickr.com

4) Petrified Forest National Park Interpretive sign