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SAN RAFAEL SWELL
A GEOLOGISTS PARIDISE

      One hundred and twelve miles south east of Salt Lake City as the crow flies and sixteen miles west of Green river, Utah. Is one of the best and most interesting sites in the world to view such geological processes as erosion, cross bedding, coal formation, deposition of ancient and modern sediment, anticlines and folds, ancient fluvial systems, ancient frozen eolian deserts known as Ergs, Dinosaur grave yards and other geological forces and events.

   Many people have traveled in or near the San Rafael swell such as the native Anasazi, Fremont, Ute and Sam Patch people. The Spanish from New Mexico and California traveled the old Spanish trail which was an old trade route north west of the Swell. The area was visited by Jedidiah Strong Smith the famous Explorer and Fur trapper in 1826 on his way to California. Smith called the area” The valley of the Colorado” because the area is drained by the Green and ultimately the Colorado River on its way to the Gulf of California. John C. Fremont also explored the area as a government surveyor and explorer.

   Later Butch Cassidy and his gang of out laws would travel the old dusty trails of the Swell. On their way to robbers roost to hide out after robbing the Pleasant Valley Coal Company of 8,000 dollars in payroll money on April 21, 1897 at Castle Gate, Utah. Archeologists are interested in the Swell to. For many petro glyphs are found in the rugged canyons and mazes of the San Rafael swell. Paleontologists have found many dinosaur bones in and around the Swell like the world famous Cleveland Lloyd dinosaur quarry at the North West end of the swell. Many dinosaur tracks and tracks of other ancient animals have been found also. Uranium was found by prospectors and miners during the cold war period and to the east of the Swell oil has been found in the brushy basin member of the Morrison formation near Cisco, Utah.

   You wonder why the San Rafael Swell, has not yet been made a National monument or Park. The San Rafael Swell or San Rafael Reef as it is some times called at is eastern edge is a large asymmetrical anticline; (large fold ) it is forty five miles wide and 82 miles long and forms a kidney bean shape.



   The fold on the east side which was formed when Precambrian rock was broken by faulting upwards and bending the Mesozoic rock is steep compared to the fold on the west side which is barely noticeable. The faulting occurred during the Laramide Orogeny (mountain building episode some sixty million years past. Almost all rocks exposed from the San Rafael swell Are of Mesozoic age with the exception of some Paleozoic Permian rocks at the center of the Swell. Erosion has played an important roll in exposing layers of rock and has peeled back the San Rafael swell for the geologists to study in great detail.



THE BREAK UP OF PANGEA IN THE EARLY TRAISSIC (245 TO 205 MILLION YEAR AGO)

At the beginning of the Triassic period the earth had one major continent called Pangaea. Some time in the early part of the Triassic period Pangaea began to break apart in to separate continents. North America began to drift to the west, as this happened a subduction zone began to form on the western fringes of the North American continent. As this happened the western fringes of North America was deformed and subsided.

  This set the stage for the deposition of the Moenkopi formation and the chinle formation. The moenkopi formation contains ripple marks and mud cracks which are commonly found in tidal flats near the sea and fluvial flood plains adjacent to the tidal flats. During this time the rivers flowed for the most part North West. In the San Rafael area the Moenkopi is divided into the moody canyon member the torry member the Sinbad limestone member and the black dragon member. An ancient sea to the west flooded the area depositing the Sinbad limestone and as the sea retreated west the mud flats left by the sea dried out in the sun forming mud cracks 



 Before the chinle formation was deposited the area was eroded for some time creating what geologists call an unconformity between the Moenkopi and Chinle formation. The rivers continued to flow in a North West direction flooding the flat plain with sediment before they entered the sea. The Chinle formation is famous for the amount of petrified wood found within it. Near the town of Boulder and approximately sixty miles south east of the San Rafael swell, Utah is found the Wolverine Petrified Forest. The species of trees found in the formation are araucarioxylon arizonicum and woodworthia arizonica. A second member of the Chinle formation The Shinarump conglomerate was deposited by huge meandering rivers.

THE DAWN OF THE JURASSIC PERIOD (205 to 145 million years ago)

   The Jurassic period on the Colorado plateau literally blew in on a gust of wind when the first of the great Jurassic deserts or ergs was laid down. This was the Wingate sandstone which forms many of the arches and natural stone monuments in the national parks which dot the Colorado plateau. The Wingate formation is one of three formations that make up the glen canyon group. Prominent cliffs form in the Wingate sandstone.

  Before the kayenta was deposited a long period of erosion occurred creating an unconformable surface on the Wingate sandstone. The slow rivers of the kayenta formation flowed west to south west when wet condition allowed but dried up due to semi arid seasons at times.

  The Navajo Sand stone which some geologist believe to be the largest fossilized dune field in the world. The deposit is eolian in origin (wind blown) this is also called an Erg because of the massive ancient and deep dunes found in this formation. The Sahara of to day is a modern example of an Erg. Rainbow Bridge, Zion National park, and the fantastic scenery found in the San Rafael swell are all examples of the Navajo Sandstone. Dinosaur tracks and petrified wood have been found in the Navajo sandstone. Cross bedding in the ancient dunes can be seen in the following picture.



   The conditions under which the Carmel formation was deposited brought desert regions of the Navajo sandstone to a close and a sea began to fill the regions of central and North West Utah. Carmel formation seen below



The Entrada sandstone of eolian origin.

   The next youngest formation the Summerville formation was deposited on and near the shore next to the Curtis Sea here sabkha conditions existed along this sea.  The Summerville formation is see in the following picture. It is the group of rocks  and cliffs just above the highway. The smooth banded  formation above that is the Morrison formation and at the top is the Buckhorn conglomerate.

 

   The Morrison formation famous for its dinosaur fauna (the real Jurassic park) which consists of the Brushy basin member (see fig blank) the Salt wash member and the Tidwell member. The Morrison formation was deposited in a large basin (148 million years ago) that drained to the north east. Many large winding rivers carried large loads of sediment to the east and north east. At times the rivers were clogged and dammed by their own sediment and broke threw the natural levees of the river out on to the plains of the Morrison basin creating a new channel for the river and dumping its load of sediment at the same time. It was by this manor that the Morrison basin was filled to over flowing. Large strato volcanoes created by the Ferrolon plate being pulled down under the continent to the west blanketed the Morrison basin with volcanic ash. Conglomerate, sandstone and mud are prevalent in the Morrison formation Seventy six miles south west of the San Rafael swell and near the town of Escalante, Utah petrified wood is found in the upper brushy basin member at Escalante state park and Dinosaur remains are found at the Cleveland Lloyd dinosaur quarry forty miles to the north. Some of the Dinosaurs found in the Morrison formation are Allosaurus a meat eating predator when full grown was about forty feet long and found in great numbers at the Cleveland Lloyd dinosaur quarry and is Utah’s state fossil, and Brachiosaurs the largest known dinosaur from the Morrison.

THE CREATACEOUS PERIOD (145 to 65 million years ago)

   Six formations or rock units were laid down during the Cretaceous period. They are from the oldest to the youngest the Buckhorn conglomerate was deposited on top of the Morrison formation and is local to the area and its presence means up lift of the earth’s surface to the west and south of the Morrison formation at the time of deposition of the buckhorn conglomerate. The buck horn conglomerate is seen below.



   The Dakota formation was deposited along the edge of a sea during the Sevier orogeny, far to the west thrust faults were forming and creating mountainous areas in what is now western Utah causing much sediment including conglomerate to be deposited at the foot of the mountains.
Cedar Mountain formation was formed by medium to large rivers moving sediment to the northeast and depositing it on flat flood plains before the rivers met the sea. Dinosaurs like Iguanodon, Utah raptor, Gastonia; acrocanthosaurus have been found through out this formation and is very similar to the Morrison formation.
 
  The advanced of the Marine invasions forming the Tununk member the Mancos shale flooded the eastern half of Utah. Large amounts of sandstone known as the Ferron sandstone formed beaches and low lands between the mountains and the interior seaway vast tracks of swamps and low forest grew along the edge of the sea which after being buried by the sandstone turned into coal. See picture of Ferron Sandstone below with coal seam.



 Many dinosaur foot print casts made of coal have been found in this formation. The Blue gate shale member was deposited in the cretaceous interior seaway that extended east from central Utah across Colorado it to Kansas. Mosasaurs and large sharks thrived in this sea. All together tununk, ferron sandstone and the blue gate shale form the entire Mancos shale formation.