Mudstone microfacies as a tool for resolving depositional ambiguities in fine-grained, deep-water successions: insights from the Taciba Formation (Itararé Group), Paraná Basin, Brazil

Fine-grained sedimentary rocks deposited in deep-water environments (i.e., below the storm-wave base) often display similar macroscopic characteristics, even when formed in different subenvironments or by distinct depositional processes. This is particularly evident in progradational wedges, where prodeltaic deposits may interfinger with or directly overlie submarine fan fringe or interlobe successions, leading to ambiguous paleodepositional interpretations of successions that may be relevant as seals or source rocks in petroleum systems. To distinguish mudstones formed in different deep-water settings and interpret their depositional processes, this study applies a microfacies approach based on outcrop samples. The early Permian Taciba Formation, Paraná Basin (southern Brazil), provides an ideal setting for this investigation, as it comprises a net-regressive succession composed of a lowermost turbidite lobe complex conformably overlain by prodeltaic mudstones and heteroliths. Our results indicate that prodeltaic mudstones are characterized by poorly organized microfacies, dominated by coarser ungraded mud, bioturbation, and discontinuous silt lenses. These features are interpreted as products of positively buoyant plumes associated with hypopycnal flows, which may occasionally evolve into secondary, bottom-hugging turbidity currents. In contrast, mudstones associated with submarine fan systems display more organized microfacies patterns, characterized by graded layers with continuous planar lamination and other sedimentary features interpreted as products of alternating shear-thinning and shear-thickening processes operating in the sub-millimeter-thick near-bed region of negatively buoyant density currents. Beyond supporting recent interpretations that emphasize depositional processes other than simple suspension settling in mudstone formation, these results allow further discussion of fluid density conditions in ancient basins and the evolution of fine-grained density flows. Overall, microfacies analysis provides robust criteria for recognizing contrasting depositional processes in deep-water mudstones, reducing interpretative ambiguity and refining depositional and paleoenvironmental interpretations.