MESH AND BUMP WEATHERING STRUCTURE FROM THE MIDDLE DEVONIAN MAHANTANGO FORMATION, PIKE COUNTY, PENNSYLVANIA: IMITATION OF ORGANIC ORIGIN

The Mahantango Formation (Middle Devonian) is present in exposures along Route 209 near Milford, Pennsylvania. Due to extensive fragmentation along cleavage, bedding, and joint intersections and weathering along vertical faces, an apron of clasts representing upper to lower portions of the exposure are present. A rainstorm resulted in slumping of dark-gray mudstones from an isolated lowermost fresh exposure. These strata represent hemipelagic deposits formed in an offshore marine setting below fair-weather base, reflecting lower energy levels.

Investigation of the slumped strata has revealed an interesting type of structure, namely mesh and bump. Evidence occurs on clasts ranging from 3-17 cm in length. Many clast surfaces exhibit a mesh-like hexagonal to pentameral structure, 2.5-4 mm in width and mesh edges 0.5-0.75 mm thick. Though at first glance similar in appearance to the trace fossil Paleodictyon, there is also evidence of smaller, circular bumps within the confines of certain meshes. Furthermore, other surfaces are composed entirely of these circular bumps (dominantly 2-3 mm in diameter). Interestingly, the mesh and bump structure also bears a broad resemblance to receptaculitid algae. However, the general lack in the present specimens of a more regular alignment of meromes in a spiral and radial array and a lack of size gradients among the plates, expected in receptaculitid algae, precludes such an assignment.

As such, spherical weathering after diagenesis is most likely responsible for this phenomenon. Initially, the spherical weathering starts from joints and bedding surfaces, which separate the volume of rocks into cube-like forms. The presence of the joints, particularly, as well as the cleavage allows the entrance of weathering agents, and accelerates the rate of chemical weathering. As a result, the corners and edges of the blocks are weathered more rapidly than the flat surfaces. Chemicals and transformations migrate toward the center of the blocks on spherical fronts which appear as circles on the surfaces. Examples of weathered spheroidal shells are preserved on several of the individual bumps. In turn, reverse processes from the center result in spheres pressing against each other and producing hexagonal patterns. As such, the mesh structure reflects the remnants of the end product of this spherical weathering.