Unravelling the Nature and Origin of Jurassic to Early Cretaceous Unconformities Offshore Southwest Britain

ABSTRACT Jurassic and Early Cretaceous times were marked by significant changes in Earth's climate and tectonics, most notably the breakup of the supercontinent Pangaea, which led to the opening of the Atlantic Ocean. In Southwest Britain, one of the most prominent features of this time is the Base Cretaceous unconformity representing widespread erosion and non‐deposition separating Cretaceous strata from underlying rocks. Despite its widespread presence in Southwest Britain, Iberia, Ireland and conjugate North Atlantic basins, the origin and nature of this unconformity remains enigmatic. To better understand its nature, seismic data was used to map the extent of the unconformities and to establish their relationships with onlapping Jurassic and Cretaceous stratigraphy. We reveal that the Base Cretaceous unconformity is a composite of at least three—Middle Jurassic, Late Jurassic to Early Cretaceous and Mid‐Cretaceous—unconformities likely generated by erosion and non‐deposition. The Mid‐Cretaceous unconformity is often assumed to be responsible for the majority of erosion, but our findings suggest otherwise. Onlap patterns of the Lower Cretaceous Wealden Formation on truncated Jurassic units indicate that the Jurassic to Early Cretaceous unconformity was the most significant. Amplitudes of uplift across different basins in SW Britain are shown to be variable. The most substantial denudation occurred following Berriasian uplift, likely linked to shortening associated with North Atlantic opening. The Mid‐Cretaceous unconformity is more subtle, primarily observed at basin margins and linked to the rift‐drift transition of the Bay of Biscay. Subsidence histories differ across basins; some (e.g., Brittany Basin) can be explained by simple post‐rift thermal cooling models, while others (e.g., Melville and South Celtic Sea Basins) require more complex explanations due to substantial missing stratigraphy, including reactivation of Variscan thrusts and sub‐plate support. Our results emphasise that spatially and temporally distinct tectonic and denudation events can combine to generate large‐scale composite unconformities.