Aber Reunion Portland: September 2021

Aber Reunion Portland 17th to 19th Sept 2021:
report by Moira

There was a magnificent view from the Height Hotel in Portland looking down over Portland Harbour, Weymouth and Chesil Beach.

I met up with a few others and we sat on the balcony outside the bar with the sun setting and the Olympic Rings visible.

Day 1: Portland & Kimmeridge 18/09/2021
The next morning we drove to Freshwater East near the end of Portland and visited old quarries in Portland Stone perched high above the cliffs, with Purbeck beds at the very top.

There were chert layers in the Portland Stone in the lower cliff, flowstone on oolitic limestone and the base of the Purbeck.

The western part of the upper quarry had good exposures of the Purbeck including fossil tree branches which had weathered out.

It was interesting to see Thrombolites grown around the fallen trees in brackish water.

[Wikipedia states: “Thrombolites are clotted accretionary structures formed in shallow water by the trapping, binding, and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria.] Tony Kirkham explained Thrombolites to us. They are Microbolites in the Purbeck beds. Like stromatolites they are cyanobacteria – blue green algae. [Thrombolites are so named because they form clots. They are radiating filaments grown in brackish water, which trap sediment and often grow around branches which have fallen in the water as seen here.] It appears sediments built up around trees and then the thrombolites grew. Elsewhere calcareous spherulites have grown eccentrically into the empty space in sea water.

We went next to the Portland Bill platform on Portland Stone with Pulpit Rock below us.

Further along we saw Purbeck Broken Beds over lying a platform of Portland Stone.

Ahead were the remains of an old crane for lowering rocks to load in boats.

At Kimmeridge Bay some of the unstable cliff face came down to be descended on by waiting holiday makers (and some geologists!). The cliff face shows 38-42 thousand year cycles. [The Earth’s tilt varies about 23 to 27 degrees in cycles.]

Beds are about 1.4m thick with shales to limestones and dolomitic (eg ledges which project into the sea). Plenty of ammonites for photographers but not easy to collect.

You cannot see the glittering iron pyrites in the photos very well – infilling desiccation cracks?

Also of note was the Clay to Limestone with organic material anaerobic to dis-anaerobic (Burning Cliff round the corner to East is black stone). The total organic content can be measured, similar to the cycles in the Blue Lias.

Day 2: Lulworth Cove 19/09/2021
John Phillips did work here and was the first to recognise that the surfaces of the chalk were thrust surfaces of a reverse fault. [Paper by Malcolm nearly ready for publication – wrench fault]

Lulworth Bank in1964 was BP first offshore drilling. There is salt offshore into Triassic salt domes along a fault line with a wedge of Wealden, Portland and Purbeck.
Stair Hole gave a good view towards Portland and the thrust line with chalk behind the Wealden.

The Crumple at Stair Hole.

Also to be seen are the Broken Beds at the west end of Stair Hole

and the thin Yellow Car Stone glauconitic Greensand with reworked Jurassic fossils.

We then walked down into the cove to look at the back wall. There is a small area with Greensand and the chalk above it.

Greensand is shallow marine by the cove with Pleistocene beds above.

Graham took us to see the oil seepage at the east side of the cove whilst a few intrepid climbers went to look for the fossil forest near the firing range – it was closed.

All our thanks to Max and Malcolm for arranging the trip and succeeded in producing sunshine, good food, interesting geology and plenty of discussion in spite of the disruption of the Iron Man cycling race.

Moira