page_title: Geology of the Capitol Reef area
section_title: Morrison Formation (Jurassic)
hierarchical_section_title: Geology of the Capitol Reef area / Primary deposition of sediments / Morrison Formation (Jurassic)
context_page_description: The exposed geology of the Capitol Reef area presents a record of mostly Mesozoic-aged sedimentation in an area of North America in and around Capitol Reef National Park, on the Colorado Plateau in southeastern Utah.
  Nearly 10,000 feet of sedimentary strata are found in the Capitol Reef area, representing nearly 200 million years of geologic history of the south-central part of the U.S. state of Utah. These rocks range in age from Permian to Cretaceous Rock layers in the area reveal ancient climates as varied as rivers and swamps, Sahara-like deserts, and shallow ocean.
  The area's first known sediments were laid down as a shallow sea invaded the land in the Permian. At first sandstone was deposited but limestone followed as the sea deepened. After the sea retreated in the Triassic, streams deposited silt before the area was uplifted and underwent erosion. Conglomerate followed by logs, sand, mud and wind-transported volcanic ash were later added. Mid to Late Triassic time saw increasing aridity, during which vast amounts of sandstone were laid down along with some deposits from slow-moving streams.
context_section_description: Again above sea level, streams laid down mud and sand in their channels, on lakebeds, and in swampy plains during the Upper Jurassic. This became the Morrison Formation, which is locally divided into three members (from oldest to youngest);
  Tidwell Member,
  Salt Wash, and
  Brushy Basin.
  The 50 to 100 foot (15 to 30 m) thick Tidwell Member is locally difficult to recognize and therefore may or may not be in the area's rocks.  Easier to identify and study outcrops elsewhere in southeastern Utah indicate they were deposited in hypersaline lagoons.
  Clay, mud, silt, cross-bedded sand, and pebbles were later deposited by meandering streams and in flood plains, forming the locally 100 to 500 foot (30 to 150 m) thick Salt Wash Member. Claystone and mudstone beds of this member erode into gray slopes that can exhibit brown, red, yellow, and green colors. Salt Wash sandstone is moderately sorted and fine to medium grained while pebble conglomerate beds are made of chert with small amounts of silica-rich limestone. Both bed types erode into ledges and small cliffs. The Salt Wash was locally mined in the 1950s to extract uranium ore.
  The 200 to 350 foot (60 to 105 m) thick Brushy Basin Member is composed of claystone, mudstone, and siltstone with small amounts of conglomerate and sandstone. Clays in this member are rich in smectite and thus tend to swell when wet and dry to a crumbled surface that looks a bit like popcorn. Fossilized dinosaur bones are abundant in this member in several places located in Utah and western Colorado. The bones are usually scattered and are thus hard to identify but nearly complete skeletons have been found in lake floor and flood-plain clays. Good exposures of the Brushy Basin Member can be seen in the Bentonite Hills.
caption: Morrison Formation from near Notom-Bullfrog Road (USGS).
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