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Pre-meeting field trip (13th December 2019)

PORTISHEAD (UK); A journey from continental to marine in clastic sedimentology.

Leaders: Dan Le Heron (Univ. Vienna, Austria) & Amy Gough (RHUL)

Aims

To study (i) some excellent outcrops of Late Carboniferous to early Devonian age (Portishead) in the morning and (ii) Middle to Late Triassic and Jurassic deposits (in Aust Cliff, Gloucestershire- time permitting) in the afternoon.

Itinerary

Morning: a traverse from Battery Point to Woodhill Bay, Portishead. Carbonates occur at the Battery Point sections  and include a rich variety of marine fossils, among which crinoids, brachiopods, bivalves, fish scales, eurypterids and a number of different types of ichnofossils occur. Stratigraphically above, and along the coast to the south, an excellent section of cross-bedded sandstones and conglomerates crop out both as tabular beds and lenticular geometries separated by mudstone beds. Our aim is to study both the carbonates and the siliciclastic rocks in this classic location. It is hoped that the mix of lithologies and environments will keep everyone happy, and we can consider the nature of the contact between the two.

Lunch:   and ensuing interpretative disagreements, will be at one of the local pubs within short walking distance from the outcrops. Lunch at the participants’ own cost.

Afternoon: Time permitting, we will visit the famous Aust Cliff section near the Severn Bridge, which is famous for its reptile, teeth and fish remains in certain Triassic levels. In spite of the short days in December in Bristol (sunset 16.01 on 13th December) expect to have some good time to visit this section after lunch, provided that no-one orders 3 course dinners. The Aust Cliff has excellent fossil hunting potential.

Logistics

Bus leaves from The Hub, Royal Holloway main campus, at 8.00 am. Bus to arrive back at Royal Holloway @18.00 (approx.). Note for safety: We will be studying coastal sections in the vicinity of Portishead. These will be coastal sections along the Bristol Channel. We will time our arrival to be mid tide. Predicted tide times for Woodhill Bay on 13th December are: High tide 7.12 am; low tide 2.18 pm. Participants arriving independently please meet at 10 am at the junction between Esplanade Road and Beach Road on the seafront. There should be public toilets available.

Duration: 1 day

Minimum / Maximum participants =  10 / 30

Cost: £25 per person

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Post-meeting field trip (16th – 17th December 2019)

Purbeck, Wessex Basin UK; Syn-rift lacustrine carbonates: cycles, microbial mounds & brackish-hypersaline facies.

Leaders: Arnaud Gallois (CGG, UK) &  Dan Bosence (RHUL)

Aims

The non-marine carbonates of the Purbeck limestones were deposited in a shallow lake on the western margin of the Wessex Basin during its late syn-rift phase. New research at RHUL demonstrates that an early brackish phase is characterised by in-situ highly porous thrombolitic microbial mounds. In cross-section, they show tabular-shaped small mounds (up to 50 cm high and 1 m across) that constitute large, more complex-shaped mounds (up to 4m high and 18m across). In plan-view these mounds have a circular shape up to 20 m in diameter and are surrounded and onlapped by an inter-mound packstone-grainstone facies. Together those facies are arranged in high-frequency lacustrine cycles capped by paleosols.

Itinerary

1st day: Waking up so early will be difficult in a cold winter morning but the rocks we will see are worth it. On this first day, we will visit 4 disused quarries across the Isle of Portland (2 in the morning and 2 in the afternoon).

  At Portland Bill we will see the transition between marine deposits of the Portland Limestone Group (oolitic grainstone with marine bivalves) organised in massive beds and lacustrine deposits of the Purbeck Limestone Group (wackestone-packstone with peloids and some ostracods) organised in thin laminated beds with paleosol intercalations.

  At Tout Quarry we will have an introduction to the succession with Portland-Purbeck boundary and the basal Mupe Member with Skull Cap, Lower Dirt Bed, Hard Cap, Great Dirt Bed, Soft Cap and Cypris Freestones. Overview of microbial mounds, their geometries and their relationships with inter-mound facies.

  Lunch at the Lobster Pot (Portland Bill) where the famous local fresh crab sandwich is served. Lunch at the participants’ own cost.

  At God Nore and Freshwater Bay we will see the basal part of the Purbeck Limestone Group where two sequences illustrating the development of in-situ microbial mounds (some around tree trunks and branches) in a brackish water lake environment and capped by paleosols.

  At Stonefield Quarry we will see pseudo 3-D cross-section view of mounds and architecture and relationship with inter-mound facies.

2nd day:

  Lulworth Cove east side we will see plan-view of mounds and folded structures in Lulworth Bay and we will discuss newly discovered offshore circular structure imaged in Weymouth Bay seafloor by multi-beam echo sounder.

  Lulworth Cove west side to see section through the Purbeck limestones showing open and closed lake deposits. These are organised as a succession of clays, oolitic and shelly grainstones and fresh- to brackish water mollusc-rich beds (coquinas of the Unio Member, Corbula Member, Broken Shell Limestone or Purbeck Marble).

Logistics

Bus leaves from The Hub, Royal Holloway main campus, at 6 am on Monday 16th and return at approx. 6 pm on Tuesday 17th.  One-night accommodation is included in the cost at The Edenhurst on Weymouth seafront. Participants need to choose single or shared rooms.

Duration: 2 days – 1 night

Minimum / Maximum participants = 7 / 18

Cost: £100 per person for single room

          £65 per person for shared room

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