Page Sandstone & Templecap Fm (Geology of Utah’s Grand Staircase)

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Geologic Cross section of the layers of The Grand Staircase from Bryce through Zion to the Grand Canyon National Parks. Note the Templecap at the top of the towers of Zion Canyon.

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Exposure:

The Temple Cap is exposed exclusively in the Southwest corner of Utah (atop the spires of Zion National Park), whereas the Page Sandstone stretches all the way into the center of the state. Both share similiar ages, but the Page is thicker and was formed by a separate erg, with far less marine influence. (ie. the slumping visible in the temple cap as the Sundance seaway covered the dune systems)

Age:  Middle Jurassic

Depositional Environment: Eolian, Marginal marine

Paleogeography:

In the Middle Jurassic, Utah lay closer to the equator creating an arid, eolian environment (Blakey 2008).  Sedimentation was controlled by sea level, climate and tectonics. The Sundance sea entered the area from the north due to a global sea level rise. This was a time of subsidence most likely due to a sag next to west lying Nevadan highlands. The Page Sandstone is confined to the south central portion of Utah and north central regions of Arizona.  Narrowing of the seaway in northern Utah encroached and inundated onto the Page Sandstone erg to cease the eolian deposition.

Tectonics:

In the Middle Jurassic, parts of Utah were in a period of mountain-building phase.  Tectonic activity, like the Nevada Orogeny, was centered in the northeast portion of Nevada and volcanic highlands were located to the south and southwest. Folding and thrusting from the orogeny led to subsidence and a foreland basin.

Features:

The Page Formation was a large dune field (erg/eolian system) ranging from northern Arizona, in the Middle Jurassic, that extended northward across a band within Utah to Wyoming (Fillmore 2000). It was separated to the west by a tidal-flat/sabkha complex and then the Sundance seaway.  The Sundance seaway expanded and extended southward and eastward and as it retreated, the Page erg followed and expanded over the sabkha sediments.  Page Sandstone and the Carmel Formation comprise a mix of marine, sabkha, fluvial systems of the Carmel (marinal marine and marine) and eolian deposits of the Page Sandstone.

The Page Sandstone is subdivided into three members.  The lowermost member, the Harris Wash Tongue, is the thickest and is composed of trough cross-bedded sands grading up into planar-bedded sands.  This indicates a transition from a dune field to a sand sheet environment, which could indicate that the environment was becoming wetter through time.  The next member is the recessive, shaley Judd Hollow Tongue, which records a wet, muddy flood plain environment.  This would have been the wettest period during the deposition of the Page Sandstone and would have been marked by the presence of lakes, fluvial channels and an absence of sand dunes.  The top member is the Thousand Pockets Tongue, composed primarily of trough cross strata that are smaller in size than the lower Harris Wash Tongue.  This indicates a return to a drier environment with sand dunes, after the wet Judd Hollow interval.
This picture shows some soft sediment deformation in the east side of the former Fremont River channel.  In the right side of the picture the bedding is trough cross stratification, fairly typical to the Navajo Sandstone.  To the left side of the picture the strata are vertical, and in between the strata curve from vertical to pointing to the upper right.  The origin of this soft sediment deformation is unknown, though it has been speculated that it is a fluid escape structure.  Fluid escape structures do not usually leave behind vertically bedded strata, so the ultimate cause of the soft sediment is still a mystery. 

Description:

The Page Sandstone is the deposit of the Page coastal dune field during the Jurassic. It overlies and in is separated from the Navajo Sandstone by the continental-scale J2 unconformity. During the Jurassic, the adjacent Carmel Sea experienced several fluctuations in sea level, affecting the position of the water table in the Page Erg. Highstands in the Carmel are associated with high watertable elevations in the Page dune field that are tied to development of polygonally-fractured sabkha surfaces, and the preservation of some portion of underlying aeolian cross-strata. The packages of lowstand aeolian strata, bounded by sabkha deposits and/or polygonally-fractured surfaces represent parasequences within the overall transgressive succession of the Page Sandstone.


The occurrence of an extensive wet sabkha surface prohibited dune field development and this lack of wind-blown sand lead to surface deflation down to elevation of the water table. As such, each parasequence bounding surface represents a reset of the dune field pattern, and provides an opportunity to correlate stratigraphic architecture to paleo-environmental conditions

Modern Analog to Utah’s Middle Jurassic

Many modern analogs have been proposed for the desert ergs, sabkha’s and limestone’s of Utah’s Middle Jurassic, but we favor Africa & the Turkmenistan/Caspian Basin as it includes all the depositional environments found for this geologic period.

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Paleogeography or Depiction of Utah during Middle Jurassic

Figure 1: Paleogeographic map of the Middle Jurassic, Page Sandstone, Carmel Formation, Entrada Sandstone, Curtis Formation, and Summerville Formation. (Blakey, 2008)

What is the Grand Staircase?

The Grand Staircase is a unique and extensive exposure of Earth’s history, showcasing over 200 million years of sedimentary rock layers. Geologists often liken these layers to a “book,” allowing for a detailed study of the Earth’s past, including changes in climate and environment.

The major sedimentary rock units exposed in the Grand Canyon range in age from 200 million to 600 million years and were deposited in warm shallow seas and near-shore environments. The nearly 40 identified rock layers of Grand Canyon form one of the most studied geologic columns in the world.

No photos No photos

Carmel Formation (Geology of Utah’s Grand Staircase)

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Geologic Cross section of the layers of The Grand Staircase from Bryce through Zion to the Grand Canyon National Parks. Note the Carmel at the top of the Zion NP sequence.

Explore unit thickness in All-in-One App

Exposure:

Within the Grand Staircase, the Curtis Formation is best exposed in east of Capital Reef National Park just south of Goblin Valley State Park.

Age:  Middle Jurassic, 160 Ma

Depositional Environment: Full marine, shallow marine to sabkha (and supratidal)

Paleogeography:

Frequent, but short-lived sea level fluctuations during the Middle to Late Jurassic caused periodically flooding from shallow extensions of the ocean. Flooding deposited gypsum, sand, and limey silt in depressed blocks of land that were bordered by parallel faults (grabens), and were periodically covered by sea water. Evaporites were deposited from repeated flooding during this time.

Tectonics:

Stable, some volcanism with subduction to the west.

Climate:

With paleolatitude range of 5 to 25 degrees north, the paleoclimate was both hot and arid (Kocurek & Dott, 1983).

Features:

The Carmel Formation is composed of 200 to 1,000 feet (60 to 300 m) of reddish-brown siltstone, mudstone and sandstone that alternates with whitish-gray gypsum and fossil-rich limestone in a banded pattern. Fossils include marine bivalves and ammonites. The Carmel formation contains massive limestone beds in varying shades of gray, but also contains brittle limestone beds that weather into hard angular chips. The limestone beds can also be sandy and some contain ripple marks. Various types of soft sediment deformation also characterize the Carmel Formation. Typical Carmel exposures occurs as a series of low cliffs and steep slopes. In some areas, the Carmel Formation forms a resistant cap and slows the erosion of the underlying Navajo Sandstone.

View of the Morrison Formation, the Summerville Formation, the Curtis Formation, and the Entrada Formation.

Description:

The Carmel Formation consists of up to 1,000 feet (300 m) of mudrock and sandstone interbedded with limestone and gypsum. It was laid down in a shallow marine to sabkha environment,[1] into which terrigenous sediment was periodically carried. This gives the formation considerable lithological complexity.[3] The formation is underlain by the Navajo Sandstone, with the regional J-2 unconformity separating the two formations, or by the Temple Cap Formation. Portions of the Carmel Formation grade laterally eastward into the Page Sandstone.[2] The Carmel Formation in turn is overlain by the Entrada Formation.[3]

In the type area of southern Utah, the Carmel Formation is divided into the Judd Hollow Member, a basal limestone member; the Crystal Creek Member, mostly mudstone and siltstone, which grades into the Page Sandstone to the east; the Paria River Member, which is also siltstone and mudstone but is separated from the Crystal Creek Member by gypsum beds; and the Winsor Member, which is mudstone, sandstone, and siltstone separated from the Paria River by a basal limestone. Further east, the limestone marker bed pinches out, and the Winsor Member and Paria River Member become indistinguishable and are informally termed the upper member. The upper member contains volcaniclastic beds of rhyolite originating in a volcanic arc just off the edge of the Colorado Plateau.[4]

The formation preserves a Jurassic hardground, rare for North America

Modern Analog to Utah’s Middle Jurassic

Many modern analogs have been proposed for the desert ergs, sabkha’s and limestone’s of Utah’s Middle Jurassic, but we favor Africa & the Turkmenistan/Caspian Basin as it includes all the depositional environments found for this geologic period.

.

Paleogeography or Depiction of Utah during Middle Jurassic

Figure 1: Paleogeographic map of the Middle Jurassic, Page Sandstone, Carmel Formation, Entrada Sandstone, Curtis Formation, and Summerville Formation. (Blakey, 2008)

What is the Grand Staircase?

The Grand Staircase is a unique and extensive exposure of Earth’s history, showcasing over 200 million years of sedimentary rock layers. Geologists often liken these layers to a “book,” allowing for a detailed study of the Earth’s past, including changes in climate and environment.

The major sedimentary rock units exposed in the Grand Canyon range in age from 200 million to 600 million years and were deposited in warm shallow seas and near-shore environments. The nearly 40 identified rock layers of Grand Canyon form one of the most studied geologic columns in the world.

No photos No photos

Entrada Formation (Geology of Utah’s Grand Staircase)

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Geologic Cross section of the layers of The Grand Staircase from Bryce through Zion to the Grand Canyon National Parks. Note the Entrada is absent in Zion but sits atop the Carmel Fm.

Explore unit thickness in All-in-One App

Exposure:

Without a doubt the most striking exposures of the Entrada Formation are in Arches National Park. Other great exposures exist in Canyonlands, Goblin Valley State Park and Lake Powell NRA.

Age:  Upper Jurassic, 80 and 140 ma

Depositional Environment: 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.

Paleogeography:

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.

Tectonics:

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.

Climate: warm and arid

Features:

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.

View of the Morrison Formation, the Summerville Formation, the Curtis Formation, and the Entrada Formation.

Description:

At its type section at Entrada Point, located in the northern part the San Rafael Swell in Emery County, Utah,[4] the Entrada consists of red silty sandstone and lesser interbedded mudstone and is a slope-forming formation. This part of the Entrada is sometimes described as the “earthy facies”.[5] Here the Entrada is overlain by the Curtis Formation, and overlies the Carmel Formation.[4] To the south and east, the Entrada transitions to cliff-forming red or white crossbedded sandstone, sometimes called the “slickrock facies”. This is actually more typical of the Entrada as a whole, and a principal reference section including both facies was designated by Peterson in 1988 at Pine Creek, 5 kilometers (3.1 mi) north of Escalante, Utah, in the Kaiparowits Basin.[5]

At the reference section in the Kaiparowitz Basin, the Entrada is 314 meters (1,030 ft) thick and is divided into three informal members. The lower member is 113 meters (371 ft) of orange-red silty sandstone, with occasional beds of red mudstone, corresponding to the earthy facies. The middle member is 132 meters (433 ft) of red to green mudstone interbedded with red to white sandstone. The upper member is 69 meters (226 ft) of crossbedded white sandstone.[6] The white color is attributed to bleaching by organic-rich fluids from overlying beds. The formation rests on the Carmel Formation and is overlain by the Morrison Formation.[7]

In the Curtis Mountains region of northeastern Arizona, the Entrada is overlain by the Wanakah Formation.[8]

In the San Juan Basin of northwestern New Mexico, the Entrada consists of upper and lower sandy members and a medial silty member, the Rehoboth Member. The Curtis Formation is sometimes absent and the Entrada then overlies Chinle Formation. It is overlain by the Todilto Formation. Southeast of Fort Defiance, Arizona, the lower sandy beds are assigned to the Iyanbito Member. Total thickness is up to 37 meters (121 ft).[8]

In the Slick Rock, Colorado area, the Entrada is divided into a “middle sandstone”, the Rehoboth Member, and the Slick Rock Member, in ascending order

Modern Analog to Utah’s Middle Jurassic

Many modern analogs have been proposed for the desert ergs, sabkha’s and limestone’s of Utah’s Middle Jurassic, but we favor Africa & the Turkmenistan/Caspian Basin as it includes all the depositional environments found for this geologic period.

.

Paleogeography or Depiction of Utah during Middle Jurassic

Figure 1: Paleogeographic map of the Middle Jurassic, Page Sandstone, Carmel Formation, Entrada Sandstone, Curtis Formation, and Summerville Formation. (Blakey, 2008)

What is the Grand Staircase?

The Grand Staircase is a unique and extensive exposure of Earth’s history, showcasing over 200 million years of sedimentary rock layers. Geologists often liken these layers to a “book,” allowing for a detailed study of the Earth’s past, including changes in climate and environment.

The major sedimentary rock units exposed in the Grand Canyon range in age from 200 million to 600 million years and were deposited in warm shallow seas and near-shore environments. The nearly 40 identified rock layers of Grand Canyon form one of the most studied geologic columns in the world.

No photos No photos

Curtis Formation (Geology of Utah’s Grand Staircase)

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Geologic Cross section of the layers of The Grand Staircase from Bryce through Zion to the Grand Canyon National Parks.

Explore unit thickness in All-in-One App

Exposure:

Within the Grand Staircase, the Curtis Formation is best exposed in east of Capital Reef National Park just south of Goblin Valley State Park.

Age:  Deposited from early to late Oxfordian time (161 – 155 Ma) (Wilcox, 2007).

Depositional Environment:

Marine and marginal-marine tidal flat. Comprises one unconformably-bound, transgressive-regressive (T-R) sequence (Wilcox, 2007).

Paleogeography:

A major transgression of the Late Jurassic seaway drowned the eolian sand sea of the Entrada Formation.  Subsequent regressive paleoflow was to the northeast into the Late Jurassic interior seaway (Kocurek & Dott, 1983).

Tectonics:

Deposition was in a retroarc to craton-margin basin as the region drifted north (Kocurek & Dott, 1983). 

Climate:

With paleolatitude range of 5 to 25 degrees north, the paleoclimate was both hot and arid (Kocurek & Dott, 1983).

Features:

The Curtis Formation shows a variety of nearshore sedimentary structures (e.g. horizontal bedding -> beach, rhythmites and sigmoidal bundles -> tidal).  The Jurassic “J3” unconformity, a regional surface of erosion atop the Entrada Formation, marks the basal bounding surface of the Curtis-Summerville T-R sequence.  A thin, lower Curtis transgressive systems tract is the finest grained facies of the Curtis Formation topped by a maximum flooding surface.  Thick middle and upper Curtis deposits represent a highstand systems tract reflected in upward coarsening cycles from marine shelf to tidal channels to shoreface envirionments.  The upper sequence is tidal flat, reddish-brown mudstones and evaporites of the Summerville Formation topped by the “J5” unconformity, the upper sequence bounding surface and the contact with overlying Morrison Formation (Wilcox, 2007).

View of the Morrison Formation, the Summerville Formation, the Curtis Formation, and the Entrada Formation.

Description:

The Curtis Formation is composed of shallow marine sandstone, with thin beds of mudstone and minor limestone and gypsum. The sandstone is grayish-green in color and flat bedded or cross bedded. The presence of glauconite and marine invertebrate fossils indicates it was laid down in a shallow marine environment that became hypersaline towards the end of deposition. It represents a high stand of the Sundance Sea in the Callovian

The formation was first described by Gilluly and Reeside in 1928 and named for exposures in the northeast San Rafael Reef at Curtis Point (39.126665°N 110.447615°W). Pipiringos and Imlay reassigned the Curtis as a member of the Stump Formation in 1979,[2] but this was rejected by Peterson in 1988.

Modern Analog to Utah’s Middle Jurassic

Many modern analogs have been proposed for the desert ergs, sabkha’s and limestone’s of Utah’s Middle Jurassic, but we favor Africa & the Turkmenistan/Caspian Basin as it includes all the depositional environments found for this geologic period.

.

Paleogeography or Depiction of Utah during Middle Jurassic

Figure 1: Paleogeographic map of the Middle Jurassic, Page Sandstone, Carmel Formation, Entrada Sandstone, Curtis Formation, and Summerville Formation. (Blakey, 2008)

What is the Grand Staircase?

The Grand Staircase is a unique and extensive exposure of Earth’s history, showcasing over 200 million years of sedimentary rock layers. Geologists often liken these layers to a “book,” allowing for a detailed study of the Earth’s past, including changes in climate and environment.

The major sedimentary rock units exposed in the Grand Canyon range in age from 200 million to 600 million years and were deposited in warm shallow seas and near-shore environments. The nearly 40 identified rock layers of Grand Canyon form one of the most studied geologic columns in the world.

No photos No photos

Summerville Formation (Geology of Utah’s Grand Staircase)

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Geologic Cross section of the layers of The Grand Staircase from Bryce through Zion to the Grand Canyon National Parks.

Explore unit thickness in All-in-One App

Exposure:

Within the Grand Staircase, the Summerville Formation is best exposed in east of Capital Reef National Park just south of Goblin Valley State Park.

Age:  Deposited Middle Jurassic, early to late Oxfordian time (161 – 155 Ma) (Wilcox, 2007).

Depositional Environment:

Marginal marine and sabkha/tidal. One unconformably-bound, transgressive-regressive (T-R) sequence, marine (saline lacustrine?) and marginal-marine (Wilcox, 2007).

Paleogeography:

A major transgression of the Late Jurassic seaway drowned the eolian sand sea of the Entrada Sandstone.  Subsequent regressive paleoflow was to the northeast into the Late Jurassic interior seaway (Kocurek & Dott, 1983, Blakey 2008).

Tectonics:

Deposition was in a retroarc to craton-margin basin as the region drifted north (Kocurek & Dott, 1983). 

Climate:

Hot & Arid. Kocurek & Dott, 1983 suggest paleolatitude range of 5 to 25 degrees north. I disagree and suggest 25-40 degrees north within a stark rain shadow of the ancient Navadaplano, similar to current Mapmi basin in Mexico.

Features:

The Summerville is noted for its thin red beds of rippled sandstones and mud cracks, overprinted with secondary gypsum veins. The Jurassic “J3” unconformity, a regional surface of erosion atop the Entrada Sandstone, marks the basal bounding surface of the Curtis-Summerville T-R sequence.  A thin, lower Curtis transgressive systems tract is the finest grained facies of the Curtis Formation topped by a maximum flooding surface.  Thick middle and upper Curtis represents a highstand systems tract reflected in upward coarsening cycles from marine shelf to tidal channels to shoreface.  The upper sequence is tidal flat, reddish-brown mudstones and evaporites of the Summerville Fm topped by the “J5” unconformity, the upper sequence bounding surface and the contact with overlying Morrison Formation (Wilcox, 2007).

View of the Morrison Formation, the Summerville Formation, the Curtis Formation, and the Entrada Formation.

Description:

The Summerville formation consists of up to 100 meters (330 ft) of red mudstone, with thin interbeds of green and red sandstone. The lower portion of the formation shows polygonal desiccation cracks and localized salt-hopper casts while the upper portion contains considerable gypsum, consistent with deposition in a sabkha on the margin of the Sundance Sea.It is exposed in the San Rafael Reef, the Waterpocket Fold, in the Henry Mountains, with additional exposures scattered across the region from the San Rafael Reef to the Paradox Basin, and in north-central New Mexico. The thin bedding is characteristic throughout the formation, but gypsum is not found in the San Juan Basin and some conglomerate is found on the south and southwestern margins of the formation. The correlation of late Jurassic beds in northwestern New Mexico with the Summerville Formation in Utah has been questioned, and it has been suggested that they be assigned to the Beclabito Formation instead.

The Summerville Formation rests conformably on the underlying Curtis Formation (Utah and western Colorado) or Todilto Formation (southwest Colorado and New Mexico) but is separated from the overlying Morrison Formation by the regional J5 unconformity.[1] It thins significantly in the Moab-La Sal area, the likely area of the divide between the marine Curtis basin to the northwest and the salina lake Todilto basin to the southeast. Here the formation is just 1.2 meters (3.9 feet) thick and rests directly on Entrada Sandstone. In many locations the Summerville is separated from the Morrison by eolian sandstones, such as the Bluff Sandstone, variously assigned to the Morrison Formation or the San Rafael Group. The Morrison Formation represents a return to more humid conditions with increased clastic input.

Modern Analog to Utah’s Middle Jurassic

Many modern analogs have been proposed for the desert ergs, sabkha’s and limestone’s of Utah’s Middle Jurassic, but we favor Africa & the Turkmenistan/Caspian Basin as it includes all the depositional environments found for this geologic period.

.

Paleogeography or Depiction of Utah during Middle Jurassic

Figure 1: Paleogeographic map of the Middle Jurassic, Page Sandstone, Carmel Formation, Entrada Sandstone, Curtis Formation, and Summerville Formation. (Blakey, 2008)

What is the Grand Staircase?

The Grand Staircase is a unique and extensive exposure of Earth’s history, showcasing over 200 million years of sedimentary rock layers. Geologists often liken these layers to a “book,” allowing for a detailed study of the Earth’s past, including changes in climate and environment.

The major sedimentary rock units exposed in the Grand Canyon range in age from 200 million to 600 million years and were deposited in warm shallow seas and near-shore environments. The nearly 40 identified rock layers of Grand Canyon form one of the most studied geologic columns in the world.

No photos No photos

Morrison Formation (Geology of Utah’s Grand Staircase)

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Geologic Cross section of the layers of The Grand Staircase from Bryce through Zion to the Grand Canyon National Parks.

Explore unit thickness in All-in-One App

Exposure:

Within the Grand Staircase, the Morrison Formation is best exposed in Capital Reef National Park.

Age:  Late Jurassic (Kimmeridgian) 155-148 Ma

Depositional Environment: Alluvial plain, fluvial channels and floodplains with paleosols

Tectonics:

Subduction to the west created a back arc rift basin (between Morrison basin and paleo Pacific Ocean).
Mountain ranges (rift shoulder) to the west were source for clastic sediment
Calderas in rift basin provided abundant ash fall during Brushy Basin deposition

Climate:

Located ~32° N (modern southern AZ)
Prevailing easterly winds (present day NE due to rotation of plate)
Warm, dry climate with high evaporation

Features:

Morrison Formation – 180-200 m thick

    Tidwell Member  (oldest) 

  •         Alluvial plain – streams, overbank deposits, paleosols; locally (in Capitol Reef area)
  •         gypsiferous, hyper-saline lagoons
  •         Varicolored mudstone with interbedded sandstone, limestone, gypsum

    Salt Wash Member (middle)

  •         Fluvial channel deposits, floodplain deposits, crevasse splays
  •         Predominately fine/medium sand – coarse sand/ pebble conglomerates; trough stratification,
  •         fining upward

    Brushy Basin Member (youngest)

  •         Lacustrine/ wetlands; local fluvial channels
  •         Varicolored mudstone
  •         Mostly ground and surface water flowing to the east (present day NE)
  •        Losing streams with associated riparian environments prograding to the east

Floodplains with paleosols; grassy savannahs

Description:

The Morrison Formation is a distinctive sequence of Upper Jurassic sedimentary rock found in the western United States which has been the most fertile source of dinosaur fossils in North America. It is composed of mudstone, sandstone, siltstone, and limestone and is light gray, greenish gray, or red. Most of the fossils occur in the green siltstone beds and lower sandstones, relics of the rivers and floodplains of the Jurassic period.

It is centered in Wyoming and Colorado, with outcrops in Montana, North Dakota, South Dakota, Nebraska, Kansas, the panhandles of Oklahoma and Texas, New Mexico, Arizona, Utah, and Idaho. Equivalent rocks under different names are found in Canada.[2] It covers an area of 1.5 million square kilometers (600,000 square miles), although only a tiny fraction is exposed and accessible to geologists and paleontologists. Over 75% is still buried under the prairie to the east, and much of its western paleogeographic extent was eroded during exhumation of the Rocky Mountains.

It was named after Morrison, Colorado, where some of the first fossils in the formation were discovered by Arthur Lakes in 1877. That same year, it became the center of the Bone Wars, a fossil-collecting rivalry between early paleontologists Othniel Charles Marsh and Edward Drinker Cope. In Colorado, New Mexico, and Utah, the Morrison Formation was a major source of uranium ore.

Figure 1: Paleogeographic map of the Middle Jurassic, Page Sandstone, Carmel Formation, Entrada Sandstone, Curtis Formation, and Summerville Formation. (Blakey, 2008)

https://web.archive.org/web/20171216143550/http://sed.utah.edu/Morrison.htm