A Tithonian (Upper Jurassic) lake shoreline in Buckinghamshire, UK: Evidence from stromatolite distribution in the Purbeck Formation

Stromatolites in the essentially non-marine Purbeck Formation (Tithonian, Upper Jurassic) west of Aylesbury (Buckinghamshire, UK) cap an erosion surface truncating lacustrine–palustrine carbonates, previously documented from an exposure close to the site of the former Bugle Pit, Hartwell. The stromatolite occurrences in this area follow a regional northwest–southeast structural grain, evidenced by local normal faults, some demonstrably active during Late Jurassic–Early Cretaceous time. Observations from associated lithofacies and biota suggest a littoral lacustrine setting for the stromatolite ‘belt’. We hypothesise that the present-day stromatolite distribution reflects a fault-controlled palaeo-shoreline, and that growth of the stromatolites was linked to lacustrine transgression driven by syn-sedimentary movement on these faults. Littoral sand associated with the stromatolites was possibly reworked from older marine sediments undergoing erosion on a nearby fault footwall, or distally, on the western margin of the Anglo-Brabant Massif.