ABSTRACT The Cretaceous-Paleocene (K/P) boundary intervals are rarely preserved in successions of shallow-water limestones. Here, we describe a shallow rocky shore on the active orogenic wedge of the eastern Alps (Austria) fringed by a carbonate platform that was largely cannibalized by erosion. We compared this succession with similar nearshore environments globally, as well as the deep sea, to gain a better understanding of the environmental response to the K/P boundary transition. In the eastern Alps, Cretaceous and Paleocene lithofacies across the K/P boundary transition are separated by a hardground that formed during subaerial exposure and that terminates Upper Maastrichtian limestone with planktic foraminiferal assemblages deposited at neritic depth during zone CF3 (ca. 66.500 Ma). Above the hardground, there are beachrocks with early Danian zone P1a(1) assemblages, which indicate the hardground spans about ~600 k.y. of nondeposition and/or erosion. During the early Danian, the marine transgressive fringe fluctuated between “shoreface to emersion” environments, depositing limestones rich in bryozoans, rhynchonellids, coralline algae, and rare planktic foraminifera along with abraded, bored, and/or encrusted clasts eroded from older rocks. Repeated short subaerial exposure is marked by vadose diagenesis and hardgrounds, including an ~1.5 m.y. interval between magnetochrons C29n to C28n and planktic foraminiferal zones P1b to P1c(2). Comparison with platform carbonate sequences from Croatia, Oman, Madagascar, Belize, and Guatemala, as well as nearshore siliciclastic environments of southern Tunisia, Texas, and Argentina, across the K/P boundary transition revealed surprisingly similar deposition and erosion patterns, with the latter correlative with sea-level falls and repeated subaerial exposure forming hardgrounds. Comparison with deep-sea depositional patterns revealed coeval but shorter intervals of erosion. This pattern shows a uniform response to the K/P boundary transition linked to climate and sea-level changes, whether in shallow nearshore or deep-sea environments, with climate change tied to Deccan volcanism in magnetochrons C29r-C29n.

Petrophysical characterization and understanding of pore systems and producibility in unconventional reservoirs remains challenging when evaluating reservoir potential. This study’s main objective is to identify and evaluate the controls on petrophysical rock types in unconventional low porosity, low permeability carbonate reservoirs in Mississippian-aged rocks of the southern Midcontinent. Representative samples selected from cores in the study area are calcareous siltstones and grain-rich packstones to grainstones. Rock fabric, pore types, and pore structure of 23 samples were investigated using multiscale image analysis of optical micrographs and scanning electron microscope (SEM) mosaics. Petrographic observations and quantified pore parameters were correlated with nuclear magnetic resonance (NMR) plug measurements of transverse relaxation times ( T 2 ), pore size distribution, and porosity. Results indicate that pore structure, permeability, and NMR response are closely linked to the dominant pore types, pore sizes, and mineralogy, which are distinctive for specific rocks—allowing for petrophysical rock type (PRT) grouping. NMR signature geometry is distinct in each of these rock type groups. Complex mixed mineralogies in these rocks homogenizes porosity and permeability relationships among rocks of different depositional facies, making it difficult to define clear-cut correlative relationships between pore architecture, rock fabric, and petrophysical response. Petrographic assessment indicates that the primary cause of pore-scale heterogeneity and varying petrophysical response is related to postdepositional diagenesis, such as silicification, cementation, dissolution, and mineralization along pores and pore throats, which produce complicated pore systems and affects matrix permeability. These observations confirm that incorporating geologic information such as mineralogy, diagenesis, and pore types/pore architecture into rock typing workflows in carbonate mudrock reservoirs is critical to understanding petrophysical response. Additionally, the distinct geometries in each petrophysical rock type group establishes the viability of using NMR as a rock typing tool based on the correlative relationships between NMR response, pore types, and facies.