ABSTRACT Middle Triassic to Lower Jurassic continental conglomerate, sandstone, and mudrock are widely exposed in the highest tectonostratigraphic terranes (internal domains) of western and central circum-Mediterranean orogenic belts. These red beds are a key tectonic facies assemblage representing onset of the Tethyan rift-valley stage in the western Mediterranean, during which plate and microplate boundaries were localized during breakup of Pangea. These red beds likely define the boundary of the Mesomediterranean microplate, which played a key role in the Cenozoic evolution of the western Mediterranean. Red beds unconformably overlie Paleozoic metasedimentary and locally plutonic rocks, and they are covered by Early Jurassic and younger sedimentary strata; they are generally mildly deformed and only locally metamorphosed. Sandstone detrital modes vary from quartzarenite to quartzolithic, reflecting a provenance from Cambrian–Carboniferous metasedimentary rocks similar to those underlying the red beds. Reevaluation of previously published petrographic databases and detailed hot-cathodoluminescence (H-CL) analysis of quartz grains indicate that most quartz grains were derived from heterogeneous metamorphic, plutonic, and volcanic rocks. Evaluation of the diagenetic evolution of red beds using chemical-mineralogical analyses and H-CL analyses indicates that compaction and cementation played key roles. Compaction consists of brittle deformation, with breakage of quartz grains and production of early quartz cement, which closed the fractures, followed by a second stage of quartz cementation. Carbonate cements consist of dolomite, ankerite, and calcite. This last cement, related to dedolomitization/calcitization processes, produced carbonate crystals with iron oxides. The sandstone experienced intense reduction of intergranular volume during early stages of burial, as indicated by contrasting compactional porosity loss (COPL; mean of 31.1%) versus cementational porosity loss (CEPL; mean of 8.5%). These data demonstrate the minor role of cementation in reducing porosity and the prevalence of compaction as the main process destroying primary pores. The diagenesis of the analyzed red beds is variable within several internal domains of the orogenic belts, suggesting local influences related to the provenance and geotectonic evolution of each basin.

The redox state of the mid-Proterozoic oceans, lakes, and atmospheres is still debated, but it is vital for understanding the emergence and rise of macroscopic organisms and eukaryotes. The Appekunny Formation, Belt Supergroup, Montana, contains some of these early macrofossils dated between 1.47 Ga and 1.40 Ga and provides a well-preserved record of paleoenvironmental conditions. We analyzed the iron chemistry and mineralogy in samples from Glacier National Park, Montana, by pairing bulk rock magnetic techniques with textural techniques, including light microscopy, scanning electron microscopy, and synchrotron-based X-ray absorption spectroscopy. Field observations of the Appekunny Formation combined with mineralogical information allowed revised correlations of stratigraphic members across the park. However, late diagenetic and/or metasomatic fluids affected primary iron phases, as evidenced by prevalent postdepositional phases including base-metal sulfides. On the west side of the park, pyrrhotite and chlorite rims formed during burial metamorphism in at least two recrystallization events. These complex postdepositional transformations could affect bulk proxies for paleoredox. By pairing bulk and textural techniques, we show primary records of redox chemistry were preserved in early diagenetic and often recrystallized framboidal pyrite, submicron magnetite grains interpreted to be detrital in origin, and red-bed laminae interpreted to record primary detrital oxides. Based on these observations, we hypothesize that the shallow waters of the mid-Proterozoic Belt Basin were similar to those in modern marine and lacustrine waters: fully oxygenated, with detrital reactive iron fluxes that mineralized pyrite during organic diagenesis in suboxic, anoxic, and sulfidic conditions in sedimentary pore waters.