The main aim of today's EVA was to travel to the radio repeater in order to carry out some routine checks and maintenance. The repeater is situated on top of Skyline Rim (waypoint 12.7.9) and in order to get there it is necessary to drive in the pressurized rover (otherwise known as the truck!) south to the main Hanksville-Loa highway. From there we proceeded west and made a brief stop after 3km at waypoint 12.7.1. Here we could see the large blocky layer of Dakota gravel overlying the softer layers of the Brushy Basin member (as at waypoint 12.5.2 on EVA5). See photo G7.1. Above this yellowish gravel strata the ground was covered in a soft gray dust upon which lay many gryphaea shells (as found on EVA4).

Photo G7.1

Photo G7.2

Photo G7.3

Driving on west another 6.5km along main the highway, we passed between towering cliffs of the Tununk Shale Member of the Mancos Formation (see time-line at end of report). We had previously sampled the Tununk Shale at Skyline Rim on EVA3 and the dark brown and yellow layers were easily recognised again. This shale layer could be seen to be capped with a more competent and less easily eroded sandstone layer (see photo G7.2). Tony and Susie had rested briefly upon this strata (known as the Ferron Sandstone Member of the Mancos Formation) after their climb to the top of Skyline Rim on EVA3. But at that stage they didn't have the energy then to study it more closely! We hoped to make a more detailed study of this sandstone on our route to the radio repeater today.

At waypoint 12.7.2 we made a right turn onto a 4WD track that runs directly north between Skyline Rim to the east and the Upper Blue Hills (and Factory Butte) to the west. The track essentially rests on the Ferron Sandstone unit, which can be seen in washes to the east of the 4WD track. The sandstone is covered with a thin layer of dark grey powder which has most probably been eroded from the cliffs of the Upper Blue Hills to the west. These cliffs are composed of another large shale strata (the Blue Gate Shale), which is also a member of the Mancos Formation. Although this shale most probably formed in a very similar environment to the Tununk shale (a deep marine anoxic environment), they show quite different erosional morphologies. The Tununk Shale forms steep cliffs with smooth talus aprons below (photo G7.2) whereas the slopes below Blue Gate Shale cliffs are deeply incised with run-off gullies (photo G7.3). The reason for this difference is not clear but may relate to the orientation of watersheds within the area.

Like the Brushy Basin and the Tununk Shale Members, the Blue Gate Shale is also capped by a resistant sandstone strata (the Emery Sandstone). It is easy to see that the Brushy Basin, Tununk Shale, and Blue Gate Shale rocks are all quite soft and very easily eroded. The only reason that they have not been washed away altogether is that each is topped by a hard layer of sandstone and/or gravel (the Dakota, Ferron, and Emery respectively). This gives rise to the series of north-south ridges that lie to the west of the Hab. These are shown schematically in the cross-section in figure G7.4 but please note that the vertical scale in this diagram is greatly exaggerated. Although the units do dip towards the east, the angle of dip is actually only very slight. Ideally we would like to make a measurement of the angle and orientation of dip but this requires a good bedding surface that has so far eluded us.

Figure G7.4

After driving 6km north to waypoint 12.7.3 we made another right-hand turn and followed the track 3km east to waypoint 12.7.4. Here we made a final right turn and parked the rover shortly after at waypoint 12.7.5. >From here we proceeded on foot in a south-easterly direction towards the repeater coordinates. As we made our way up the gradual incline we came across a number of bluffs of the Ferron Sandstone unit. Up-close, this could be seen to be a fine to medium sand with some massive strata (up to 15 feet thick) inter-layered with thinner beds of silt. The sandstone is yellowish in colour and contains a fairly high proportion of feldspar material (an arkosic sandstone). At waypoint 12.7.6 we noted many small nodules protruding from the bedding surfaces of the rock. These result from fluids passing through the sandstone and preferentially cementing some areas and making them more resistant to weathering than the surrounding rock. A little further on at waypoint 12.7.7, we observed much larger nodules measuring up to 40cm across. These were a deep orangey-red in colour indicating the presence of iron in the fluids that formed them (see photo G7.5). Lying lose on the ground at this site we also found spherical rocks which, when cracked open, exhibited impressive concentric color-banding (photo G7.6).

Photo G7.5

Photo G7.6

From this location we then pressed onto the Skyline Rim and the radio repeater to commence our maintenance checks. These took some time since the large space-suit gloves seriously impeded our ability to manage the intricate wiring etc. However we were determined to remain in Sim and pretty much completed the task despite these difficulties. It was only the copper wires, which needed twisting to ensure a clean electrical contact, that defeated us and finally required me to take my gloves off briefly. (See EVA report for full details.)

On the walk back down to the pressurized rover (waypoint 12.7.11), we noted many fractures/raised veins within the rock - many of which had been cemented by the same fluids that created the nodules observed earlier. I managed to take an orientation measurement of one of these fractures (bearing of 310 degrees), and this is consistent with the pattern of faulting that has been documented for this area. At waypoint 12.7.12 it was also noted that layers of the Ferron Sandstone appeared to be interspersed with organic-rich layers resembling the Tununk Shale. This suggests that the transition from on type of environment/desposition to the next was not abrupt, but instead happened gradually over a period of time. In fact, the Ferron Sandstone is interpreted as a large delta system that swept into the region to overlie the Tununk Shale. This was accompanied by a regression in sea-level. The sea then later inundated the land once more and this resulted in the deposition of the Blue Gate Shale.

So all in all, it was an interesting excursion geology-wise even though that wasn't our primary objective. It has to be said though, that with views as good as these, it's difficult to keep your eyes (and your mind) on the task-in-hand. I would quite happily have sat up on the Rim all afternoon just gazing at the landscape. The patterns, shapes and colors defy my powers of description but I hope you enjoy our photographs.