Location of the Isle of Portland View north from Portland along Chesil Beach
Palaeogeography of the Jurassic Pangaea rifting to form Gondwana and Laurasia. Tethys begins to segregate from Panthalassa ocean at this time forming a wedge shape in the east. North America begins to rift from South America and Africa. Gondwana begins to rift into eastern and western components and Tethyan seaway forms linking Tethys and the Pacific. Northern and central Atlantic now take shape.
Kimmeridge Clay Formation Classic black shale. Laminated Mono/bispecific benthic (sea floor) communities Pelagic (water column) fauna: ammonites, Plesiosaur, Saccocoma (pelagic crinoid) Pyrite nodules, pyritised shells and phosphatised fish scales
Kimmeridge Clay Formation Generally anoxic/dysoxic sea floor with euxinic periods. Periodic oxygenation Fossil identification: Gallois 2007, Townson 1979
Portland Sandstone Formation Outcrops inaccessible Represents fall in sea level (regression) Documented the absence of stenohaline organisms (belemnites, crinoids and corals) (Townson 1975, Meville & Freshney 1982) Abnormal salinity? (Leeder 1999, Townson 1975)
Portland Limestone Formation Cherty Series Micritic limestone with scarce benthos Chert formed from sponges Storm deposited shells Soft sediment above wave base, periodic storms
Portland Limestone Formation Cherty Series being quarried for aggregate, Crooks Quarry Horizon of Titanities Bedded chert “Icicle” horizon Nodular chert
Portland Limestone Formation Freestone Series Oolitic limestone, sparse fine shell debris Oysters, bivalves, gastropods Titanites ammonite Patch reefs with oysters, Solenopora (sponge) and lithophagid (boring) bivalves Large subaqueous dune system behind main ooid shoal Shallow marine environment with wave influence
Portland Limestone Formation Roach Top of the Freestone Series Aptyxiella (“Portland Screw”) with Trigonia (“Horse’s Head”) and oysters Current aligned Aptyxiella Occurs in north west only Strandline marking sea level highstand or storm event (Townson 1975) OR Colonisation in sheltered environment behind patch reefs
Purbeck Formation Micritic limestone deposited in lagoonal environment Stromatolites (algal mounds) indicate hypersalinity, low energy and scarce grazing Ostracod, bivalve and gastropod occurrence mutually exclusive with stromatolites Evaporites, similar to modern day sabkha environment
Purbeck Formation Purbeck Formation, Mutton Cove Skull Cap Top of Portland Limestone (with Roach) Hard Cap (stromatolites) Basal Dirt Bed Lower Dirt Bed Great Dirt Bed (soil with wood and charcoal) Soft Cap (Fossil Forest) Broken Beds (evaporites) Hard Slatt (dinosaur footprints)
Purbeck Formation Seasonal climate can be seen in lagoonal sucession
Purbeck Formation Dinosaur footprints, burrs, silicified wood and palaeosols (fossil soil) Calcrete and charcoal in soil indicates semi-arid, but seasonal climate (Francis 1984) Lagoon with stromatolite barrier at seaward side. Seasonal semi-arid climate with intense summer evaporation. Tide and storm influenced
Dinosaurs of the Purbeck Ornithopod Small sauropod Two-toed theropod Three toed theropod Iguanadon Dromaeosaur?MegalosaurusDiplodocus Footprint identification reference: West 2006, Benton 2005
From field observations to palaeoenvironment reconstruction Walthers Law “Stratigraphic sequences vertically adjacent to one another would have occurred laterally adjacent to one another in the geological past” Prinipal of uniformitarianism “The present is the key to the past”
References http://www.medair.org.uk/images/uk_map.jpg http://www.britannica.com/eb/article-70644/community-ecology Benton, M.J. 2005. Vertebrate palaeontology. Blackwell Publishing, Malden, Oxford & Carlton. 455 PP Francis, J.E. 1984. The seasonal environment of the Purbeck (Upper Jurassic) fossil forests. Palaeogeography, Palaeoclimatology, Palaeoecology, 48, 285-307 Gallois, R.W. 2007. Report on the geology of the Portland Gas Storage Site, Upper Osprey, sle of Portland. Gallois Geological Consultancy report for Portland gas Ltd, 70PP Leeder, M. 1999. Sedimentology and Sedimentary Basins: From Turbulence to tectonics. Blackwell Publishing, Malden, Oxford & Carlton. 592 PP Meville, R.V. and Freshney, E,C, 1982. British Regional Geology-The Hampshire Basin and adjoining areas. 4 th ed. Her Majesty’s Stationery Office, London, 132 PP Townson, W.G. 1975. Lithostratigraphy and deposition of the type Portlandian. Journal of the Geological Society of London, 131, 619-638 Townson, W.G. 1979. Information for a geological field excursion to Dorset, Shell U.K. Exploration and Production.38pp West, I.M. 2006. The Isle of Portland: Portland Bill. Geology of the Wessex Coast (part of Jurassic Coast, Dorset and East Devon World Heritage Site). Internet field guide. http://www.soton.ac.uk/~imw/portbill.htm. version 25 December, 2006.