Magnitude Of Sea-level Fluctuations Research Articles

Reply to: Comments on: “Anatomy of a late Quaternary carbonate island: Constraints on timing and magnitude of sea-level fluctuations, West Caicos, Turks and Caicos Islands, BWI” by Wanless and Dravis [Quat. Sci. Rev. DOI:10.1016/j.quascirev.2020.106216

We address concerns raised by Wanless and Dravis (2020) on our paper documenting the late Quaternary evolution of West Caicos island. We negate the claim that omitting their paper (Dravis and Wanless, 2017) had any consequences for our interpretations. We also provide reasoning and data to demonstrate that our interpretation of the Quaternary stratigraphy of the island was derived from detailed field work and high-resolution remote sensing data, which was subsequently corroborated by AAR and U-Th age-dating control, the opposite of the claim by Wanless and Dravis.

Comments on: “Anatomy of a late quaternary carbonate island: Constraints on timing and magnitude of sea-level fluctuations, West Caicos, Turks and Caicos Islands, BWI” by Kerans et al. [Quat Sci. Rev. 205 (2019), 193–223

We comment on interpretations of the evolution of late Pleistocene geology on West Caicos Island, Caicos Platform, B.W.I. by Kerans et al. (2019), which omits our documentation of the interrelationship of carbonate facies in outcrop and in boreholes (Dravis and Wanless, 2017). We further disagree with their geochronological dating assumptions/results that are the basis for interpreting late Quaternary sea-level fluctuations.

Anatomy of a late Quaternary carbonate island: Constraints on timing and magnitude of sea-level fluctuations, West Caicos, Turks and Caicos Islands, BWI

The data set of the island of West Caicos consists of a combination of high-resolution lidar and digital imagery, radiometric data, and amino acid racemization (AAR) D/L values as well as extraordinary preservation of sedimentary and bio-constructed deposits on the island. Together with a well-established regional stratigraphic framework from the neighboring Bahamian island chain and detailed field mapping, the West Caicos data set provides an opportunity to advance our understanding of the link between carbonate stratigraphy and the controlling parameters of sea level, sediment supply, tectonics, and climate. The exceptionally well-preserved stratigraphic record of the mid–late Pleistocene from West Caicos allows recognition of marine isotope stages (MIS) 9/11, 5e, and Holocene. The west coast of the island provides a continuous 8.4 km exposure of the MIS 5e record that enhances our understanding of the intra-MIS 5e bipartite succession in this region and supports a sea-level lowering that occurred during the last interglacial (LIG).West Caicos accreted as a series of eolian, foreshore–shoreface, barrier-reef, and fringing-reef facies. Shoreline positions for all but the MIS 7 and MIS 5a/c deposits are now captured in the sub-meter geodetic-quality topobathy lidar survey. The Star Town unit of MIS 9/11 age forms the southern margin of the island as well as the central karst-modified spine, with uranium–thorium (UTh) coral ages of between 368 and 365 thousand years ago (ka) and a D/L Glu average of >0.60. Early MIS 5e strata include the Railroad Ridge unit in the center of the island, which was previously identified as one of the older units. The Railroad Ridge unit is a north–south linear dune ridge that has a D/L Glu average of 0.52 and indicates the onset of sediment sources from the east associated with a flooded central Caicos Platform. Approximately synchronously, the early MIS 5e South Reef unit (+3.75–4 m sea-level elevation) nucleated on the western coast of West Caicos, attached to the middle Pleistocene Star Town unit. The South Reef corals yield an average coral UTh age of 126.5 ka, with an average D/L Glu from skeletal matrix of 0.46. This transitional barrier/fringing reef is unconformably overlain by the late MIS 5e Boat Cove unit. Corals in the basal transgressive phase of this upper MIS 5e Boat Cove unit yield an average 120.6 ka UTh and average D/L Glu of 0.45. Shallow-marine, foreshore, and eolian facies of the late MIS 5e rest on a wave-cut terrace that cuts the early MIS 5e South Reef unit by as much as 3 m down to a +0.5 m above present sea-level elevation. The upper MIS 5e Boat Cove unit displays a maximum of +4.5 m elevation above present-day sea level, measured from the middle of the foreshore. This estimate may be a minimum value as the exposures on the west coast are part of a forced regressive foreshore sequence. Last stages of the upper MIS 5e deposition are seen in a series of regressive dune-beach ridge deposits referred to as the Northeast Ridges unit, with sea-level positions ranging from +4.5 m early in the central part of West Caicos, to +0.5 m on the far northeast coast, eventually building down below present-day sea level. This sea-level history fits well with previously documented records in the Bahamas and globally, and the extensive erosion supports a short-lived (between 126 and 120 ka) intra-MIS 5e relative-sea-level fall. Holocene deposition on West Caicos is restricted to northern and eastern margins of the island as a combination of strandplain and dune ridge mixed skeletal–oolitic units.Using the high-resolution digital elevation model (DEM) and linked digital geologic map makes it possible to compare areas of the different Pleistocene and Holocene units quantitatively and to constrain volumetric assessments and accumulation rates on the island for different units. The peak sediment volume production/accumulation rate—between 6.5 and 8 × 106 m3 or between 2.2 and 2.7 × 106 m3 per ka—is observed in the upper MIS 5e unit. This rate is 5 times greater than the 0.5 × 106 m3 per ka estimated for the Holocene ooid dune-ridge/strandplain complex, suggesting that the combination of higher sea level, greater wave and wind energy, and warmer ocean temperatures may have fostered the higher rate of chemically precipitated sediment supply.

An incised valley fill and lowstand wedges in the Upper Devonian Foreknobs Formation, central Appalachian Basin: Implications for Famennian glacioeustasy

The supposition that the Late Devonian Gondwanan glaciation should be recorded in more temperate regions by black shales, incised valleys, and lowstand wedges is tested with reference to the Foreknobs Formation (late Frasnian to early Famennian) of the central Appalachian Basin (USA). The upper (early Famennian) portion of the Foreknobs Formation in a proximal strike belt in eastern West Virginia contains an erosionally based, coarse, conglomeratic, incised valley fill (IVF) that is underlain and overlain by strata deposited in a relatively deep-marine, ramp setting. A signal of sea-level drawdown is in the form of conglomeratic, hummocky cross-stratified (HCS) storm event beds underlying the IVF, suggesting the proximity of a gravely fluvial point source up-paleoslope. Within a more distal outcrop 66km (41miles) to the west, and at about the same stratigraphic horizon, the Foreknobs Formation exhibits a relatively thin succession of black shale that was deposited as sea-level fell, resulting in the concentration of nutrients that caused anoxic conditions. Overlying the black shale are several erosionally based lowstand wedge deposits comprised of cyclic conglomeratic, braided-stream facies which fine-upward to dark gray shales deposited within estuarine environments. The genetically related proximal IVF and the lowermost distal lowstand wedge formed during a major 3rd-order eustatic sea-level fall (35–45m) consistent with published 3rd-order sea-level curves. Lowering of sea-level (base-level) resulted in steeper stream gradients which caused incision and transportation of gravel from the wedge-top depozone of the foreland basin down to the basinward-migrated shoreline. Inferred depositional environments of the distal lowstand wedge are consistent with interpretations from regional subsurface mapping west of the most distal outcrop belt. From inferred magnitudes of sea-level falls and the physics of ice-sheets, calculations of areal extents of ice over Gondwana suggest that an early Famennian inferred sea-level fall of 35–45m translates to an area of ice which is only half of the reported area of late Famennian ice. The data are consistent with step-wise increases in magnitude of sea-level fluctuations during deposition of the Upper Devonian Foreknobs Formation within a period of time transitional between Middle Devonian greenhouse conditions and the later icehouse conditions of the Late Paleozoic.

Regional seismic stratigraphy and controls on the Quaternary evolution of the Cape Hatteras region of the Atlantic passive margin, USA

Seismic and core data, combined with amino acid racemization and strontium-isotope age data, enable the definition of the Quaternary stratigraphic framework and recognition of geologic controls on the development of the modern coastal system of North Carolina, U.S.A. Seven regionally continuous high amplitude reflections are defined which bound six seismic stratigraphic units consisting of multiple regionally discontinuous depositional sequences and parasequence sets, and enable an understanding of the evolution of this margin. Data reveal the progressive eastward progradation and aggradation of the Quaternary shelf. The early Pleistocene inner shelf occurs at a depth of ca. 20–40 m beneath the western part of the modern estuarine system (Pamlico Sound). A mid- to outer shelf lowstand terrace (also early Pleistocene) with shelf sand ridge deposits comprising parasequence sets within a transgressive systems tract, occurs at a deeper level (ca. 45–70 m) beneath the modern barrier island system (the Outer Banks) and northern Pamlico Sound. Seismic and foraminiferal paleoenvironmental data from cores indicate the occurrence of lowstand strandplain shoreline deposits on the early to middle Pleistocene shelf. Middle to late Pleistocene deposits occur above a prominent unconformity and marine flooding surface that truncates underlying units, and contain numerous filled fluvial valleys that are incised into the early and middle Pleistocene deposits. The stratigraphic framework suggests margin progradation and aggradation modified by an increase in the magnitude of sea-level fluctuations during the middle to late Pleistocene, expressed as falling stage, lowstand, transgressive and highstand systems tracts. Thick stratigraphic sequences occur within the middle Pleistocene section, suggesting the occurrence of high capacity fluvial point sources debouching into the area from the west and north. Furthermore, the antecedent topography plays a significant role in the evolution of the geomorphology and stratigraphy of this marginal system.

Rapid onset of late Paleozoic glaciation on Gondwana: Evidence from Upper Mississippian strata of the Midcontinent, United States

Direct evidence of the late Paleozoic glaciation of Gondwana from glacial deposits suggests that geographically extensive continental glaciation began some time in the Namurian (Late Mississippian). However, the timing and characteristics of the onset of glaciation are poorly understood because of a lack of reliable paleontological control and reworking of initial glacial deposits by subsequent glacial advances. Indirect evidence of glaciation preserved in unconformity-bounded, low-latitude ramp sequences in the Illinois basin, United States, suggests that geographically extensive continental glaciation of Gondwana actually began in the late Visean. An abrupt change from carbonate-dominated sequences bounded by disconformities with little evidence of erosion to mixed carbonate-siliciclastic sequences bounded by unconformities with deep incised valleys was likely produced by a three-fold increase in the magnitude of eustatic sea-level fluctuations. The increase in the magnitude of sea-level fluctuations was likely driven by an equally abrupt increase in ice volume and marks the onset of the geographically extensive late Paleozoic glaciation of Gondwana. A possible explanation for the rapid onset of glaciation is the closing of the equatorial seaway between Laurussia and Gondwana. Closing of this seaway would have led to an abrupt change in oceanic and atmospheric circulation patterns that could have initiated major continental glaciation in the Southern Hemisphere.

Chronological correlations between the Pilbara and Kaapvaal cratons

The early geological development of the Pilbara and Kaapvaal cratons has many features in common. Attempts have been made to correlate geologically similar features of the two cratons, and it has been postulated that they originated as contiguous components of a single continent, ‘Vaalbara’, during this time. The early geological histories of the Pilbara and Kaapvaal cratons are here compared in detail and the evidence that they were initially contiguous is assessed. These comparisons indicate significant differences in the chronologies of magmatic events within the granite–greenstone crusts of the Pilbara and Kaapvaal cratons. In addition, igneous correlatives emplaced during ca 2985 and 2782 Ma magmatic events on the Kaapvaal Craton have not been identified on the Pilbara Craton, and a well-defined 2760 Ma magmatic event, manifest as widespread emplacement of granitic rocks into the Pilbara granite–greenstone basement and eruption of flood basalts of the lower part of the Fortescue Group, is absent from the Kaapvaal Craton. Furthermore, similarities in first- and second-order transgression–regression cycles within the sedimentary supracrustal sequences may be attributable to global sea-level fluctuations, and thus may be irrevelant to the question of former contiguity. However, similarities in some aspects of the geological development of the Pilbara and Kaapvaal cratons imply that there were periods, extending for between 60 and 200 Ma, of the Archaean era during which the style of crust formation, intensity of volcanism and subaerial erosion, and magnitude of sea-level fluctuations may have varied on a global scale. Such similarities include the overall duration of formation of the granite–greenstone crusts from ca 3650 to 3100 Ma, the onset of craton-wide erosion in the interval ca 3125 to 3000 Ma, the major episodes of flood basaltic volcanism between 2760 and 2680 Ma, the predominance of chemical (carbonate and banded iron-formation) sedimentation between ca 2630 and 2440 Ma and the transition to widespread clastic sedimentation within the interval 2440 to 2200 Ma.

High-Resolution Sequence Stratigraphic Analysis of the Shannon Sandstone in Wyoming, Using a Template for Regional Correlation

ABSTRACT Recent interpretations of the lower Campanian Shannon Sandstone in the Powder River Basin, Wyoming have suggested that it was deposited in falling stage and lowstand incised shorefaces, rather than in stacked shelf ridges up to 160 km from shore. Despite these detailed studies, there is no published correlation of the outcrop deposits at Salt Creek Anticline with the subsurface deposits in the Hartzog Draw-Heldt Draw (HD-HD) area. A complete high-resolution sequence stratigraphic analysis of the Shannon requires that the outcrops at Salt Creek be correlated with the subsurface fields at HD-HD. There is little core control in the approximately 30 km between these areas, so correlations are based almost entirely on well logs. A template has been constructed in the HD-HD area, where ther is good core and well log control, which embodies data from facies, facies successions, well log signatures, and bounding discontinuities. The template consists of a regressive surface of erosion (RSE) that forms the base of an incised shoreface sandbody, and a transgressive surface of erosion (TSE) that truncates the top of the sandbody and the RSE. Two cross sections have been constructed, and the template has been used to guide correlations between Salt Creek and the HD-HD area. From the cross sections, six progradational events and six transgressions have been defined during Shannon time. The positions of the RSEs and TSEs allow estimates of between a few meters and 60 m for the magnitude of sea level fluctuations that controlled Shannon deposition. The proposed correlations confirm and reinforce the interpretation of the Shannon as a series of incised shoreface sandbodies, separated by transgressive systems tract mudstone deposits.

Sequence Stratigraphy and Facies Variations within Neogene Turbidite Systems, Northern Deep Gulf of Mexico

The Neogene strata of the northern deep Gulf of Mexico are characterized by turbidite systems of different stages of evolution and depositional setting. These Neogene sequences are the basinal, downdip equivalent strata to the petroleum-producing intervals in the continental shelf and slope to the north and northwest. The middle to late Miocene Upper Mexican Ridges sequence (12.5-5.5 Ma) and Cinco de Mayo sequence (5.5-4.5 Ma) are interpreted to consist primarily of unchannelized to partially channelized basinal turbidite systems. Many small cut-and-fill features are present in the sequences; small channel-levee systems may be present only in a few places. The overlying Mississippi Fan (4.5 Ma to present) consists of 17 seismic sequences that have a much larger areal distribution than the two underlying Miocene sequences. Each fan sequence is characterized primarily by sediments deposited in channel, levee, and overbank settings. The differences among these turbidite sequences are caused by both the overall progradation of the continental margin and differences in the magnitude of sea-level fluctuations and volumes of sediment deposited in the deep Gulf of Mexico during the Neogene. The Upper Mexican Ridges and Cinco de Mayo sequences are interpreted to be the first coarse-grained turbidite systems deposited in this areamore » of the deep Gulf of Mexico, with the overlying Mississippi Fan being part of a continuum in deep-water depositional systems. The oldest sequences in the Mississippi Fan were relatively small and approximately the same size as the Upper Mexican Ridges and Cinco de Mayo sequences. The magnitude of sea-level fluctuations, which controlled the timing of deep-water sedimentation, was not as large during the middle to late Miocene as during the Plio-Pleistocene.« less

Capital Reef Complex (Permian), Guadalupe Mountains, Southwestern United States: A Classic Sedimentologic Model in Flux: ABSTRACT

The Capitan reef complex of west Texas and New Mexico has been an important sedimentologic model since a reef origin was proposed for the Capitan Limestone 1929. The Capitan's magnificent exposures the Guadalupe Mountain area; its large scale; its variety of carbonate, sandstone, and evaporite facies; and its relationship to major petroleum resources of the Permian basin have made it a justly famous sedimentary geologic model for academic and industrial geologists alike. Since 1950, extensive research has yielded markedly contrasting sedimentologic interpretations of key features, such as the nature and origin of the Capitan massive (reef wall); the back-reef pisolite, sandstone, and evaporite facies; the depositional profile of the shelf and shelf edge; the impor ance and magnitude of sea level fluctuations; and the role of submarine, vadose, and phreatic diagenesis. Early views of a barrier reef depositional profile have been replaced by a shelftop marginal mound profile, which the mound's gentle crest coincided with the backreef pisolite and tepee facies. The Capitan massive, earlier considered an ecologic barrier reef, is now interpreted as an outstanding example of massive limestone formed at a submerged shelf edge where extensive submarine cementation lithified sponge wackestones and formed massive cement boundstones. Permian vadose diagenesis, earlier accorded much importance and inferred as being related to major sea level falls, appears negligible. Phreatic diagenesis by mixed meteoric and marine fluids was at least locally important the Capitan massive and foreslope strata. The famous Guadalupe pisolite, interpreted until the mid-19 0s as lagoonal, and then widely accepted as Permian caliche of vadose origin, is now reinterpreted as largely synsedimentary, formed by subaqueous precipitation from hypersaline waters of a peritidal shelf crest; associated vadose fabrics are minor, overprint isopachous pisolite fabrics and are unrelated to major intraformational erosion surfaces. Subaerial erosion surfaces within the reef complex are largely localized high on the shelf marginal mound. Unequivocal evidence of emergence of the Capitan massive or its underlying foreslope has not been recognized, suggesting that any sea level lowering during Capitan deposition did not exceed a few tens of meters. Although a sabkha origin of the back-reef evaporites and eolian transportation of sand across a sabkha surface has conceptual appeal, a lagoonal origin of the evaporites and subaqueous deposition of the back-reef sandstone sheets better fits the available field evidence. Vast amounts of siliciclastic sand had to bypass the Capitan massive and upper foreslope facies, but neither channels nor scoured erosion surfaces have been identified. The Capitan reef complex can serve not only as a world class sedimentologic model, but also as a much studied model not yet well understood. The model is one whose scientific investigators have frequently been afflicted by over-reliance on Holocene models (modern model mesmerism), by overthrust of established authorities or dogma (dogma reverence), by uncritical acceptance of new concepts (bandwagonitis), by overuse of superficial look-alike features for End_Page 1441------------------------------ genetic analogies, and by just not discerning what was available for viewing the field or laboratory. Capitan remains a model in flux awaiting more critical field and laboratory research. End_of_Article - Last_Page 1442------------