Beware: Premature withdrawal leads to slumping
Jelle Wiersma and I were checking out an outcrop of Morrison between two sections we (Deb Jennings and I) measured the previous summer. On our way along the contact between the Sundance FM and the overlying Morrison, we ran across some sauropod tracks in the upper portion of the Sundance. This is a foreshore(?) deposit capped by beach dunes (no thin sections of this eolian sand yet, but the ones about 3 miles down strike are composed of oolites and sand).
The image below shows the cross section of the track in the foreshore sandstone. There is a block at Jelle’s feet, it contains the other half of the footprint. If I can get a permit and figure out how to slice of a section (big ass portable rock saw?) it would make a cool museum display and maybe a tight little paper (including a detailed measured section of Sundance!)?
Jelle Wiersma, Sundance FM, sauropod track in cross section (w/outline of deformation).
The image above has been traced out to better show where the shear is. The breccia is not very visible in the picture unfortunately, but it is quite apparent on outcrop. The micro detail is amazing, you can see a series of normal faults with 1-3 cm displacement on the back side (heel) and the breccia is in the middle and towards the front (toes) most likely from withdrawal of the foot. The surface feature produced (at the time of deposition) was covered and it is very difficult to see these prints on the surface (née impossible!).
I wonder if they’d show up in a GPR study? Tracks from the Upper Sundance aren’t unheard of, but this is unique in terms of preservation and environment. ‘Normally’ the tracks are in a limestone that typifies the Sundance/Morrison contact. Oh, and there are at least 7 tacks in cross section along the small section we walked that day, one of which was likely a juvenile or possibly another taxa.
Happy Sauropods! …er Seds! Oh wait… this is the best of both worlds!
Happy Sauropods! AND Seds!
