Frank Peel naturally begins his AAPG Distinguished Lecture about the Gulf of Mexico’s Louann Salt with a brief review of the history of the universe – an explanation expressed in terms of toilet tissue. Special focus goes to the evolution of the solar system, in a single roll.
He then finds an empty parking space in the Jurassic and pulls in for a closer look at the pre-Louann basin landscape.
It’s a dynamic scene, in Peel’s interpretation: semi-arid but with flows from rugged highlands in a setting dominated by a deep lake, with a meandering and boisterous river akin to today’s Congo River.
Peel gives this waterway the technical Mesozoic designation “Big Old River,” although he later alternates to the less restrictive term “That Big River.” And the big lake he’s named Lake Jackson, in honor of a former colleague.
The magic that lets Peel “see” this landscape is high-quality 3-D seismic. Data from TGS Geophysical reveals the pre-Louann scene “frozen like a fly in amber,” said Peel, senior research fellow in the Bureau of Economic Geology at the University of Texas at Austin.
“On these data you can actually see the base of the salt, the edge of the salt. You can see it in beautiful, exquisite detail,” he observed.
Peel, who earned his doctorate at the University of Oxford, later held industry roles including structural geologist at BP and chief geologist for BHP Billiton Petroleum. Today, he’s also co-director of Appeel Geosciences Ltd.
New Insights on an Enduring Mystery
He has titled his lecture “A Lost World Rediscovered,” but the details come in the subtitle: “3-D Seismic Data Reveal Spectacular Images of a Jurassic Landscape on the Eve of Louann Salt Deposition in the Gulf of Mexico, with Implications for Salt Deposition.”
Understanding the Louann provides a key to deciphering the geology and subsea topography of the Gulf, one of the world’s leading exploration and production provinces. Up until now, the pre-salt realities have been uncertain, challenging and mysterious.
One reason for that, Peel notes, is the absence of useful analogs. The contemporary Dead Sea is also a hypersaline lake expanse, but not a helpful analog because “it’s way too small. It’s in a totally different tectonic setting,” he said.
Also, the Louann salt body contains no fossils, no interbedded, non-salt sediment layers as clues to deposition, no Usiglio evaporite sequences, no other obvious pointers to investigate basin paleogeography, he explained.
With collaborator Gillian Apps, a BEG research fellow, Peel takes a fresh look at the Louann setting based on 3-D seismic interpretation, geochemistry and other available information. He concluded by rejecting some previous speculation about the Louann Salt’s origins.
He noted one striking feature about this Jurassic rift basin scene is the contrast between the rugosity of the eroded uplands, cut with remnants of rivers and stream drainage, and the smoothness of the adjacent lowland. In Peel’s envisioning, you take the rough with the smooth.
“The boundary between the two terrains is exactly on a depth contour,” Peel observed.
“We are interpreting this as a water level. This is a lakebed,” he said. “There’s good evidence we have one-kilometer depth here. It’s a deep lake, deepening as it goes off the data.”
Peel has written that the “pre-salt lake margin was steep, with fault scarps that were eroded above lake level, and fan-deltas deposited below lake level. Water level in the lake lay about one kilometer below global sea level.”
As analogs for this topography, he names the area around Lake Powell in southern Utah and the surroundings of a large lake of – probably – liquid methane on Saturn’s moon Titan.
A ‘Seriously Sunken Basin’
What’s also remarkable is the true depth of the lake’s surface, which Peel generally puts at 750 meters to a full kilometer below coeval global sea level. By comparison, the lowest land expanse in North America today is 86 meters below sea level. The Dead Sea area, the world’s deepest, is 431 meters below sea level.
So if Peel is right, it’s a seriously sunken basin. And that helps explain what Peel thinks is about to happen to this basin in the Middle Jurassic – something extraordinary and possibly unique in geological history, something truly remarkable.
Meanwhile, in the ancient world, the Louann landscape is about to undergo a major change.
“In the Late Triassic we have rift basins forming in the (present-day area of the) Gulf of Mexico and gradually opening up,” with seawater propagating through the rift system, Peel said.
“At 170 million years ago, this basin opened up. It was flooded. Water came in from the global sea system,” he added.
After the Gulf of Mexico basin flooded catastrophically, this semi-desert world was buried beneath an inland sea several kilometers deep and then under kilometers of deposited Louann Salt, Peel said. And it all happened very quickly, in a few tens of thousands of years, he estimated.
Peel rejects the prevailing shallow-water, gradual-deposition model for the Louann. He favors a depositional model where salt was precipitated in deep hypersaline water in conditions leading to rapid deposition, where evaporation is matched by seawater influx in a steady-state system.
“The message I want you to take is, you really can’t get around that volume of salt being deposited in a very, very short time,” Peel said.
“As far as I know, this is the fastest steady-state deposition known to science,” he said.