(in San Rafael Group)
Figure 1: Paleogeographic map of the Middle Jurassic, Page Sandstone, Carmel Formation, Entrada Sandstone, Curtis Formation, and Summerville Formation. (Blakey, 2008)
Upper Jurassic, 80 and 140 ma
Eolian, sabkha, and tidal flat. The Entrada Sandstone preserves terrestrial environments. Within the field trip area, the deposits generally indicate a high water table with some dunes (wet eolian) present. The bedding contains sandstone laminations, sand lenses with some lenses starved and encased in mud. This environment is broadly interpreted as a tidal regime (tidal flat) in this region.
In the Upper Jurassic, the supercontinent, Pangea, was beginning to break up with North American and Eurasia pulling apart from South America. Utah was closer to the equator with eastern Utah as a dry Sahara-like desert, with shallow seas that intermittently covered the area (Blakey 2008).
At around 170 Ma, the Goblin Valley State Park area was a wide tidal flat between the sea to the north and continental mountains and hills to the west. Tidal channels migrated across the tidal flats, routing flowing water to the open sea. Coastal sand dunes also covered parts of the tidal flats. Oscillatory tide motions were a dominant force in the deposition of this area. Silts, sands, and clays were primarily sourced from erosional debris shed from granitic highlands of Northwestern Utah and then were re-deposited in seas, shorelines, river channels, and playas.
Figure 2: These goblins form in flat-lying, fine-grained sandstone beds that are interbedded and underlain by shale and siltstone. Joints within the Entrada are conduits for erosion along zones of weakness, and subsequent spheroidal weathering.
Goblin Valley State Park is near the edge of a regional system of faults that cut across the San Rafael Swell (Fillmore 2000, Milligan 2003). Several sets of microfaults divide the Entrada Sandstone into yard sized rhombohedral blocks. The blocks exhibit reduced grain size, decreasing porosity and permeability within the fractures. In Goblin Valley only small-scale fractures with small offsets may be visible.
warm and arid
The dark reddish color of the Entrada Sandstones comes mainly from the mineral hematite (an iron oxide and principal ore of iron) staining the sandstone. This is the same formation that makes up the natural arches of Arches National Park in southeastern Utah. A synthesis of the Entrada Sandstone is given in Carr and Kocurek (1993).
Joint or fracture patterns in the Entrada Sandstone create initial weak zones that become enlarged over time. Joints intersections are susceptible to weathering because of increased surface to area volume ratio. These joints weather quickly, creating spherical-shaped goblins, from spheroidal weathering (Milligan 2003). Interbedded and underlying shale and siltstone beds are capped by the sandstone beds. The soft shale and siltstone beds help create the smooth shaped pedestals in Goblin Valley State Park.
Sites Best to See it:
Stop 1-2: Goblin Valley State Park
For a complete list of references please go to the References page.
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Disclaimer: The information is property of the University of Utah. Unless cited, images and files found on this site have been taken or created by the Geology and Geophysics Department at the University of Utah. Any use of these images should be cited appropriately. The stratigraphic column is from: Mathis, A. C. 2000. Capitol Reef National Park and Vicinity Geologic Road Logs, Utah, in: P.B. Anderson and D.A. Sprinkel (eds.) Geologic Road, Trail, and Lake Guides to Utah’s Parks and Monuments Utah Geological Association Publication 29. http://www.utahgeology.org/uga29Titles.htm
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