The uppermost Triassic Cotham Member of the Lilstock Formation preserves two regionally extensive microbialite horizons – the well-known stromatolitic Cotham Marble and the older, largely overlooked Estheria Bed – offering new insights into environmental change during the Rhaetian. Integrated field, facies and literature review data demonstrate that both horizons are far more geographically widespread than previously recognized, extending across multiple depocentres in southwest England and the English Midlands. The Estheria Bed comprises thrombolitic–stromatolitic bioherms with variable mesostructures, including vugs, grainstone infill and locally preserved lamination, indicating fluctuating hydrodynamic and early diagenetic conditions. The Cotham Marble exhibits a consistent architecture of laminated stromatolites and columnar branching forms, initiated by shallow burrows or borings and variably accompanied by Microtubus microfossils. Combined evidence from microbial fabrics, associated fauna, sedimentary structures, geochemistry and biomarker studies indicates deposition within a laterally extensive, shallow freshwater–brackish lagoon at least 9500 km2 in area. This lagoon experienced two mudstone-dominated packages culminating in microbialite formation. Together, the Cotham Marble and Estheria Bed provide useful horizons for regional correlation, revealing a dynamic lagoonal system sensitive to tectonic and hydrological change.