Dorset Overview
The coastline of Dorset has attracted the attention of geologists for two centuries because the exposure and extent of the fossiliferous Jurassic rocks is quite possibly the best in the world. Together with the coast of East Devon, the Dorset coast was granted UNESCO World Heritage Site status in December 2001, and is known as the Jurassic Coast. However, the entire geological succession includes spectacular Triassic, Jurassic and Cretaceous rocks, representing a range of palaeoenvironments from arid deserts, through lush floodplains to warm tropical lagoons and seaways. These Dorset coastal exposures allow us to easily access and study 150 million years of geological history and to gather evidence for changing environments and the organisms that lived there. The cliff, bays and beaches also demonstrate the close connection between the underlying geology and the coastal landforms and hazards that form as a result.
Lyme Regis: the Blue Lias shallow seaway
We start the virtual field trip at Lyme Regis towards the western end of the Jurassic Coast world heritage site. As the Wessex Basin sedimentary succession generally dips to the east, the exposed stratigraphy along the coast becomes younger in that direction, and in effect we are able to ‘walk’ eastwards through time. This allows us to travel through over 150 million years of Earth history, from the Triassic (~240 Ma) in the west to the uppermost Cretaceous (~75 Ma) in the east. If we travelled further towards Bournemouth and the Isle of Wight we would encounter another 50 million years of Earth history, taking us well into the Cenozoic.
To the west of Lyme Regis there are latest Triassic rocks that record the transition from non-marine Triassic environments to the initial marine flooding of the UK, which established warm shallow seaways that persisted for most of the Jurassic, spanning a time interval of around 50 million years. This part of the UK was a subsiding, depositional basin (Wessex Basin) for much of this time and saw the accumulation of a range of different sediment types reflecting varying relative sea levels and climate. Time intervals of relatively higher relative sea level and deeper water were characterised by mudrock-dominated successions, e.g., the lower Jurassic Blue Lias, middle Jurassic Oxford Clay and upper Jurassic Kimmeridge Clay. Intervals of lower relative sea level and shallower water were characterised by either sand-dominated successions, e.g., the lower Jurassic Bridport Sands, or by carbonate-rich successions, e.g., the middle Jurassic Inferior Oolite and upper Jurassic Corallian and Portland Limestone.
The Blue Lias section at Lyme Regis allows us to look at the earliest part of the Jurassic marine history of the basin. More specifically, this locality allows us to examine a succession of alternating shallow marine clay-rich and carbonate-rich sediments and to think about the potential controls on these sediment compositions and how quickly they might have accumulated.
Triassic - Jurassic Boundary Section by NHM_Imaging on Sketchfab
Ammonite pavement section by NHM_Imaging on Sketchfab
The Isle of Purbeck: from tropical lagoons to coastal river floodplains
At the eastern end of the Jurassic Coast is the Isle of Purbeck, which is made up of the youngest Jurassic and Cretaceous rocks of the Dorset succession (Kimmeridgian-Campanian), including the final filling of the Jurassic marine seaway. This is the story of shallowing relative sea-level, from the relatively deep-water, clay-rich Kimmeridge Clay succession to the shallow marine Portland Limestone, and finally the termination of marine sedimentation by an unusual lagoonal and lacustrine succession, the Purbeck Limestone of earliest Cretaceous age. After this, the area saw terrestrial environments for 20 million years, with a sequence of river floodplain deposits known as the Wealden. Marine environments were re-established once more by a mid-Cretaceous transgression (Aptian, 125 Ma) which saw the accumulation of shallow water sandstones (called ‘greensands’, because of the abundance of the mineral glauconite) followed by deeper marine sediments, including a thick succession of deeper water limestones, known as the Chalk. This succession is beautifully exposed at Durdle Door and Lulworth Cove, where the geology has shaped the geomorphological evolution of the coastline to form a series of stacks, arches, proto-coves and coves. The Kimmeridge Clay is best seen at Kimmeridge Bay and the Portland and Purbeck limestone are best accessed on the Isle of Portland where the building stone of the Portland Freestone has been quarried since Roman times.
The Isle of Purbeck succession allows us to examine a wide range of clastic and carbonate sedimentary rocks and to use sedimentological and palaeontological observations to reconstruct the rapid changing environments.