The Colorado Plateau An OUGS Severnside Field Trip 2015 The Colorado Plateau 2015 Introduction This was to be another Severnside trip, but one with a new organiser! Janet Hiscott and Tom Sharpe had devised the idea for this trip when talking about what he would do in his retirement from the Museum of Wales. The large group of 27 members that collected in Denver at the end of September 2015 were the beneficiaries of this chat. We were all eager to explore the Colorado Plateau with Janet and Tom to guide us. The Colorado Plateau is an area between the Rockies and the Basin and Range Province and extends through parts of 4 states, Utah, Colorado, New Mexico and Arizona. (Fig. 1) It contains some of the most famous and most visited National Parks of the USA and we were here for 17 days to be introduced to the geology, paleontology and human interaction with and exploitation of the region. The Rockies have been through several cycles of uplift and erosion since the Precambrian, the most recent period of uplift started 5Ma ago and the area has seen a change in elevation of 5000ft. There was associated volcanic activity, building volcanoes and causing a redistribution of metals by heated fluids. The latter having been exploited by man in the Colorado Mineral Belt. The debris eroded from the Rockies during these successive periods of upift was deposited in the plains surrounding them. These sediments, and marine deposits from a period of inundation by an inland sea, form the level beds that comprise the Colorado plateau. During this deposition the local flora and fauna left its mark with footprints frozen in soft lacustrine sediments, shoals of carcases washed against a riverbank, forests preserved on floodplains or buried to create coal and other plant life contributing to the production of oil and gas. It was during the last period of uplift that the rivers of the region cut down through the layers in some dramatic ways creating the hearts of the parks that we were to visit. Fig..1 Outline map of field trip route. (Tom Sharpe 2015) 2 The Colorado Plateau 2015 Day 2 Denver to Steamboat Springs The group had gathered in an airport hotel in Denver on the evening of Monday, 28th September and we set out in bright sunshine first thing on Tuesday morning to cross the Rockies. This day would be unique on our trip, not only for the altitude we would be at but also the type and nature of the rocks we would see. Our route took us north and then west on the US34, the Trail Ridge Road, following a thousand year old Indian trail. The road rises to 12,183 ft and traverses climate zones from prairie grassland to alpine tundra. It is cooler and windier at the top than in Denver! We had a brief stop at Estes Park (elevation 7522 feet) to shop for supplies before continuing the climb, seeing the hotel that featured in The Shining in the distance. The Rocky Mountains of Colorado are part of a huge mountain chain, the Western Cordillera, which extends from Alaska to Patagonia, close and parallel to the Pacific coast. The Colorado Plateau, where we would spend most of the trip, lies between them in the east and the Basin and Ranges in the west. The Rockies form the continental divide of North America. The Front Range, formed of Precambrian rocks, rises up west of Denver with steeply dipping Palaeozoic and Mesozoic strata exposed as ‘hogbacks’ or ridges along the eastern flanks. The Front Range rises to peaks of 14,000 feet and is a dramatic boundary. The Precambrian rocks include gneisses of about 1750 Ma that are intruded by large granite masses of about 1450 Ma. By the end of the Precambrian, the overlying rocks had eroded down to the top of these granite batholiths. The area experienced uplift in the Carboniferous to form the Ancestral Rockies which were themselves subsequently extensively eroded. The Laramide Orogeny, in the Cretaceous, faulted and uplifted the area and erosion then reduced the mountains to 3,000 feet above the plains. Continuing uplift in the last 5 Ma has raised the mountains about 5,000 feet to their present altitudes - a complicated story! We spent the day in the Rocky Mountain National Park working our way up to the Visitors Centre which lies at 11,796 feet, the highest Visitors Centre in the National Park Organisation. En route we stopped at Many Parks Curve (elevation 9,620 ft) where there were views of the surrounding mountain peaks. Through binoculars it was possible to make out the schistic nature of many of the peaks but there were also granite domes visibly cut by pegmatites. In the valley below lateral moraines could be identified (Fig. 2.1) together with an alluvial fan formed when a dam burst in the valley above. Fig. 2.1 View from Many Parks Curve of McGregor Mountain and the valley of the Fall River between two lateral moraines. Driving on, we stopped at the Forest Canyon Overlook (elevation 11,716 feet) to follow 2 short trails. Visitors are instructed not to leave the marked trails here in order to protect the sparse vegetation which is low lying and fragile (Fig. 2.2). Whilst hardy enough to withstand the intense cold they also have to contend with as little as 2 inches of moisture a year in this alpine desert. The tundra plants are Fig. 2.2 The alpine environment needs low lying, hugging the ground and their roots penetrate 4-5 protection from the numerous feet of visitors to feet into the rocky ground. Forest Canyon Overlook. An early autumn shower had blanketed the tundra. 3 The Colorado Plateau 2015 Evidence of glaciation was clearly visible in the surrounding peaks, with ice shattered, brecciated debris filling the ice carved hollows in the mountains. The weather changed in the afternoon and we watched thunder and lightening storms pass over, with dramatic changes in light (Fig. 2.3). Some of the accessible tors allowed us to examine the interaction between the granite and the schist it had intruded, now deeply eroded into mushroom shapes because the granite has eroded more rapidly (Fig. 2.4). Fig. 2.3 A dramatic view of The Rockies in rapidly Fig. 2.4 Differential erosion of granite surmounted changing weather. by schist has produced these mushroom shaped formations. We enjoyed a brief stop at the visitors centre. This building is shut for 7 months of the year and is surrounded by snow poles that extend high above the roof level. It is dependant on its own generator for power. After leaving the visitors centre, we continued to drive through the park seeing the Lava cliffs and the Never Summer Mountains. Unfortunately, during the descent one of our group became ill and needed to be whisked away by ambulance to hospital to be assessed. Fortunately, with no other obvious cause for his illness, the medics diagnosed altitude sickness and he was able to rejoin us the following morning. We reached Steamboat Springs slightly later than anticipated. Report by Janet Hiscott Photos by Averil Leaver 4 The Colorado Plateau 2015 Day 3 Steamboat Springs to Vernal, Utah Steamboat Springs Once 25 of us had decided upon our enormous American lunch sandwiches from a frightening selection of combinations, courtesy of the Deli in Steamboat Springs, we visited Black Sulphur Spring, one of the geothermal springs on the banks of the River Yampa, after which the settlement was named. Apparently, the noise of the hot springs bubbling originally sounded like a steamboat, but quietened once a railroad cut was made nearby in 1908. The hot waters emerge at temperatures of 80-100°F and deposits of travertine, CaCO3, were seen coating the rocks where the spring waters emerged. The heated waters emerge via a fault line between Precambrian and sedimentary Mesozoic rocks, some of which were observed dipping at 60° near the springs and the fault line. The sedimentary rocks, of mainly tan coloured sandstones and shales, formed part of the Mesaverde sequences, deposited along the coast of the Western Interior Seaway, in the late Cretaceous Period. The sulphurous odour emanating from the springs did not diminish our appetites for giant sandwiches later in the day! (Fig. 3.1) Many of us identified with and applauded Tom’s use of the Fahrenheit scale for high temperatures and the Celsius scale for cooler temperatures. We followed the Yampa River, downstream and westwards, travelling by bus on Cretaceous Mancos Shale and overlying Mesaverde sandstones and coals, seeing much evidence of coal powered electricity generation. The tertiary Uinta basin formed ideal circumstances for the accumulation of oil and gas reserves. Evidence of oil and gas extraction and previous uranium mining (Uranium from interbedded tertiary volcanic ash sequences) were seen from the coach. Onwards and westwards on the bus, on very bumpy road surfaces, which Tom explained as relating to the underlying Mancos Shale. This formation contains bentonite clay rich layers, derived from weathered volcanic ashes, which are especially susceptible to expansion and contraction under varying weather conditions. Picnic lunch was taken outside the Colorado Welcome Centre, at the town of Dinosaur, which indicated what was in store for us later. (Fig. 3.2) Fig. 3.1 Black Sulphur Spring, Steamboat Springs Fig. 3.2 Road sign, Dinosaur, Colorado Dinosaur National Monument Permian to Cretaceous strata are upturned against the Uinta uplift at the northern edge of the Colorado Plateau, exposing a remarkable fossiliferous bed or lagerstätten of the uppermost Jurassic sediments, the Morrison Formation.