Melissa has selected the Cretaceous Dakota Sandstone in the field area for the subject of her honours thesis. This resistant unit forms the north-south ridge behind the MDRS Hab near Hanksville, Utah, and today's EVAs were spent examining the geology of this unit. We have identified at least six different layers within the unit, beginning with a medium-to-coarse grained, sub-rounded yellow sandstone marking the beginning of the Cretaceous. (Below this layer we have identified a contact with paleosols of the Jurassic Morrison Formation) The next layer, a medium-to-coarse grained brown sandstone varies in thickness from 8 - 40cm, and is not present everywhere. These two layers show weak cross-bedding in a few locations. They are full of at least 5 different types of burrows, and contain some randomly oriented bivalve fossils, and gastropod molds.

Overlying the sandstones there is a layer of coarse gravels extremely rich in bivalve fossils - mostly intact Gryphaea spp. ranging in size from 0.2 - 4cm. This indicates a life assemblage. No other fossils have been identified in this layer, however different borings in bivalve shells indicate the presence of some gastropods and sponges. As the layer features predominantly Gryphaea spp., which are tolerant of low salinities, the paleoenvironment is likely estuarine. This layer also shows undulated bedding. Now things get complicated: At a certain point along the ridge south of the Hab there exists a fault where we believe the Dakota sandstones to be offset by at least 4m. On the north side of the fault the coarse gravel layer is directly overlain by layers of volcanic ash, thought to be bentonite. On the south side of the fault there are at least 3 layers of interfingering crossbedded conglomerates and sandstones overlying the previously mentioned sandstones, although a contact was not located. These interfingering layers are likely channel deposits. (We found what might be a dinosaur bone within the top conglomerate layer!) The same bentonite layer from the north side of the fault directly overlies the top-most sandstone of the interfingering crossbedded sandstones.

Tomorrow we will return to this area to take a closer look at the fault, and the fossil assemblages and burrows, to sample the sandstones, to measure paleocurrents, and to search for a contact between the sandstones on the south side of the fault.

Lith Canyon lies in the northern part of the study area and is famous as a locality for both petrified wood and vertebrate remains. An EVA team with Melissa as the geologist was dispatched yesterday to this area on a reconnaissance. The rock containing these fossils consists of intensely crossbedded conglomerates and sandstones. It is possible that partially intact bones exist within these layers. Bone fragments were also found along with a large amount of petrified wood in a gulley just south of the main canyon.

Another EVA team with Rocky Persaud and Jonathan Clarke as geologists yesterday carried out a scout mission to the spectacular cliffs of west of Hanksville and are visible from the road. The cliffs consist of shales, gypsum, and anhydrite of Summerville Formation capped by the basal conglomeratic sandstones of the Morrison Formation, both of Jurassic age. The cliffs are cut by short box canyons, flat floored features that are terminated by a cliff. These are thought to be cut by groundwater discharge undermining the cliff rather than being cut by running water. The canyon we examined had a small stream that flowed over the lip of the head of the canyon, but it was much smaller than the size of the canyon would require for it to be the main agent responsible. Evidence for the groundwater model was present in the form of a distinct active groundwater seep at the base of the Morrison feature. Groundwater sapping is a major landscape shaping process in many arid environments on earth, including the American west and inland Australia. It is also thought to be an important process on Mars, carving some of the network valleys.

This team also examined the stratigraphy and sedimentary structures in these two units. The Summerville consisted of decimetre scale beds of alternating red-brown calcareous shale, chicken wire anhydrite and occasional green siltstones. These were cross-cut by thin bedding subparallel veins of selenite and subvertical veins of powdery gypsum with breccia fragments of shale. Despite their different appearance the two veins cross-cut each other and are therefore coeval. Up section there is an increase in the abundance of sulphate minerals with the appearance of gypsum and anhydrite nodules, the largest are at the top of the succession and are up to 1 X 2 metres in size. Like the chicken wire anhydrite, these nodules formed soon after deposition. These cliffs illustrate that there are different generations of evaporite minerals and it is not sufficient to say that gypsum is present and that this indicates evaporitic conditions, the context of the gypsum has to be recognised for the correct interpretation to be made. Also appearing at the top of the succession are lenses of cross bedded sandstone. These features are consistent with deposition of mud and anhydrite on an evaporitic flat with the intensity of evaporitic conditions increasing up section. The sandstone lenses represent the progradation of a higher energy channel facies across the flat. The appearance of the basal Morrison Formation presents the culmination of this facies. The Morrison consists of coarse gravelly sandstone to conglomerate with well exposed trough cross beds indicating current flow to the east and south east.