Colorado and its turbulent past: Prologue

This prologue introduces a series of posts that will hopefully capture the look and feel of Colorado (hereinafter CO) from the perspective of an off road enthusiast. I am joining some fellow off roaders to traverse CO from east to west along the Trans America Trail (TAT) as modified for four-wheel-drive vehicles. We are picking up the TAT from MacKenzie CO rather from the SE corner of the state because of time constraints but I hope this post will introduce the parts we miss on our trek. Here is a track of our path across the Rockies overlain on a Google Earth image.

Our starting point at MacKenzie CO is indicated by the white square. I did drive along I25 (US87 on the image)east of the Sangre de Cristo Mtns and I will integrate this as best I can in this Prologue. We won’t be missing too much, however, because we will be going through the San Juan Mtns., which are also a major part of the geologic history of CO.

My primary source for the geology of CO is the excellent book by Halka Chronic and Felice Williams, Roadside Geology of Colorado. I will also use general sources that are appropriate. I am relying on geologic maps from the USGS. I will also use the Colorado Geologic Survey’s available data where I can, including maps.

This voyage didn’t start as far east as previous posts because I have moved to Baton Rouge, only about a mile from the Mississippi River. The first part of the journey in time and space is a retread of a previous trip, as I discussed in a previous post. It begins by crossing the Mississippi River and encountering multiple shorelines as sea level has changed in the last 2.5 my. This journey takes us though the Eocene (56-34 my) sediments of NE Texas as we approach Dallas on I20. As I travelled west along I20, the marine sedimentary rocks; which varied between sands, mud, and limestone (deep water); become older, finally reaching early Cretaceous (~120 my) in Fort Worth.

After heading NW before daylight, I crossed over the angular unconformity between the early Cretaceous sediments and mostly Permian (300-250 mya)sedimentary rocks along US 287 between Dallas and Amarillo (shown by the white line in the map below).

Note how the contour of the Cretaceous (140-65 mya) sediments near Dallas (brown color) cuts across the contours of the Permian sediments to the NW, which are shades of blue. This gap between Permian and Cretaceous sediments represents erosion of this area during an interval that includes the Ouachita Orogeny (~318-271 mya) of the Arkansas, Texas, and Oklahoma, the Marathon Uplift of West Texas, and probably the Llano Uplift of Central Texas. The Permian sediments include most lithologies, from sand and shale to evaporates and carbonates. This area was undergoing rapid changes in the shallow water environments during this time, ultimately leading to uplift and erosion.

As I travelled NW from Amarillo, I saw some Tertiary sediments that formed a cap rock, which formed small ledges where recent erosion had cut into this surface. Tertiary basalt flows started capping older sediments near the NM border as I entered a volcanic field that is contemporary with the Rio Grande Rift over the last 70 my.

My route took me across the NE corner of NM and through this volcanic field. There were several cinder cones and many basalt flows capping Paleozoic sedimentary rocks in this area.

This cinder cone was one of several spaced several kilometers apart. These were isolated whereas a much larger volcano only a km or so away had flows originating from near its base.

The basalt creates resistant surfaces and mesas form. The underlying rock is Mesozoic sandstones. These mesas only covered several miles along I25 as I headed north so this is not a giant field but probably an outlier from the much more extensive volcanics to the SW in NM and west in CO.

The subjacent Cretaceous sandstones are thin to massive bedded with cross bedding visible from the highway in road cuts. It also contains coal seams, which were common from this geologic period. These are still mined and train loads were headed south.

I drove north up I25 on top of the Cretaceous Pierre Shale, which forms most of the Great Plains. This mostly mud rock was deposited by the Western Interior Seaway that covered the middle of N. America. Thick sections of the slightly older Niobrara Limestone were visible east of the interstate whereas the Cretaceous sandstone was dominant until Colorado Springs, with granite plutonic rocks forming the back bone of the Spanish Peaks and the Front Range in the north, but that is another post.