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Devon Geology

Field Days in Devon

IGI regularly take groups to geological localities in Devon (and north Somerset), typically as part of geochemical training courses for oil companies or as guides for A-level school parties. Most of our staff are familiar with these localities, and a suitable field excursion based at Hallsannery in North Devon can be built from the following examples, or with the inclusion of other localities not listed below. Relevance of these field excursions to a basin modelling course are included in the descriptions.

Sedimentology of the Culm: Syn-orogenic sedimentation

Visiting the Taw-Torridge estuary with spades, not hammers, modern mud flats with sand bars and meandering channels can be investigated. Modern features such as roots, worm burrows, channel erosion, lag deposits and slumping can be contrasted with similar palaeo-features in the cycles of Carboniferous (Namurian to Westphalian) 'basin-fill' sandstones and mudstones of the Bideford Formation at Westward Ho!. The Carboniferous depocentre is generally termed the Culm Basin and was located on the continental shelf lying north of the rapidly closing Proto-Tethyan (Rheic) ocean. The sandstones and shales of the foreshore and cliff record a delta building out into the Culm Basin, with quiet-water rippled mudstones contrasting with high energy burrowed delta-top sequences. Eight or nine repeats of these cyclic sequences are seen.
Modelling theme: Building a 1-D model - stratigraphy, sedimentation, compaction

Compressional tectonics: Syn-orogenic folding and faulting

Travel to Hartland Quay to see the turbidite facies of the Crackington Formation, the deeper water facies of the Culm Basin: ripples, flutes and tool marks show a dominance of east-west axial currents in the trough. Compressional tectonics are illustrated by differing fold styles, faults and thrusting. On to Millook Haven to find graded turbidites and marine goniatites deeper in the Crackington Formation, and to view spectacular horizontal zig-zag folds. Return via Upton (just south of Bude) to see the massive shallow water welded sands and associated early extensional and later compressional styles in the overlying Bude Formation.
Modelling theme: Temperature calibration, geothermics (temperatures, gradients, heat flow, thermal conductivities), mixing lithologies, scales of anisotropy.

Late-orogenic intrusions and mineralization

During the journey to Mid Devon, the roof of the 'Cornubian granite' massif is exposed on Dartmoor. Though primarily feldspar, quartz and micas, a variety of granite types are recognised, with varying ?flow textures and mineralogy. Sedimentary xenoliths are seen within the granite mass and the contact zone and metamorphic aureole is also well exposed. Evidence for the migration and expelling of exotic mineralising fluids is preserved, with old mine dumps offering opportunities to collect associated mineral samples.
Modelling theme: Temperature - Radiogenic heat flow component and hydrothermal flux

Post-orogenic New Red Sandstone molasse (including Devonian carbonate reefs)

Along the South Devon coast (Torquay), mid Devonian carbonate reef build-ups possibly centred on sub-aqueous volcanics (pillow lavas) show off-reef, reef-front, reef-top and back-reef facies. At Shaldon Ness, Permian coarse clastic conglomerates illustrate high energy flash floods eroding the reefs, pillow lavas, the lost 'roof' of the granite batholith as well as 'country' rock. These alluvial fans fringed the Variscan mountains, and moving on to Dawlish, a contrasting lower energy mid- and distal fan facies can be seen. These are interbedded with aeolian sands, good analogues for the gas reservoirs of the Southern North Sea.
Modelling theme: Application of modelling - migration to reservoirs, and timing of hydrocarbon charge to trap.

Post-orogenic transgression

Along the North Somerset coast, the Triassic to Lower Jurassic transgressional sequence (Norian - Rhaetian - Hettangian) is perfectly exposed. This site records a dramatic change in sedimentary facies and environment and has been proposed as a Global StratoType location contrasting changes in bio-, litho- and chronostratigraphy. The section starts with rarely exposed Norian-hosted evaporites (gypsum) in a low energy, meandering fluvial system attesting to the erosion of the Variscan hinterland to near base level. This red-bed landscape was then flooded, forming a series of lagoonal laterally discontinuous shales and algal carbonates associated with massive beds of single species bivalves and including the winnowed Rhaetian 'bone beds'. Once fully marine conditions were established, the organic rich shales of the Lias (Lower Jurassic) produce an ideal opportunity for a discussion on petroleum source rocks, maturity, generation and petroleum systems. Overall, the transgressional sequence allows collection of bivalves, brachipods, ammonites, and corals. Structural inversion tectonics (normal, reverse, thrust and strike-slip faults) of the Mesozoic Bristol Channel Graben are exposed in cliffs (cross-sections) and foreshore (map view) all along the coast and lead to a discussion of the trapping potential of the numerous exposed structures.
Modelling theme: A historical review of kerogens and kinetics leading to generation and expulsion and entrapment of hydrocarbons.