Siliciclastic and carbonate sedimentology, Lusitanian Basin, Portugal 2018

BSRG goes international! The second BSRG postgraduate of 2018 took place from 26–30 November in Portugal. We visited the Jurassic and Cretaceous siliciclastic and carbonate infill of the Lusitanian rift basin, which is exposed along the west coast of Portugal. The trip was led by Matthew Watkinson, Plymouth University, and organised by Kévin Boulesteix and Maarten Heijnen, University of Manchester and NOCS/University of Southampton respectively. A diverse group of in total 24 individuals from different backgrounds, affiliations (including institutes in Poland and Thailand), age groups, and career stages, but with a common interest in sedimentology participated on the trip. Amazing outcrops, a great field guide (on paper and in person), stimulating discussions, and Portuguese cuisine made the trip a great success!

We all met on Lisbon airport at 6PM on Friday 26 November, picked up three vans and drove to Ericeira, an old, traditionally fishing village that got a make-over into a buzzing surfing town. After the usual chaos of dividing rooms, and fighting for the top bunks, we had an introduction round, followed by dinner in smaller groups, during which everybody could explore the trendy restaurants and bars of Ericeira in their own pace.

The next morning (Saturday 27 November) Matthew provided us with the geological framework of the Lusitanian Basin, needed for the trip. The basin was formed by rifting that separated Iberia and Eastern Canada and formed the Central North Atlantic. This rifting occurred in several phases spanning from the Upper Jurassic to the Lower Cretaceous, before shifting westward. The Lusitanian Basin is, besides the opening of the Atlantic, also influenced by the western Tethyan Ocean, making it an analogue to Middle Eastern petroleum systems.

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Dark organic rich shales at Portinho do Areia do Norte.

After the introduction in the hostel, it was time to study the outcrops. We drove north to the peninsula of Peniche where we started our really windy day with Jurassic pre-rift deposits. We headed to Papoa Headland, to see the inner ramp grainstones of the Coimbra Formation. These are overlain here by a textbook example of a transgressive hardground with borings and large bivalves (oysters) remains. Only a 2-minute drive brought us to Portinho do Areia do Norte, where we saw the expression of anoxic bottom waters, resulting in dark coloured shales with high organic carbon content. The rocks just above where very rich in benthic and nektonic fossils, and especially the amount of belemnites was memorable. The weather was slightly challenging this day, so we headed to Peniche to find some shelter for a comfortable lunch. After a brief visit to the GSSP for the Toarican-Pliensbachian boundary, we made several stops along the cliffs towards Cabo Corvoeiro. These sections have been interpreted as a rare example of a prograding submarine fan, but this has recently been questioned and might be a simple shallow marine succession. Several remarkable scours are visible here too and the possible interpretations of these scours and the outcrops as a whole were discussed, while being sprayed with salt water blown up onto the cliffs by heavy gusts. As the BSRG postgrad fieldtrips have already shown during last year’s storm Brian (during fieldtrip to Devon and Cornwall), a little bad weather doesn’t stop us! The field day ended at the disused quarry near Sobral de Lagoa, where the timing of a limestone cave collapse, salt tectonics, and later deposition of Upper Jurassic rocks can be very nicely discussed, using differences in bedding and detailed observation of structures. We all warmed up during the drive back and ended the day with a lovely group dinner at one of Ericeira’s nice restaurants.

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Matthew discussing the remarkable scours at the cliffs towards Cabo Corvoeiro.

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Discussing the interplay between the architecture of a channel-flood plain fill and accommodation space creation and what the implication are for reservoir connectivity at Porto Calada.

The second field day brought us to the Late Jurassic syn-rift/salt clastic deposits of the basin. We started at Praia de Santa Cruz with its iconic boulder on the beach, where we saw a broad range of deposits including a salt diaper wall, marine slope consisting of hemipelagic muds, debrites, and turbidites, and a delta front deposit on the top of this shallowing upwards sequence. This succession is interpreted to represent a progradational syn-rift to early post-rift succession in the hanging wall of a Late Jurassic half graben. We learned that in the days of the initial work, rifting was a fashionable subject in geology, and that the role of salt kinematics maybe has not been considered well enough, leading to interesting discussions. In the afternoon we visited the stunning fluvial successions at Porto Novo. This outcrop shows beautiful sedimentary structures, including lateral accretion surfaces. The last stop of the day was at Santa Rita, where we studied the Upper Lournha Formation, which is interpreted as gravelly and sand-rich fluvial deposits. This interpretation was questioned by some of the participants, leading to a good discussion.

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Lunch on a tropical Cretacous beach. The pineapple and pomegranate contributed to the tropical atmosphere. The weather didn’t.

We started the third day at Porto Calada, where we visited large-scale outcrops of channels. We discussed the interplay between the architecture of a channel-flood plain fill and accommodation space creation and what the implication are for reservoir connectivity. The rest of the day mainly focused on nearshore clastic systems of the Lower Cretaceous. We studied sequence stratigraphy on a tropical Cretaceous beach at São Lourenço, which was also an ideal lunch stop (we doesn’t want to lunch on a tropical beach?). Then we visited a beautifully 3D exposed spit with amazing sedimentary structures at Praia do Coxos. Our day ended with the transition from clastic bars into carbonate shelf with, among other fossils, beautiful nereneid gastropods. Then, we headed to a nice beach bar near Ericeira, before heading back. Since this was our last night as a complete group, we decided to have another group dinner. We ended at a nice restaurant at the old fishing harbour of Ericeira.

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Well-deserved drink after a day in the field

We visited several outcrops further south, between Ericeira and Lisbon (Cascais area) on our last day. We were back in the carbonates again. The day started at the beautiful Praia Abano, where we studied Upper Jurassic deep water carbonates, including carbonate gravity flows. Some later magmatic processes made interesting looking dyke structures through the carbonates here. On one exceptionally stunning beach we made this year’s group photo (with Miquel unfortunately missing). After a chaotic visit to the train station of Cascais to drop off one of the participants, we went to Forte Crismina. We learned the difference between elevator rudists and recumbent rudist and recognised them in the field. Also sponges and corals where plentiful. We ended the geology part of the trip by seeing the start of the Cretaceous orbitolinids (forams) bloom. We drove back to Lisbon airport where we brought back the vans. Some participants flew back in the evening, while another group explored Lisbon’s nightlife to celebrate another successful BSRG postgraduate fieldtrip.

We would like to thank the trip leader Matthew Watkinson for showing us the amazing geology of the Lusitanian Basin. Also, we would like to thank all the participants for their enthusiasm and contributions.

Maarten Heijnen and Kévin Boulesteix, BSRG representatives.

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Group photo at Praia Abano

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