Cretaceous sedimentary rocks along the Rio Puerco near Cabezon Peak

I volunteered to lead one day of the 2024 NMGS field trip, which will be based out of Bernalillo and focusing on the “nexus” of three physiographic provinces: the Rio Grande rift, southern Rocky Mountains (i.e., Nacimiento Mtns), and the southeastern Colorado Plateau. The day I am leading, we will go down the middle Rio Puerco Valley near Cabezon. Larry Crumpler (NM Museum of Natural History) will lead two stops focusing on the Rio Puerco volcanic necks near Cabezon Peak. My stops pertain to Cretaceous sedimentary rocks, Holocene geomorphic processes, and how climate change and geomorphic processes, namely arroyo cutting, have affected the groups of people that lived along the Rio Puerco in the past (i.e., Hispanic farmers and Ancestral Puebloans).

I am including some photographs from the lower part of the Point Lookout Sandstone, which is a Late Cretaceous unit at the base of the Mesa Verde Group. The Point Lookout Sandstone represents nearshore facies involved in a major northeastward progradation (>200 km) of the shoreline of the Western Interior Seaway. The first photograph is an outcrop of a stratigraphic interval studied earlier by Robyn Wright-Dunbar, formerly of UNM. Note that overall there is a coarsening-upward in terms of the proportion of sandstones vs. mudstones, consistent with a long-term shallowing (regression) trend. However, Robyn recognized several smaller transgressions that occurred during this shallowing, labeled T2 and T3 in the photograph (T1 is the top of the Mancos Shale). The R1, R2, R3 units represent successive, smaller regressions that occurred during the overall progradation of the Point Lookout Sandstone. The unit-pairs of T1-R1, T2-R2, and T3-R3 can be considered as parasequences. What caused these parasequences (transgressive-regressive couplets) to occur include climate-controlled sea level changes or short-term changes in sediment supply (i.e., reductions of sediment brought into the nearshore environment by nearby rivers).

The other photographs in this post illustrate some sedimentologic features found in the R2 unit. These include groove-like toolmarks + horizontal burrows (pictured in the 3rd photo), symmetrical ripplemarks (pictured in the fourth photo), and vertical burrows (pictured in the 2nd photo). The vertical burrows in the second photo are the knobby features in the upper part of the photograph, about 3-4 ft above my hat. These burrows are probably either ophiomorpha or skolithos. They do not occur in R1, so the environment in R2 must have been amore favorable place to live – perhaps shallower and more oxygenated.