This peripheral unit of the Wasatch Formation was first described by
Oriel and Tracey in 1970. It is not exposed in the monument, but forms a
belt of outcrops around the edge of Fossil Basin and in channels cut at
right angles to the basin edge.
The Tunp Member is seen to intertongue with nearly all of the Wasatch
and Green River formations. The member grades laterally basinward from
coarse, angular conglomerates to fine mudstone. Two limestone tongues of
the Green River Formation are interbedded with the Tunp, indicating that
at least twice there was major expansion of the lake.
Lithologically, the Tunp Member is a diamictite. This is a sedimentary
rock with a wide range of particle sizes. The Tunp is best described as
a red, conglomeratic, sandy mudstone with angular, poorly rounded to
smooth, well-rounded clasts with a size range from pebble to boulder.
There is no bedding or orientation of clasts.
The Tunp Member probably originated from mudflows and gravity sliding
(Tracey et al. 1961). Environmental studies indicate that the area had a
warm, humid climate. This would cause deep weathering of the surrounding
slopes. This weathered material would then be a prime source to be acted
upon by rain, gravity, and possibly earthquakes. These agents of
deposition would then cause the material to flow and be deposited with
little chance for sorting and rounding of the rock particles. The result
was a belt of coarse, unsorted detritus on the basin edges now called
the Tunp Member.