LATE QUATERNARY SEA-LEVEL FLUCTUATIONS AND DEPOSITIONAL SEQUENCES, SOUTHWEST LOUISIANA CONTINENTAL SHELF

1987 ◽  
pp. 199-219 ◽  
Author(s):  
JOHN R. SUTER ◽  
HENRY L. BERRYHILL ◽  
SHEA PENLAND
Keyword(s):  
Continental Shelf ◽  
Sea Level ◽  
Late Quaternary ◽  
Depositional Sequences ◽  
Sea Level Fluctuations ◽  
Louisiana Continental Shelf

The Origin of Modern Atolls: Challenging Darwin's Deeply Ingrained Theory

2021 ◽  
Vol 13 (1) ◽  
pp. 537-573 ◽  
Author(s):  
André W. Droxler ◽  
Stéphan J. Jorry
Keyword(s):  
Sea Level ◽  
Late Quaternary ◽  
Direct Interaction ◽  
Evolutionary Model ◽  
Reef Growth ◽  
Sea Level Fluctuations ◽  
Fringing Reefs ◽  
Volcanic Islands ◽  
Karst Model ◽  
The Tropical Pacific

In 1842, Darwin identified three types of reefs: fringing reefs, which are directly attached to volcanic islands; barrier reefs, which are separated from volcanic islands by lagoons; and ring reefs, which enclose only a lagoon and are defined as atolls. Moreover, he linked these reef types through an evolutionary model in which an atoll is the logical end point of a subsiding volcanic edifice, as he was unaware of Quaternary glaciations. As an alternative, starting in the 1930s, several authors proposed the antecedent karst model; in this model, atolls formed as a direct interaction between subsidence and karst dissolution that occurred preferentially in the bank interiors rather than on their margins through exposure during glacial lowstands of sea level. Atolls then developed during deglacial reflooding of the glacial karstic morphologies by preferential stacked coral-reef growth along their margins. Here, a comprehensive new model is proposed, based on the antecedent karst model and well-established sea-level fluctuations during the last 5 million years, by demonstrating that most modern atolls from the Maldives Archipelago and from the tropical Pacific and southwest Indian Oceans are rooted on top of late Pliocene flat-topped banks. The volcanic basement, therefore, has had no influence on the late Quaternary development of these flat-topped banks into modern atolls. During the multiple glacial sea-level lowstands that intensified throughout the Quaternary, the tops of these banks were karstified; then, during each of the five mid-to-late Brunhes deglaciations, coral reoccupied their raised margins and grew vertically, keeping up with sea-level rise and creating the modern atolls.

Onset of the Late Paleozoic Glacioeustatic Signal: A Stratigraphic Record from the Paleotropical, Oil-Shale-Bearing Big Snowy Trough of Central Montana, U.S.A.

2019 ◽  
Vol 89 (8) ◽  
pp. 761-783 ◽  
Author(s):  
Justin P. Ahern ◽  
Christopher R. Fielding
Keyword(s):  
Sea Level ◽  
Ice Age ◽  
Late Paleozoic ◽  
Time Interval ◽  
Relative Sea Level ◽  
Depositional Sequences ◽  
Sea Level Fluctuations ◽  
Shark Bay ◽  
Facies Associations ◽  
Central Montana

Abstract In the Big Snowy Mountains of central Montana, USA, late Visean to Bashkirian strata preserve a nearly complete, but poorly documented, paleotropical stratigraphic succession that straddles the range of current estimates of the onset of the Late Paleozoic Ice Age (LPIA). Sedimentologic and stratigraphic investigation of the Otter (late Visean to Serpukhovian) and Heath (Serpukhovian) formations, with secondary focus on the overlying Tyler (late Serpukhovian to Bashkirian) and Alaska Bench (Bashkirian) formations, facilitated an appraisal of paleotropical environmental change preserved in this succession. Three facies associations reminiscent of environments currently forming in Shark Bay, Australia, were identified in the Otter Formation: shallow semi-restricted littoral platform, intertidal platform, and supratidal plain. Five facies associations broadly comparable to modern environments present in the Sunda Shelf and southern coast of the Persian Gulf were identified in the Heath Formation: offshore outer ramp, mid- to outer ramp, inner ramp, coastal plain, and sabkha. Facies associations preserved in the Heath Formation are here explained in the context of a protected, homoclinal carbonate ramp situated in a partially silled epicontinental embayment. A shift from low-magnitude relative sea-level oscillations preserved in the Otter Formation to a cyclothemic stratigraphic pattern entailing ≥ 6 fourth-order, high-frequency and high-magnitude relative sea-level fluctuations in the Heath Formation is here interpreted to record the main eustatic signal of the LPIA in central Montana. Current published biostratigraphic constraints for the observed stratigraphy estimate the main eustatic signal of the LPIA to have occurred approximately between 331 (base Serpukhovian) and 327 Ma in central Montana. A distinct upward transition from coal and paleosol-bearing depositional sequences in the lower Heath to evaporite and limestone-bearing depositional sequences in the upper Heath preserves a broad humid to arid paleoclimate shift during deposition of this unit, which influenced hydrographic circulation patterns and the resultant distribution of anoxic environments in the Big Snowy Trough during this time interval. Improved depositional and sequence stratigraphic models of the Heath Formation proposed in this study permit new insight into the theoretical distribution of, and water depth necessary to preserve, black, organic-rich claystone and shale in partially silled intracratonic basins, in addition to new temporal constraints on LPIA onset in paleotropical western Laurentia.

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