A study of the Coll de Montllobar cliffs to understand the updip extent of transgressive–regressive events and how they affect transitional zones

The Tremp–Graus Basin, located in Northern Spain, is a foreland piggyback basin situated at the foot of the Pyrenees Mountains, north of the Montsec Thrust belt. The basin is the result of an east–west trending synclinal area between the Boxial and Montsec Thrusts, located north and south of Tremp, Spain, respectively. Toward the end of the Cretaceous Period, syntectonic sedimentation began and continued through the Miocene, resulting in the formation of the basin. The Tremp–Graus Basin is structurally open in the west and closed in the east, possibly due to structural events and the presence of a shallow sea that inundated the area during the Ypresian (56–47.8 Ma). The Pyrenees Mountains to the north served as the main sediment source as the thrust sheets became proximal to the basin. The Boxial Thrust supplied localized alluvial fans during the Maastrichtian (72.2–66 Ma) (Arevalo 2022, Busquets 2022).

This study focuses on a vertical section in the Coll de Montllobar cliffs to identify sequence boundaries in outcrop. Analyzing high resolution sedimentary architecture as the rocks shift from continental/terrestrial deposits to marine-influenced deposits within the Upper Àger Group through the Lower Montanyana Group is necessary to understand how channel characteristics differ through vertical section. The study includes six detailed measured sections, fluvial mapping conducted using drone footage, 3D modeling, and stable isotope analyses on carbonates to infer paleoclimate and depositional environment. The conclusion of the study will provide insight to the updip extent of transgressive events within the region, describe how transgressive–regressive cycles influence channel behavior, and define characteristics of fluvial channels as the system transitions from tidal influence to non-tidal influence.