Pankaj Khanna | Geology Reports

USGS2025

Mill Springs Limestone Submember (new) of the Point Peak Member, Wilberns Formation, Moore Hollow Group, Texas

This study proposes a new mappable unit, the Mill Springs Limestone Submember, within the Upper Cambrian Point Peak Member of the Wilberns Formation of the Moore Hollow Group in the Llano Uplift, central Texas. The submember is characterized by thick microbial boundstones that distinguish it from the thinly bedded heterolithic mixed clastic and carbonate strata of the Point Peak Member. The submember contains discontinuous individual bioherms up to 4.5 meters (m) thick, and cluster up to 15 m thick. The Mill Springs Limestone generally occurs in the upper and middle portions of the Point Peak, from west to east in the study area, and when present is mappable at the quadrangle (1:24,000) scale. In the western part of the study area the Peak Member contains facies that reflect a shift from tidal-flat deposition in the lower part of the Point Peak, to more open, outer-shelf conditions in the upper part of the Point Peak and development of thick bioherms characteristic of the Mill Springs Limestone Submember. The new designation aids geologic mapping and future paleoenvironmental studies.

Professional Paper↗

USGS2024

International Ocean Discovery Program Expedition 389 preliminary report: Hawaiian drowned reefs

Our understanding of the mechanisms controlling eustatic sea level and global climate changes has been hampered by a lack of appropriate fossil coral records over the last 500 ky, particularly into and out of the glacial periods. This problem was addressed by International Ocean Discovery Program Expedition 389 by drilling a unique succession of Hawaiian drowned coral reefs now at 110–1300 meters below sea level (mbsl). The four objectives are to investigate (1) the timing, rate, and amplitude of sea level variability to examine cryosphere and geophysical processes, including the assessment of abrupt sea level change events; (2) the processes that determine changes in mean and high-frequency (seasonal–interannual) climate variability from times with different boundary conditions (e.g., ice sheet size, p CO 2 , and solar forcing); (3) the response of coral reef systems to abrupt sea level and climate changes; and (4) the variation through space and time of the subsidence and the volcanic evolution of the island. To achieve these objectives, 35 holes at 16 sites ranging 131.9–1241.8 mbsl were drilled during the expedition. A total of 425 m of core was recovered, comprising reef (83%) and volcanic (17%) material. Average core recoveries were 66%, with numerous intervals characterized by very well preserved mixtures of coralgal and microbialite frameworks with recoveries >90%. Some science-critical shallow sites were not drilled due to a failure to secure permits to operate in Hawaiian state waters. Furthermore, apart from one site the target penetration depths were not achieved. Preliminary radiometric dates indicate that the recovered reef deposits are from 488 to 13 ka in age. The Onshore Science Party took place in February 2024. Cores were CT and hyperspectral scanned and described. Standard measurements were made, and samples were taken for postcruise research. Preliminary assessment of the age and quality of the reef and basalt cores suggest that many of the expedition objectives will be met.

Hawaii↗