6 Humble Point to Charmouth

 

Cretaceous

66 to 89 million years ago (MYA).

Upper chalk. The Chalk Group exposed within the Jurassic Coast World Heritage Site (WHS) comprises up to 400 metres of chalks composed dominantly of calcareous nannofossils. Traditionally, the succession is divided into the Lower, Middle and Upper Chalk, although this classification is inappropriate in east Devon. On the coast of Purbeck the Chalk forms spectacular cliffs at Bat’s Head and White Nothe between Worbarrow Bay and Lulworth Cove and at Ballard Head. In East Devon it is about 100 metres thick, and is seen within the cliffs between Lyme Regis and Branscombe, and occurs as isolated outliers between Branscombe and Sidmouth. The Upper Chalk consists of variously nodular, flinty, slightly marly and smooth white chalks which display conspicuous bedding that is picked out by lines of flints, marls and beds of harder chalk. Diverse calcitic faunas of bivalves, brachiopods, belemnites, corals, echinoderms, bryozoans, serpulid worms, fish and a few reptiles have been found in the Upper Chalk within the Jurassic Coast WHS. The sections in the vicinity of Beer, and between White Nothe and Ballard are particularly valuable as reference sections for the chalks of southern England.

90 to 94 MYA.

Middle chalk. The Middle Chalk is present throughout the area, the lower part comprising very fossiliferous hard nodular limestone. In east Devon this yields important ammonite faunas of the latest Cenomanian and earliest Turonian age that are not found elsewhere in the UK, in addition to abundant bivalves of the genus Mytiloides. The Chalk in the Beer area includes the famous Beer Stone, a soft inoceramid-rich chalk that hardens into a fine ornamental building stone on exposure to the air. The overlying, softer, New Pit Formation is locally flinty in east Devon and while poorly fossiliferous in the field, nevertheless contains an highly diagnostic and abundant fauna of planktonic foraminifera, ostracods and calcareous nannofossils, all of which can be used for regional and international

correlation.

95 to 100 MYA.

The Lower Chalk comprises marly chalks, often rhythmically bedded, which are rather sparsely fossiliferous in the field, but which contain abundant microfossils typical of the Cenomanian Stage. The basal bed of the Lower Chalk (also known as the Basement Bed or Glauconitic Marl) locally contains an abundant macrofauna of beautifully preserved phosphatised ammonites and other molluscs which are very important for inter-regional correlation. The Lower Chalk shows progressive onlap to the west with a basal conglomerate of the chalk facies younging from West Purbeck (Middle Cenomanian) westwards to Devon (late Cenomanian). The Lower Chalk is absent in west Dorset and east Devon, where it is replaced by a succession of thin, fossiliferous, sandy limestones called the Beer Head or Cenomanian Limestones.

101 to 109 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

186 to 187 MYA

Dyrham Formation. Three tiers: Well exposed below Stonebarrow and Golden Cap, this unit comprises three well-cemented levels within a fine-grained sandstone. Pliensbachian. 

188 MYA

Charmouth mudstone. Green Ammonite Member: Seen below Stonebarrow and Golden Cap, this unit comprises marine shales, which become more silty and less calcareous upwards. Named after the colour of calcite filling ammonite moulds. Pliensbachian: Davoei Zone. Taken as the top unit of the Lower Lias although the topmost bed, Beds 39-41, are referred to the basal Middle Lias, Margaritatus Zone.

189 MYA

Charmouth mudstone. Belemnite Marls: This light grey unit is more calcareous (marly) than the preceding unit. It is characterised by small-scale cyclicity showing as light and dark couplets thought to be due to climatic modifications controlled by precessional orbital changes. The sedimentology and geochemistry of the cycles has been studied in detail. The Belemnite Stone represents the topmost unit. Pliensbachian: Jamesoni and Ibex Zones.

190 MYA 

Charmouth mudstone. Black ven marls: This unit is best seen in Black Ven and below Stonebarrow. It comprises blue-black mudrocks mostly in the form of calcareous shales with occasional thin limestones and nodules. It is famous for ammonites preserved in translucent yellow calcite. (Late Sinemurian: Turneri-Raricostatum Zones).

191 to 198 MYA

Charmouth mudstone. Shales with beef: This unit is well exposed between Lyme Regis and Charmouth and shows a similar rhythmicity to that of the Blue Lias but the thin limestones are mostly missing and probably replaced diagenetically by fibrous calcite or ‘beef’. It is thought to represent a deeper marine facies than the Blue Lias. The unit is also the source of fine fossils, especially reptiles, ammonites and belemnites. (Mid Sinemurian: part of Semicostatum and Turneri Zones).

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.