LIDAR ANALYSIS OF MUD MOUNDS IN THE THUNDER SPRINGS MEMBER OF THE MISSISSIPPIAN (TOURNAISIAN ) REDWALL LIMESTONE, GRAND CANYON, ARIZONA

Enigmatic carbonate buildups occur deep within the Marble Canyon portion of the Grand Canyon, in the early Carboniferous (Tournaisian) Thunder Springs Member of the Redwall Limestone. Most of these are similar to Waulsortian mud-mounds which grew in transgressions across the globe at this time. Recent sequence stratigraphic interpretations suggest that Thunder Springs Member was deposited in a transgression. The geometries of these mounds, timing of deposition and sequence stratigraphic position, support interpreting some of these structures as mud-mounds.

Mound shape and geometry were documented using a Riegl VZ-1000 terrestrial laser scanner to collect point cloud data (average point density of 172 pts/m²) of a mound located on river left near Nautiloid Canyon (River Mile 35). At the thickest, the mound is 15.55 m, with a width of 154.0 m, and mound flank angles range from 19 to 22 degrees. Beds adjacent to the mounds do not onlap but are gently inclined with minor thinning over the mound crest. These draping beds persist throughout the overlying 28.3 m of the Thunder Springs Member resulting in 2.2 m of relief along the top surface. Crinoidal grainstones of the lower 7.4 m of the Mooney Falls Member thin from 9.6m on flanks to 7.4m over the mound crest, and ultimately flattened this relief on the seafloor. The total thickness at this location of the Thunder Springs Member is 40.7 m and the Mooney Falls Member is 92.3 m.

On-going research includes the analysis of 27 hand samples taken of strata below, in, and above the mound-bearing interval which are being analyzed for faunal composition and diagenetic history. Initial results are that core samples contain biomoldic finely crystalline euhedral dolomite.

At the same stratigraphic horizon in the Thunder Springs Member in this area, mound-like features of folded beds or “core-less” mounds are estimated to range from 5 to 12m high, and up to 50m wide. These may be formed by draping beds which are associated with mound cores behind the wall of the exposure.