During our two weeks here at the Hab, crew 12 have explored a large variety of sedimentary rock strata. These strata are inclined at a low angle to the west and are exposed in north-south bands, with the oldest deposits being found in the east and the youngest in the west. This report aims to bring together our observations of the different units and to place them in some kind of historical/environmental context (with help from several additional sources listed at the end of this report). The units are described starting with the oldest (Triassic) first and working forward in time.

Navajo Sandstone (Triassic): This is a pale colored cross-bedded sandstone that weathers into rounded buff colored cliffs and knobs. The cross-bedding is aeolian in origin and this desposit resentents ancient sand dunes of a large desert (approximately the same size and latitude as the Sahara Desert). We observed this unit along the southern margin of the San Rafael Swell just north of Goblin Valley. The swell is a major topographic feature in which Paleozoic and Mesozoic rocks are arched upwards in a 80 mile long and 40 mile wide anticline (dome). The San Rafael Swell was also the site of a lot of uranium mining in the past.

Carmel (Jurassic): These green, gray and red beds of limestone, shale and gypsum are not very well exposed in the Goblin Valley area but can be seen from the road to drape the southern boundary of the Navajo sandstone. They too are tilted by the swell (dipping towards the south), in contrast with the overlying foramtions that are sub-horizontal.

Entrada (Jurassic): These fairly structureless white and brick-red sandstones are interlayered with thin beds of muddy siltstone. The sandstones were deposited in a desert that covered much of southern Utah at this time, while the muddy siltstone denotes marine reworking of the aeolian sands during high tide in a mud-flat environment. This unit therefore represents the transition betweeb sand dunes and a shallow sea (this is known as a sabkha environment). Due to the sediment type and degree of cementation by siliceous and limy fluids, the Entrada layers erode at different rates. This gives rise to the hour-glass shaped formations seen by the crew at Goblin Valley. These "goblins" are also commonly known as hoodoos. Following the deposition of the Entrada formation, there was a period of tectonic activity in the region which lead to widespread erosion and the J3 unconformity that underlies the Curtis formation.

Curtis (Jurassic): The Curtis formation can be seen clearly from highway 24 on the drive from the Hab to Goblin Valley. The pale green marine sand, silt and limestone contrasts starkly against the deep red of the underlying Entrada. These sediments represent shallow sea deposition in a narrow seaway that extended south toward Capitol Reef during this period.

Summerville (Jurassic): This unit comprises thin continuous beds of orange and brown mud and siltstones, interspersed with white layers of gypsum. These deposits mark the return to an extensive tidal-flat environment and the gypsum layers were left behind as mineral-rich sea water periodically evaporated from low-lying pools that dotted the tidal flat. The strata are also cross-cut by numerous gypsum veins that were precipited after the formation had been deposited,from mineral-rich groundwaters passing through fractures in the rock. Also preserved within this formation are ripple marks and mud cracks, again consistent with a broad tidal flat periodically inundated with shallow water followed by a period of evaporation and exposure to the air. The Summerville formation is particularly well exposed in vertical cliffs at Candor Chasma, Tank Wash Canyon and along highway 24 west of Hanksville.

Tidwell (Jurassic): This unit, which forms the base of the Morrison formation, is not well exposed in the region. This is because the shallow marine/mud-flat siltstones and sandstones were exceptionally rich in gypsum. The gypsum dissolves upon contact with water leaving powdery slopes when exposed at the surface. Despite this, a greenish layer can be observed draping the tops of the vertical cliffs of the Summerville formation. The desert-like conditions that prevailed in the area during the Early and Middle Jurassic were too harsh for the growth of any significant flora or fauna. For example, dinosaurs were not able to exist in this environment.

Salt Wash (Jurassic): The Salt Wash unit forms the middle member of the Morrison formation and marks a distinct change in the prevailing conditions to a wetter environment. The J5 unconformity lies between the Tidwell and Salt Wash. The Salt Wash member consists of lenses of fluvial sandstones and conglomerates, representing the channels of an ancient river system. These rivers underwent tubulent flow (indicating steep topography within the region at that time) and the conglomerate clasts are varied (brought in from well outside the Hab area).The unit also contains red and green layers of siltstone from quiet flood-plain deposition between river channels. The Salt Wash unit can be seen clearly in the walls of Lith Canyon and White Rock Canyon. The Morrison formation is also rich in uranium ore. At the time of the Salt Wash deposition, the area underwent intra-basinal uplift (a reactivation of the San Rafael Swell) and this causes the Salt Wash member to vary in thickness and pinch-out altogether in some places.

Brushy Basin (Jurassic): This, the upper-most member of the Morrison Formation, is present continuously across the area indicating that uplift had ceased by this time. The Brushy Basin unit, upon which the Hab sits, comprises deep red, green and white mudstones and siltstones that were deposited in extensive mudflats and only cut by rare streams. Some thin limestone ledges were also formed where fresh water ponded on the mudflat. The formation is very clay-rich and contains a good quantity of volcanic ash. The sediments are very soft and erode easily into low rounded hummocks. A distinction between the Salt Wash and Brushy Basin members can sometimes be difficult/confusing because the Salt Wash includes layer of red and green silt/mudstone (like the Brushy Basin) and the Brushy Basin is also cut by occasional river channels. Both the Salt Wash and the Brushy Basin members contain petrified wood, dinosaur bones, agates and chert - all of which were sampled by crew 12.

Dakota (Cretaceous): The Dakota is the basal Cretaceous unit and forms a hard, resistant layer at the top of the Morrison formation. The Dakota formation gets undercut as the softer Brushy Basin deposits are easily eroded and this causes large blocks of Dakota to fall down from the skyline (for example, at Bobs Rock Garden). There are three different units within the Dakota and each represents a different environment. The bottom unit consists of ocre colored cross-bedded sandstones and massive conglomerate deposits containing rounded pebbles. These are an indication of the strong current forces in the rivers at that time. The middle unit consists of sandy coal - remnants of a densely vegetated swamp. The top unit is a shallow marine sandstone from a shallow sea/beach setting, indicating a rise in sea-level (marine regression). Fossil oysters (bivalves known as gryphaea) are found in this unit together with burrows from organisms living in the sand. The basal sandstone/conglomerate can be seen widely in the Hab area and caps the Hab ridge. Gryphaea shells were found by crew 12 at various locations in Mid Ridge Planitia, but they were lying lose on the ground and a good sample of bedrock was not located. Ferns, petrified wood, bone and agates have also been found within the Dakota (but not necessarily by this crew). As stated previously, the Cretaceous climate of western North America differed considerably from the arid conditions of the Jurassic period. Various lines of evidence point toward wetter conditions in the Cretaceous including coal deposits and vast quantities of erosion and deposition.

Tununk Shale (Cretaceous): To the west of Mid Ridge Planitia lies the Skyline Rim. The cliffs of this rim are made up of the dark brown Tununk Shale member of the Mancos formation, which is easily eroded to form the steep talus slopes at its base. The Tununk Shale denotes the continued rise in sea-level that began during the Dakota deposition. As the sea expanded, drowning the alluvial plains, Dakota sands gave way to deep sea anoxic muds - rich in organic material. This harsh muddy bottom environment did not contain any living organisms but fossil amonites, that dropped down from the overlying waters, are found within the formation. Crew 12 did find some fossils in the Tununk Shale but are unsure of their identification.

Ferron Sandstone (Cretaceous): The marine regression that occured during the Tununk deposition was relatively short-lived and the Ferron Sandstone (that caps the Tununk Shale unit) is the product of a large delta that deposited sand over the deep-sea muds. This sand was derived from the Sevier Orogenic Belt that lay to the west of the area during this time. The Ferron Sandstone did not occur as an abrupt facies change. Rather, alternating layers of Tununk Shale and Ferron Sand (on the back of Skyline Rim) suggest that initially small spurts of sand were washed into the shallowing sea. These were then followed by increasingly large inputs of sandy sediment. Near its base the Ferron Sandstone (also part of the Mancos formation) contains iron-rich nodules which possibly give it its name (?) Near the top of the unit, the beds exhibit extensive bioturbation and have been completely churned up by organsims living in the sand. These organisms indicate that transport of sand to the area was sporadic (otherwise the organisms could not have survived). Like the Dakota formation, the Ferron Sandstone is relatively resistant to erosion and provides a protective cap to the underlying Tununk Shale - thus forming Skyline Rim. Thin coal layers have been reported at the top of the Ferron Sandstone, together with remnants of twigs, leaves and stems. These coal swamps formed as the sea returned to a flood delta. Crew 12 did not locate this coal horizon.

Blue Gate Shale (Cretaceous): West of Skyline Rim lie the steep cliffs of Factory Butte and the North Cainville Mesa. These cliffs are comprised of the Blue Gate Shale Member of the Mancos Formation which, like the Tununk Shale, is soft and easily eroded - into rills and ridges. The Blue Gate Shale represents another marine inundation and environmental setting very similar to that of the Tununk Shale.

Emery Sandstone (Cretaceous): The inaccessible summit of Factory Butte and the top of the North Caineville Mesa are composed of blocks of Emery Sandstone (uppermost member of the Mancos Formation). It is not possible to sample this unit directly but blocks lying on the lower slopes of the Blue Gate Shale were studied by the crew. The erosion of this layer is controlled by prominent verticle joints - formed by tectonic stresses associated with the San Rafael Swell. This pale yellow sandstone marks another inward surge of sediment from the Sevier Orogenic Belt to the west. Like the Ferron Sandstone, the Emery unit represents a return to shallow water conditions and provides a resistant cap to the underlying shale cliffs. The sandstone mambers of the Mancos formation have been found to contain petrified wood, while the shale members contain sea fossils that include shark teeth. Despite a hopeful search, crew 12 did not find any shark teeth (except the one in the 40 cents tray in the Hanksville gas station!)

Sources:

• The Geology of the Parks, Monuments and Wildlands of Southern Utah by Robert Fillmore (University of Utah Press, 2000)
• Roadside Geology of Utah by Halka Chronic (Maintain Press Publishing Company, 1990)
• Geology of Utah by William Lee Stokes (Utah Museum of Natural History and Utah Geological and Mineral Survey, 1987)