Dorset Geologist’s Association Group (DGAG) will host a talk on the Tuesday 23rd September 2025.
Title: Diamond ‘Stratigraphy,’ SDRs & LIPs*, A 1200 km ‘Restricted’ Margin and a new oil province: The Orange Basin Offshore Namibia & South Africa: An Aggressive Passive (Passive Aggressive?) Margin Basin
Speaker: Allan Scardina (Upstream Decisions LLC) who has forty plus years in exploration geology (Oil/Gas) in many parts of the globe.
Time: Talk will start at 7pm; finish approximately 8pm
Venue: Activity Meeting Room: Dorford Centre, Bridport Road, Dorchester, DT1 1RR
Lecture Entry Cost: All welcome: £6 non-members (£5 for DGAG members), cash best.
Booking a seat: contact Chris Webb at email: events@dorsetgeologistsassociation.org
Talk Description: This promises to be an intriguing mix of “hard rock” diamonds and “soft rock liquid gold” – Editor comments!
Speakers summary:
Passive margins are named for their notional lack of significant post-rifting tectonic activity. A ‘standard’ well-behaved passive margin goes through progressively slowing tectonic subsidence related to the cooling of the underlying crust. That is, most of the time.
However, in some cases, passive margins are anything but. The extensive Orange Basin offshore Namibia and South Africa is one such ‘aggressive’ passive margin. The rifting phase appears to have been heavily dominated by the development of seaward dipping reflectors (SDRs), probably in two different phases. However, the origins of these SDRs may not be what is commonly assumed by the “SDR” nomenclature. Post-rift uplift (rebound?) created one-to-several regional ridges that significantly impacted future clastic and carbonate facies development. The boundaries of the greater Southern Africa margin were, in the Aptian (exact timing may vary along the margin), geographically restricted by hot spot-generated ridges, leading to a Mediterranean-scale silled basin and the development of a thick organic-rich marine layer that is the source rock for the recent petroleum discoveries. Significant uplift (ca. 1000 – 2000+ m) of the southern African Plateau in the Late Cretaceous- Early Tertiary and linked tilting and erosion of the margin resulted in the development of at least two interconnected updip extensional-downdip compressional systems and contributed considerably to the development of both the rich placer deposits of high-quality diamonds that makes Namibia the world’s leading sources of “marine” diamonds and the turbidites containing some of the recent large hydrocarbon discoveries. The active nature of this margin continues into the Tertiary with present-day earthquakes up to M5 and seamounts probably linked to Late Tertiary volcanics in the deep offshore, both of which might be related to a reactivated transform system or underlying deep mantle processes.
This talk will illustrate the above events, place them in a regional context, and highlight several remaining uncertainties along the margin. It will also touch on the geopolitical impact of geology.
*Large Igneous Provinces = LIPs
Seaward Dipping Reflectors on seismic records = SDRs
Photo: Courtesy of the Encyclopedia Britannica pending better from the speaker
Additional Links: Pending more information
Dorset Geologist’s Association Group (DGAG) will host a talk on the Tuesday 22nd April 2025.
Title: The Variscan Orogeny; it’s Causes and Consequences
Speaker: Alan Driscole (DGAG member) who has forty plus years in exploration geology (Oil/Gas) in many parts of the globe including the Atlantic margin.
Time: Talk will start at 7pm; finish approximately 8pm
Venue: Activity Meeting Room: Dorford Centre, Bridport Road, Dorchester, DT1 1RR
Lecture Entry Cost: All welcome: £6 non-members (£5 for DGAG members) cash
Booking a seat: contact Chris Webb at email: events@dorsetgeologistsassociation.org
Talk Description: The talk is another from Alan and on a topic that there have been discussions on at previous talks and field trips. If you’re exploring geology in the southern UK, Ireland or much of Europe it is a subject that comes up regularly so it is timely to review the orogeny, it’s causes, history & influence on later structure through the Mesozoic and Tertiary age rocks.
The Variscan orogeny culminated in final consolidation of the Pangea supercontinent, with closure of the Rheic Ocean and collision between Laurasia and Gondwana. Rather than a simple single collision, the Variscan incorporates several tectonic phases as small micro-continents, originally split off from the margin of Gondwana, which then collided with Laurasia during a 100 million year period from the early Devonian to the end of the Carboniferous. As can be imagined, these events had a profound influence on the structural and environmental development of southern and central Europe during late Palaeozoic, including in England, Wales and Ireland. In this talk we will look at the story of this prolonged orogeny in Europe; how it influenced Carboniferous geology in the southern UK and its subsequent geological development through to the Alpine orogeny, from the mid Cretaceous to mid Tertiary.
Photo: Courtesy of the speaker –is from Millock Haven, Cornwall & shows Variscan chevron folds in Carboniferous Culm. (Can you spot the “Z” or “S” folds! – Editor)
Additional Links:
1. A good summary by the geological society of UK main orogenic phases
https://www.geolsoc.org.uk/Plate-Tectonics/Chap4-Plate-Tectonics-of-the-UK/Variscan-Orogeny
2. An introduction for geological enthusiasts to SW England and the wonderfully named Variscan Coast.
https://variscancoast.co.uk/
Dorset Geologist’s Association Group (DGAG) will host a talk on the Tuesday 20 February 2024.
Title: A Geological Tour of Namibia
Speaker: Alan Driscole (DGAG member) – forty years in the oil & gas business including the Atlantic margin from the Barents Sea south to …… Namibia
Time: Talk will start at 7pm; finish approximately 8pm
Venue: Activity Meeting Room: Dorford Centre, Bridport Road, Dorchester, DT1 1RR
Lecture Entry Cost: £6 (£5 for DGAG members) collected on room entry
Booking a seat: Contact DGAG events at email: cwebb48578@aol.com
Talk Description: From some of the oldest fossils and highest dunes in the world to the largest meteorite, Namibia is a geological treasure house. In this talk we’ll explore the history of Namibia; from the Precambrian to the present day, making a diversion offshore to see how this history has been controlled by the formation and dismemberment of Gondwanaland. We’ll also discuss Namibia’s emerging important for both “old” energy and the energy transition.
Photos A & B: Courtesy of the speaker
A: Dunes at Sossusvlei
B: The Hoba meteorite (with the speaker for scale)
