New Mexico Geological Society Annual Spring Meeting — Abstracts

Structural and stratigraphic observations on the Waulsortian mud mounds in the Sacramento Mountains of South-central Mexico, suggest syn-depositional deformation as an important factor in the mounds enigmatic evolution. Two end-member evolutionary models have been proposed, referred as to biohermal and allochthonous models. Recent detailed mapping of the entire Mississippian has shown that syn-depositional deformation is restricted to mound systems, where it is pervasive. This relationship is inconsistent with both of the proposed models. The biohermal model generates relief by biotic buildup and does not account for the deformational features documented in around mound systems. Although the allochthonous model implies deformation related to mass transport processes, the deformation should extend away from the mounded areas. However, such implied deformational patterns have not been found.

Waulsortian mounds have been described as containing a mudstone core facies surrounded by flank facies of crinoidal and bryozoan debris. Our detailed observations of mound facies in the Sacramento Mountains show mound cores as chaotic accumulations of mudstone to packstone beds, with the coarser facies being most abundant. Remnant bedding is observed throughout and is chaotically oriented. Mud-filled injection dikes cut across these core facies which often show soft sediment deformation such as slump folds and scars. Stratal geometries of the flanks indicate syn-depositional structural growth. The flank facies display both thinning and onlapping of beds as they approach the mound core, as well as stratal truncation patterns. These observations suggest that the cores of mounds were moving upward as the flank facies were being accumulated.