Field Geology - Lyme Regis, Dorset, UK

Location: Lyme Regis, Dorset, UK

Cliffs below Devonshire Head (SY:328-910 to SY:332-914) between Seven Rock Point and Monmouth Beach
Access: Monmouth Beach is accessed via Cobb Road, just to the west of the Boat Building Academy (SY:335-915)

Title: Clay Mineralogy, Organic Matter and Climate of the Blue Lias Formation, Lyme Regis - By Andrew Green 

The Blue Lias (Hettangian-Lower Sinemurian) exposed in the cliffs and foreshore between Seven Rock Point and Monmouth Beach, west of Lyme Regis, represent the topmost part of the formation belonging predominantly to the lower Sinemurian. With regard to the biostratigraphic ammonite biohorizons, commonly used throughout the Jurassic to divide and date the rock succession, the visible Blue Lias at this location belongs to the bucklandi and semicostatum ammonite zones. Within the cliff section, the junction with the overlying Charmouth Mudstone Formation (Shales-with-Beef Member) can also be identified through a bioturbated erosion surface.

The term Blue Lias as noted by Lang (1924), was originally only applied to the blue-gray argillaceous limestones. Stratigraphically, however, the phrase has evolved to now reference the entire Lias section below Lang’s named Bed 49-Grey Ledge, consisting of rhythmically alternating hemipelagic shales, marls and limestone in cycles 0.5-2.0m thick (see image 1A).

The Blue Lias sediments, deposited in a warm shallow epicontinental sea, represent the first marine incursion since the formation of the supercontinent Pangea. Following the Triassic-Jurassic mass extinction, which occurred 4Ma before the Blue Lias and wiped out ~12% of all marine taxonomic families, the Lower Lias was characterised by expanding, deepening seas dominated by ammonites, ichthyosaurs and plesiosaurs.

The Blue Lias mudstones are comprised of a variety of clay minerals: illite (30-60%), mixed illite-smectite (15-45%) and kaolinite (10-30%). The clay mineral distribution has been shown to vary stratigraphically, with the Lyme Regis section representing a transition from an illite-rich to kaolinite dominated composition through the bucklandi zone (Deconinck et al., 2003).

The Blue Lias Formation at Lyme Regis is rich in organic matter, with TOC (total organic content) content ranging up to 12% (4% average). Elevated organic richness is not continuous within the section, but instead is confined to thin laminated, pyritic shales with little bioturbation. Such shales occur rhythmically through the Blue Lias Formation in marl-shale-limestone cycles with sharp contacts between lithologies (see image 2).

At Lyme Regis (as opposed to other Blue Lias outcrop locations - see image 1B) the Blue Lias Formation TOC profile does not hold any stratigraphic relationship. This is believed to result from the condensed nature of the section twinned with diagenetic carbonate cement overprinting the primary link between organic matter and clay mineralogy.

With the aid of Blue Lias TOC profiles from other locations, (St Audrie’s Bay, Somerset, UK) and the known association between clay mineral assemblage and palaeoclimate, (illite - arid/kaolinite – humid & wetter) the deposition and preservation of organic matter throughout the Blue Lias Formation is noted to have preferentially occurred during more stable arid climates.

References:
DECONINCK, JF., HESSELBO, S.P., DEBUISSER, N., AVERBUCH, O., BAUDIN, F. & BESSA, J., 2003. Environmental controls on clay mineralogy of an Early Jurassic mudrock (Blue Lias Formation, southern England). International Journal of Earth Science, 92, 255-266.
LANG, W.D. 1924. The Blue Lias of the Devon and Dorset coasts. Proceedings of the Geologists’ Association, 35, 169-185.