RECONSTRUCTION OF GRAND MESA VOLCANIC FIELD, WESTERN COLORADO

Grand Mesa, an erosional landform, is capped by Miocene lava flows that rest on Cenozoic sedimentary rocks. The present-day in-situ lava-field footprint is about 155 km²; however, because of extensive slumping, the original extent is unknown. This study is an attempt to reconstruct the areal extent of the field as it may have existed prior to Pliocene-Pleistocene degradation and to document the movement of the slump blocks.

The largest surviving part of the Grand Mesa lava field has a "Y-shaped" outline, with Crag Crest forming the stem, and the Palisade and Flowing Park lobes forming the branches. Additional outliers of the field exist to the east, such as Leon and Crater Peaks. The flow sequence ranges in age from 9.45 to 10.99 Ma based on 28 Ar-Ar dates and rests on a paleo-topographic Miocene (?) surface. The informal Goodenough formation (Miocene?) underlies the flow sequence and consists of weakly consolidated, clay-rich mudrock, lithic sandstone-conglomerate, and cherty limestone. The low mechanical strength of this unit has caused numerous retrogradational slump blocks on the north and south flanks of the Mesa, creating east-west trending landslide benches, and radiating slump fields surrounding the outliers to the east. The north bench is about 28 km long and 6.5 km wide, the south bench is about 29 by 5 km, and the eastern field is about 19 by 11 km. Slump blocks in the north landslide bench have moved in a northerly direction, in the opposite direction in the south bench, and radially in the eastern area. Block rotation ranges from a few degrees to near 90º.

Using published maps, topographic maps, and aerial photos, each block (polygon) was moved back to its inferred original (pre-slumping) position, much like doing a jigsaw puzzle.

The mean migration distance for the north group (N = 158) is 0.98 km, for the south group (N = 88) it is 1.5 km, and for the east group (N = 211) is 1.4 km. Maximum block size in the north and south benches is about 7.6 km², and approximately 1.7 km² in the eastern field. The combined surface area of the north blocks is about 65 km², for the south blocks it is about 72 km², and for the east blocks about 48 km². Adding these values to the footprint of the in-situ field (155 km²), suggests that the total footprint was at least 340 km².