All of the formations are given here, in chronological order.

MYA = millions of years ago.


ERA

MYA

NOTES

CRETACEOUS

66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89

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 91 92 93 94

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 96 97 98 99 100

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.

101 102 103 104 105 106 107 108 109

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.

110 111 112

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.

113 114 115 116 117 118 119 120 121 122 123 124 125

The Lower Greensand, Gault Clay and Upper Greensand are seen at outcrop within the Jurassic Coast WHS between Sidmouth and Swanage. These exposures display key features of earth history during over c. 15 million years during the Aptian and Albian Stages of the Cretaceous. They represent complex changes in sedimentary environments during the early stages of one of the greatest periods of continental flooding by the oceans that the planet has experienced, culminating in the deposition of the overlying Chalk. The Lower Greensand Formation consists of a grey-black sandstone with ironstone levels. These horizons are fossiliferous, with marginal marine bivalves including Eomiodon, Cuneo corbula, and the gastropod Cassiope, and including ammonites such as Deshayesites spp. The formation includes, at Swanage, the Punfield Marine Band with a unique estuarine fauna, which becomes fully marine on the Isle of Wight.

126 127 128 129 130 131 132 133 134 135 136 137 138 139

The Wealden Group, of early Cretaceous age, comprises a succession of mudstones and sandstones laid down in non-marine (fluvial) environments. Exposures of Wealden strata are of international interest because they provide data on terrestrial environments during the early Cretaceous, a time of major environmental change. The Wealden exposures within the Jurassic Coast WHS are the most stratigraphically extended succession seen within Europe at a single locality. The formation is exceptionally thick, up to 600 metres at Swanage, and thins rapidly to zero west of Lulworth Cove.

140 141 142 143 144

The Purbeck Group consists mainly of fossiliferous thin-bedded limestone and mudstone. It has long been recognised as a distinctive unit, and it differs obviously from the massive Portland limestone beneath not only by the thin-bedding but also by the generally non-marine fauna, rarity of thick-shelled molluscs and absence of ammonites. The lagoonal and lacustrine fauna with vertebrates of continental origin contrasts with the marine faunas of the underlying rocks. The separation from the non-marine clastic Wealden Group above is lithological. Up to 246 numbered beds have been described in some detail, and five geological members can be mapped in the field. Many are named and the ostracod and gastropod content of each is known.

                    JURASSIC

145 146

The Portland Sand and Portland Limestone formations represent the final shallowing on the last major sedimentary rhythm of the Dorset Jurassic (J5) which commenced with the deeper facies of the Oxford Clay; in the overlying Purbeck Group lacustrine and non-marine environments are finally developed. Well-exposed on the Isle of Portland and in Purbeck, the area was recognised as a key area in earliest days of investigation and in 1829 Brongniart coined the term Portlandian by which rocks of this age have been known internationally. The excellent Portland Stone has been quarried and exported nationally and internationally for over three centuries.
The Portland Sand comprises impure siltstones and with oyster beds grading up from the Kimmeridge Clay below; levels are full of rhaxellid sponges. It contains an important ammonite fauna, which provides the last link with Russian faunas before restriction of the Boreal seas. The overlying Portland Limestone has a lower unit full of cherty levels and a Basal Shell Bed, which has yielded rich bivalve and gastropod faunas. The Portland Freestone above is the famous building stone of shelly limestones and oolitic levels with local stromatoporoid occurrences. The giant ammonite faunas have been described at least since 1668; the biostratigraphy has been well described (Wimbledon and Cope, 1978).

147 148 149 150 151 152 153 154 155 156 157

The Kimmeridge Clay is represented in Dorset by a succession of mudrocks of variable organic carbon content, together with minor amounts of carbonate, siltstone, fine-grained sandstone and ironstone. The formation is remarkable in that it has some very high total organic carbon values averaging around 10 per cent but reaching up to about 60 percent. In contrast most mudrocks have a total organic carbon content of 0-2 percent. The individual mudrock beds of variable organic carbon content are generally tens of centimetres thick and reflect the changing chemical, physical and biological conditions in seawater at that time. The beds contain an abundant well-preserved fauna of mainly ammonites, bivalves, gastropods and vertebrates. It is the standard section for the Boreal Lower Tithonian (=Bolonian) ammonite zones and is probably the only section world wide with good exposure of the Rotunda and Fittoni Zones. Recent work on samples from the Kimmeridge Clay Formation in Dorset presents the only radiometric date for the marine Tithonian world-wide. The Eudoxus to Fittoni Zones are exposed uninterrupted between Brandy Bay and Chapman’s Pool. Further sections, particularly of the lowermost zones, occur at Ringstead Bay, Osmington Mills, Black Head and West Weare Cliffs on the Isle of Portland.

158 159 160 161 162 163

Corallian. The deeper water facies of the Oxford Clay give way to shallower-water facies of sandstones, limestones and oolites in the Corallian Beds, the former name of this unit. Excellent examples of a variety of trace fossils can be found in many of the sandstone and limestone units. One of the sandstone units (the Bencliff Grit Member) shows an uncommonly preserved sedimentary structure termed swaley cross-stratification. This is one of the best exposures of such structures in western Europe. The Bencliff Grit Member contains mature hydrocarbons at the western end of the Bran Point exposure. This represents a dissected hydrocarbon reservoir: the hydrocarbons are most likely from the Lower Lias, similar to the nearby producing Wytch Farm and Kimmeridge Bay oil fields. In the middle of the Oxfordian succession there is an oolitic grainstone. The overlying Clavellata Member has a beautifully preserved ammonite/bivalve fauna. The Sandsfoot Grit Member and overlying Ringstead Waxy Clay probably represent a beach barrier complex and associated lagoonal facies. The top of the Corallian Group and base of the Kimmeridge Clay Formation contain a complex succession of very condensed beds with a rich, well-preserved fauna.

164

Oxford clay. The boundary between the Callovian and Oxfordian Stages is well exposed on the coast at Ham Cliff and is well-known for its ammonite faunas and contribution to Oxfordian biostratigraphy. Important individual features include the well-preserved aragonitic ammonites and nodule beds. In particular the Red Nodule Bed exposed at Furzy Cliff is associated with a beautifully preserved three dimensional ammonites and bivalves. The Dorset Oxfordian sections have contributed to significant publications and ongoing work in many fields. The Oxford Clay (Callovian) is hydrocarbon-rich and has produced fine faunas of marine invertebrates and reptiles. Callovian: upper Macrocephalus to Lamberti Zones.

165

The Kellaways Formation and Lower Oxford Clay are poorly exposed along the Fleet shore from East Fleet to near Tidmoor Point. They represent the initiation of the fourth major sedimentary rhythm (J4), which ends with the carbonates of the Corallian Beds (upper Oxfordian).

166

The Cornbrash is exposed at Shipmoor Point, Berry Knap and near East Fleet. This thin limestone comprises a shallow-water facies with abundant brachiopods, bivalves and other fauna. The faunas show a major change within the Cornbrash at which the Bathonian/Callovian boundary is drawn. Lower Cornbrash (Barry Member), Bajocian: Discus Zone: Upper Cornbrash (Fleet member), Callovian: Macrocephalus Zone.

167

Forest Marble. Discontinuously exposed just west of West Bay, between Burton Bradstock and Abbotsbury Castle, and along the shores of the Fleet. The overlying sequence from the mudstones of the Fuller’s Earth through mudstones and calcareous sands of the Forest Marble to the limestones of the Cornbrash represent the third major sedimentary cycle (J3) of the Jurassic and the Bathonian Stage.

168

The Frome Clay Limestone forms the uppermost reservoir in the Wytch Farm Oilfield. It includes richly fossiliferous horizons and fossil oyster beds. Bathonian: Zigzag Zone to lower Discus Zone.

169

Fullers Earth. In the English Channel Basin the formation is predominantly mudstone with carbonaceous material at some levels and shell beds.

170 171

The Inferior Oolite is exposed along the coast on top of the Bridport Sands between West Bay and Burton Bradstock. This thin limestone unit of Aalenian and Bajocian age is a condensed unit but shows a very rich fauna of invertebrate fossils. It is characterised by local unconformities and discontinuities but a remarkably full ammonoid biostratigraphy is known. It is especially famous for the work by Buckman (1860-1929) on the fossil brachiopods and ammonites, both from along the coast and outcrops inland. Buckman used the ammonite sequence to establish a zonation, which can be correlated in many other areas of the world. Following the pioneering studies of Albert Oppel (1831-1865), Buckman’s detailed ammonoid work established a new precision, which led to modern chronostratigraphy. Although much criticised in his day, the detailed work of Buckman has been replicated in recent years by Callomon and Chandler and others. The topmost unit of the Bridport Sands and up to the Yellow Conglomerate is assigned to the Aalenian Beds (4-7 at Burton Bradstock), and Beds 8-16c are assigned to the Bajocian.

172 173 174

Bridport Sands. Best seen between West Bay and Burton Bradstock but also below Thorncombe Beacon. The base of the Aalenian and Middle Jurassic lies in the topmost bed. These microrhythmic sandstones have been much studied at outcrop because they form the middle reservoir of the Wytch Farm Oilfield. The change in age of the sandstones of the upper Lias southward from the Midlands was first documented by Buckman and is an example of diachroneity much quoted in textbooks. Toarcian: Levesquei Zone.

175 176 177 178 179 180 181 182

Down Cliff clay: Best seen in Down Cliff east of Eype Mouth. Thought to belong to the Toarcian Levesquei Zone.
Beacon Limestone: Best seen immediately east of Eype Mouth. These thin condensed limestones comprise the top unit of cycle J1. They show evidence of contemporary fault movement. The Marlstone includes condensed faunas of the Pliensbachian Spinatum Zone and these pipe down into the beds below. The overlying Junction Bed (sensu stricto) comprises condensed faunas of the Toarcian Falciferum to earliest Levesquei Zones. Dyrham Formation (29-176 metres).

183

Dyrham Formation. Thorncombe sands: Best seen below Thorncombe Beacon this unit comprises cross-stratified sands with bioturbated levels. Topmost Pliensbachian.

184

Dyrham Formation. Down Cliff sands: Best seen below Golden Cap and Thorncombe Beacon. These are laminated sandstones with the Starfish Bed yielding the fossil brittle star Palaeocoma egertoni at the base. The Margaritatus Stone forms the top unit. Pliensbachian: Margaritatus Zone.

185

Dyrham Formation. Eype clay: Best seen around Eype Mouth. This is a light grey micaceous clay with nodules and the extraordinarily fossiliferous Day’s Shell Bed within it, which has yielded about sixty species, mostly of molluscs, close to the top. (Pliensbachian: Margaritatus Zone).

186 187

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

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

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

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).

191 192 193 194 195 196 197 198

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).

199 200

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

201 202 203 204 205 206 207 208

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.

209 210 211 212 213 214 215 216 217 218 219 220 221 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237

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.

238 239 240 241 242 243 244 245 246 247 248 249 250 251

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.

252 253

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.