Wessex OUGS Field Guide to Durdle Door, Dorset, May 2018 The Geology Durdle Door, Dorset Chalk stratigraphy, sedimentology and tectonic structure New marker beds Durdle Cove, Dorset looking west over Scratchy Bottom and Swyre Head to Bat’s Head. The line of caves at the foot of the cliff in Durdle Cove is formed on the Durdle Cove Thrust (see also Rowe 1901, Plate III, pp. 16-17). Open University Geological Society Wessex Group Field Excursion Sunday 13th May 2018 Leaders: Rory Mortimore and Jeremy Cranmer Field guide prepared by Rory Mortimore www.chalkrock.com Based on the paper Late Cretaceous stratigraphy, sediments and structure: gems of the Jurassic Coast of Devon and Dorset, England just going into press in the Proceedings of the Geologists’ Association, 2018. 1 Wessex OUGS Field Guide to Durdle Door, Dorset, May 2018 Wessex OUGS Field Trip Durdle Door, Durdle Cove, Scratchy Bottom and Bat’s Head Late Cretaceous stratigraphy, sediments and structure: gems of the Jurassic Coast of Devon and Dorset, England Introduction Extraordinary, long-distance litho-marker beds such as the Lewes and Shoreham Tubular Flints and associated marl seams and fossils (Fig.2), recognised in cliff exposures and cliff-fall boulders, are keys to unlocking the stratigraphy and tectonic structures in the Late Cretaceous (Fig.1) of the Jurassic Heritage Coast. Durdle Cove is a special gem exposing the Lewes and Seaford Chalk stratigraphy where these and new marker beds are identified and where sediments and tectonic structures provide clues to timing of movements that produced a Late Cretaceous pericline which grew into a Miocene monocline along the line of the underlying Purbeck Reverse Fault. During ‘inversion’ along this fault (Fig.4) some Late Cretaceous Chalk formations were in part or completely ‘lost’ (e.g. Middle Turonian New Pit Chalk Formation) and others were condensed (e.g. Late Santonian and Early Campanian Newhaven Chalk Formation). Other features of this section include the presence of the very large bivalve fossil Platyceramus sp. which provides clues to Chalk sea-floor environments. Figure 1. Cretaceous (D’Halloy, 1822) series and stages after Birkelund (1984) Although much of Britain was covered by a shallow shelf sea during the Late Cretaceous, Wessex, including the Jurassic Coast, provided shallower conditions than the main basinal areas to the east. A rich, shelly, shelf-fauna especially in the Albian, Cenomanian and Early Turonian provides many 2 Wessex OUGS Field Guide to Durdle Door, Dorset, May 2018 palaeontological gems including ammonites rarely preserved in more basinal areas associated with a rich diversity and abundance of echinoids and brachiopods in highly condensed successions. Figure 2. Stratigraphical position of South Dorset and Durdle Door Upper Cretaceous Chalk sections in relation to the standard ‘basinal’ stratigraphical column in the Wessex Basin and eastwards. A special Cretaceous gem of the Jurassic Coast is the spectacularly folded Chalk forming the Ridgeway and Purbeck Hills in south Dorset within which there is a history of tectonic movements spanning the Jurassic to Cenozoic (e.g. Underhill and Patterson 1998). The spine of this folding is located in the near vertical to overturned bedding seen in all the bays from Ballard Point through Worbarrow and Mupe Bay to Lulworth Cove and Durdle Cove. Within the folded Chalk the smaller- scale structures, especially the thrusts and inclined shear fractures, have been the subject of several studies. Arkell (1936, 1938, 1947), classified the fractures in the Chalk into seven groups and these were given a genetic interpretation related to the evolution of the Purbeck fold by Phillips (1964). 3 Wessex OUGS Field Guide to Durdle Door, Dorset, May 2018 However, Bevan (1985) and latterly House (1989, 1993), recognised that the Arkell classification and Phillips’ interpretation was over-complicated as the conjugate, inclined fractures probably predated the monoclinal folding having formed through bedding-slip during the Late Cretaceous and then been rotated into their present position during later Cenozoic folding. These structures are comparable to similar fractures and subsequent rotation in folds seen in Sussex and elsewhere (Mortimore, 2011). Further studies of the meso-fractures and faults (Ameen, 1990; Ameen and Cosgrove, 1990) have emphasised the origins of the Purbeck fold as a ‘forced’ anticline due to uplift over the Purbeck Reverse Fault causing folding in the ‘cover’ rocks (Fig. 3). They also illustrated that the famous Ballard Head Thrust Fault (e.g. Rowe, 1901 Photo Plate VIII), formed prior to the main monoclinal folding as a thrust ramp structure which has itself been subsequently folded in the monocline (Ameen and Cosgrove, 1990, 1991). Figure 3. Map showing the broadly east-west line of faults (Purbeck, Needles, and Sandown faults) which underlie the steeply dipping north monoclonal folds and along which inversions from pre- Gault-Greensand (Albian mid-Cretaceous) basins and highs (Portland-Wight Basin and Hampshire- Dieppe High) became the Portland Wight High and Hampsire Basin in the Late Cretaceous – Cenozoic. Based on Chadwick and Evans 2005. LC=Lulworth Cove WB=Worbarrow Bay 2. Albian – Cenomanian Gault, Greensands and Grey Chalk palaeogeography Within the narrow strip of Gault, Upper Greensand and Grey Chalk coastal exposures in Dorset between Punfield Cove in the east and Lulworth Cove in the west is a remarkable change in lithologies and thicknesses of these deposits . These lateral changes, recognised by Strahan (1895, 1898), have been investigated in detail (Tresise, 1960, 1961; Drummond, 1967, 1970) and the causes related to a contemporaneous tectonic structure between Mupe Bay and Lulworth Cove. A parallel study in inland Dorset, showing similar lateral variations from east to west, has led to the idea of a tectonically controlled Mid-Dorset Swell between a Wessex shelf further to the west and a basin to the east (Drummond, 1967, 1970; Kennedy, 1970; Bristow et al., 1995; Gallois, 2004). House (1989, p. 99) considered that east-west growth faults were a more likely cause of the lateral changes including thinning in the sediments (see also discussion in Hart, 1994). Whichever interpretation is correct the stratigraphic and sedimentary complexity creates a special, multifaceted-geological-gem, which requires detailed centimetre by centimetre analyses to fully unravel. Latterly, Gallois and Owen (2018) have provided detailed measured field sections illustrating the stratigraphy and lithology of the Albian Gault and Upper Greensand in the Jurassic Coast. A special feature of the South Dorset coastal area is the presence of an extensive hardground, mineralised with calcite, green galuconite and orange iron staining at the boundary between the Albian Upper 4 Wessex OUGS Field Guide to Durdle Door, Dorset, May 2018 Greensand and the Cenomanian Grey Chalk (simultaneously the boundary between Early and Late Cretaceous series, Fig.1). This Worbarrow Bay Hardground (Fig. 4), contains extensive Thalassinoides burrow networks . In many places, bioerosion has reduced the top layer of this hardground to a rubble of bored and mineralised nodules in a silty glauconitic matrix. Figure 4. Durdle Cove section 1 Albian Upper Greensand, Cenomanian Grey Chalk, Plenus Marls and basal Melbourn Rock and Turonian Holywell Nodular Chalk with the Grit Beds. 3. Cenomanian Grey Chalk from basin to shelf and rare fossils The relatively simple Grey Chalk succession in the Wessex Basin and its extension into the Wealden area with two formations (West Melbury Marly Chalk and Zig Zag Chalk formations, Fig.2) becomes highly complex in the condensed, marginal facies of the Wessex Shelf. What makes the highly condensed sections of the shelf a particular Cretaceous ‘gem’ of the Jurassic coast is the abundance and richness of a diverse fauna and the preservation of otherwise rare ammonites, especially the key index ammonites of the Late Cenomanian (Neocardioceras juddii ) and the Early Turonian (Watinoceras devonense (Wright and Kennedy)) on top of the Haven Cliff Hardground in southeast Devon (Wright and Kennedy, 1981). The diversity of echinoids and brachiopods in a shelly, shelf fauna within a gritty sandy matrix has made field sections at Seaton Hole and Wilmington Sand Pit famous. In other exposures along the Dorset coast such as Worbarrow Bay and White Nothe much of the information comes from cliff-fall boulders. 4. Late Turonian and Early Coniacian Lewes Nodular Chalk Formation Like the Holywell Nodular Chalk the nodularity and hardground development in the Lewes Nodular Chalk increases/strengthens westwards from Sussex-Hampshire into south Dorset and southeast Devon. Such lithological development probably relates to extensive submarine winnowing and erosion across the relatively shallower seas of the Wessex Shelf. Despite the condensed sections and increased nodularity many of the key marker beds including marl seams, flint bands, hardgrounds, 5 Wessex OUGS Field Guide to Durdle Door, Dorset, May 2018 trace fossils and fossil abundance horizons can be recognised in south Dorset and southeast Devon (Figs. 6-18.). A particularly well developed mineralised phosphatic hardground present in Durdle Cove at the entrance to Scratchy Bottom is named the Durdle Door Hardground (Figs. 15 and 18). With more research it may be possible to correlate the Durdle Door Hardground with the Sussex succession (possibly the Hope Gap Hardground) as both hardgrounds are associated with