96-12 Perplexed about the Subsalt Rubble Zone? Well that makes Two of Us! Recent Investigations into the Timing of Rubble Formation and Possible Correlations.

Session: Twenty-Seven Years of Advances in Understanding Salt-Sediment Interaction: A Legacy of Katherine A. Giles

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Abstract:

During the past 30 million years in the northern Gulf of Mexico, there have been three two-million year intervals during which there has been significant allochthonous salt emplacement as recorded by well data. These intervals occur between 14.5-12.5 Ma, 10.5-8.5 Ma, and 4-2 Ma. Seventy-nine wells from a 109 well database had sufficient subsalt paleodata to accurately define a time of salt extrusion. Fifty-seven wells (72%) fell within the three defined intervals, 12 from 14.5-12.5 Ma, 26 from 10.5-8.5 Ma, and 19 from 4-2 Ma. Of the seventy-nine well subset, forty-three wells (54%) encountered subsalt rubble zones, which represent slumped carapace, and 36 (46%) did not. Plotted against the Haq et al. 1987 eustatic curve, twenty-seven wells (34%) documented salt emplaced during high-stands and fifty-two (66%) during low-stands. Wells with rubble (35% vs 65%) or without (33% vs 67%) were similarly split between high-stands and low-stands.

Plotting of these data against the eustatic curve or multiple other geometric and salt-tectonic parameters yields no statistically significant correlations to presence and thickness of rubble. A rubble zone will be absent where there is an overturned megaflap beneath the sheet, since this represents preserved diapir roof. But why are there no apparent correlations in other scenarios?  Ultimately, generation of slumped carapace depends on many factors, including 1) eustasy, 2) regional/local extension/contraction, 3) salt budget, 4) sheet merger/canopy formation, 5) carapace thickness, 6) bathymetric relief, 7) style of salt breakout, 8) and sediment influx/erosion. Most allochthonous salt is part of regionally extensive canopies with significant gravity-driven basinward translation of the overburden. This can generate high-relief scarps with thick suprasalt strata, such that slumping is common and somewhat randomly distributed regardless of variations in sediment-accumulation rate, ramps or flats in the base salt, distance from feeder and sutures, or other variables. It is notable that only two wells out of sixteen in the Lucius/Hadrian area record rubble presence. These wells are at the distal end of the canopy system in close proximity to many large rafted carapace blocks. One might expect rubble to be widespread in the area given these circumstances. Perhaps lateral suturing of the canopy leads to less slump failure than simple forward motion.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10640

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