2 Straight Point to High Peak

 

Cretaceous

101 to 109 million years ago (MYA).

The Upper Greensand Formation within the Jurassic Coast World Heritage Site (WHS) is a glauconite-rich succession of sandstones and calcarenites, with thin sandy limestones (usually concentrations of shell debris) and dark brown splintery cherts. In east Devon the chert-rich sandstones reach their maximum thickness (c. 25 metres). In this area, the cherts are characterised by the presence of sedimentary bedding. Trace fossils and other macrofauna visible within the silica concentrations confirm that the cherts are replacement features within the diagenetic history of the sediment. The uppermost sandstones in west Dorset and east Devon are characterised by glauconite-rich cross-bedded sandstones that form a quite distinctive building stone. The macrofauna is dominated by the bivalves, especially Exogyra spp., although gastropods and echinoids are also well known. The Perinflatum Subzone is an important phosphatised horizon at the top of the formation in Punfield Cove. The microfauna is quite restricted due to preservational problems, although the sandstones of east Devon have yielded Orbitolina sefini.

110 to 112 MYA.

The Gault Clay and the Upper Greensand formations within the Jurassic Coast WHS are generally sandy in character. At Swanage and in the Lulworth area the Gault is a poorly-fossiliferous silty clay and the sand content increases both westwards and up-succession. In east Devon the Gault is inseparable from the overlying Upper Greensand. Ammonites are often abundant: over 100 species have been recorded from the Gault and Upper Greensand in Purbeck. Microfossils are quite rare, with much of the formation decalcified and badly weathered.

Jurassic

199 to 200 MYA

Blue Lias. This unit is well exposed near Lyme Regis. The Triassic/Jurassic boundary is drawn 2.5 metres above the base of this unit which comprises small-scale rhythms of anoxic shale (produced under low oxygen conditions), oxic shale and thin limestones. The limestones are full of neritic and benthic fossils indicating small shallowing-upward rhythms probably resulting from environmental control following orbitally-forced climatic changes. The unit is richly fossiliferous. A level at the top (Saurian Shales) was a major source of fossil reptiles. The limestones were formerly a source of hydraulic cement. The Hettangian Stage is represented by the lower half of the Blue Lias (Planorbis Zone to Angulata Zone) and the Sinemurian commences in Bed 21 with the Bucklandi Zone. Note that the base of the Jurassic still has to be defined by the International Union of Geological Sciences (IUGS). Succeeds the Penarth Group, probably without a significant stratigraphic break; it comprises grey, fossiliferous, calcareous mudstones and limestones of marine origin. The base of the Jurassic is placed within Blue Lias Formation at the stratigraphic appearance of ammonites of the genus Psiloceras. This major stratigraphic boundary is exposed in the section between Pinhay Bay and Seven Rock Point, west of Lyme Regis at a level 2.5 metres above the base of the group. Beds in the group below this level (Pre-planorbis Beds or Ostrea Beds) are assigned a latest Rhaetian (latest Triassic) age.

Triassic

201 to 208 MYA

Penarth group. Exposed discontinuously in the faulted and landslipped area between Culverhole Point and Pinhay Bay; it rests disconformably upon the Mercia Mudstone. The group comprises black, fossiliferous shales (Westbury Formation) overlain by grey-green, brackish-water to marine mudstones and marine limestones (Lilstock Formation) and is dated biostratigraphically as Rhaetian age.

209 to 237 MYA

Mercia Mudstone. Exposed between Sidmouth and Branscombe, between Seaton Hole and Culverhole Point, and at Charton Bay; it succeeds the Otter Sandstone conformably. It is overlain, above an angular unconformity, by westward overstepping Cretaceous rocks; continuity of exposure of the group is broken between Branscombe and Seaton by a combination of faulting and the effect of this unconformity. Between Weston Mouth and Branscombe, and at Charton Bay, exposure is affected by landslip. The group consists largely of red-brown mudstones, with some grey-green or silty beds. Fossiliferous dolomitic sandstones and grey-green mudstones (Weston Mouth Sandstone Member) are exposed around Weston Mouth, and higher beds near Branscombe contain large amounts of gypsum. The highest unit in the group, seen east of Seaton, comprises mainly grey-green sediments (Blue Anchor Formation). The dominant sediments of the group accumulated in playas and sabkhas under subaerial and subaqueous conditions; water was of mixed continental and marine origin, and evaporitic conditions resulted in the formation of gypsum and, elsewhere in the Wessex Basin, halite. The Weston Mouth Sandstone represents a brief estuarine episode. The Blue Anchor Formation represents a transition from dominantly continental to dominantly marine influences. Magnetostratigraphic work indicates that the base of the group is Ladinian (Late-Mid Triassic) in age. Carnian-Rhaetian ages are indicated by palynomorphs from the middle and upper parts of the Group.

238 to 251 MYA

Otter Sandstone. Rests disconformably upon the Pebble Beds and consists largely of fine to medium grained sandstones, some of aeolian origin but most deposited by shallow, northward-flowing, braided rivers. Remains of arthropods, fish, amphibians and reptiles represent the fauna of a range of terrestrial and fresh-water habitats and indicate an Anisian (early Mid-Triassic) age, which is supported by magnetostratigraphic evidence. Internationally important reptile remains.
Budleigh Salterton pebble beds. Exposed at Budleigh Salterton. They rest unconformably upon the Aylesbeare Mudstone Group and comprise gravels and sands deposited in a northward-flowing, braided river system. Some of the pebbles contain fossils indicative of provenance in outcrops of Ordovician and Devonian rocks. At the top of the Pebble Beds a ‘reg’ type palaeosol with numerous ventifacts represents subaerial exposure. The unit has no indigenous fossils; its age is constrained by those assigned to the under and overlying deposits.

252 to 253 MYA

Aylesbeare mudstone group. Exposed between Orcombe Rocks and Budleigh Salterton. It consists largely of mudstones, which accumulated in a sabkha-playa environment; a minor component is sand, which accumulated in small aeolian dunes.