Highstand Shedding Research Articles

Modern Periplatform Highstand Shedding of Two Semidrowned or Drowned Shallow Carbonate Systems, Pedro Bank and the Southern Shelf of Jamaica, Northern Nicaragua Rise: ABSTRACT

Water depth as well as bank-top sediment cover and thickness are the most commonly used criteria to determine whether a modern carbonate bank is drowned or semidrowned. Because neritic carbonate production appears to drop by a factor of two in water depths ranging between 10 and 20 m, it is believed that the limit of neritic carbonate production is the main constraint on the drowning of modern carbonate platforms. Because the shallow isolated carbonate banks on the northern Nicaragua Rise, on the Nicaragua/Honduras and southern Jamaica carbonate shelves, and on many other modern carbonate banks worldwide are covered by an average of 20 to 30 m or more of water and usually by coarse carbonate sediment, carbonate sedimentologists have considered these banks good examples of semidrowned or even drowned carbonate banks. Based on recent research on the northern Nicaragua Rise, however, the authors can demonstrate that these banks are today still healthy producers of large volumes of periplatform sediment (fine aragonite and magnesian calcite), which are almost totally exported to the deep surrounding slopes. These sediments, deposited during the past 9,000 yr, form periplatform wedges that are defined on 3.5-kHz profiles. Radiocarbon ages of the wedge surface sediment, ranging betweenmore » 230 and 610 yr ago, are clear evidence for contemporaneous production of sediments on the shallow bank and shelf, and their instantaneous export to the upper slopes. For the last 5,000 yr, sedimentation rates ranging from 2,000 mm/k.y. off Pedro Bank to 1,300 mm/k.y. off Jamaica are comparable to sedimentation rates on the slopes of Great Bahama Bank, ranging between 2,700 and 6,000 mm/k.y.« less

Exposure and Flooding of Carbonate Platforms Recorded in Carbonate Ooze and Calciturbidites: ABSTRACT

The tops of carbonate platforms are sensitive yet imperfect gauges of sea level because of their numerous lowstand unconformities. Deep-water sediments on the platform flanks can be used to complement the bank-top sections. Periplatform sediments accumulate continuously and combine information on plankton stratigraphy and oceanography in their pelagic fraction with information on exposure and flooding of the banks in the neritic fraction. Neritic input is governed by the principle of highstand shedding. During highstands of sea level, sedimentation rates are high and turbidites abundant bank-derived aragonite dominates in the perennial sediments and grains from the platform interior dominate in the turbidites. During lowstands, rates are low and turbidites rare; plankton dominates in the perennial sediment and skeletal grains from the bank margin in the turbidites. In this way, the Bahamas and the Maldives preserved a detailed record of Quaternary glacio-eustasy. The sea level signal in turbidites is especially useful because it is not erased by diagenesis. Triassic turbidites from the Alps display rhythmic variations in the ratio of biota from the bank margin and the bank interior. These variations fall in the same frequency range as the exposure cycles of the platform. Periplatform turbidites also record unique events, such asmore » the transformation of the Bahama Banks from reef-rimmed atolls to flat-topped platforms in the Pliocene or the drowning of platforms on the Moroccan continental margin in the Early Cretaceous.« less

Carbonate Platforms, Sequence Stratigraphy, and Sea Level: ABSTRACT

Besides tracing reflectors, the mapping of stratigraphic sequences marks a major advance in seismic interpretation. However, construction of sea level curves from sequence stratigraphy is complicated by other factors besides sea level influencing sequence geometry. One such factor is lithology. This point is examined by comparing siliciclastic systems and carbonate platforms. During the Pleistocene, the siliciclastic sediment supply to the deep sea was at its maximum during glacial lowstands of sea level. Pleistocene carbonate platforms were exactly in antiphase to this rhythm. They produced and exported most sediment during interglacial highstands when the platforms were flooded (highstand shedding). In the Bahamas and other platforms, accumulation of both bank-derived fines and sandy turbidites is higher by a factor of 2 to 9 during the interglacials, and turbidites tend to cluster in these highstand intervals. Highstand turbidites also differ in composition from their lowstand counterparts. Turbidite abundance and composition together provide a faithful record of the Pleistocene sea level cycles not easily erased by diagenesis. Geometrically, platform respond to sea level by forming highstand turbidite wedges and lowstand drapes of pelagic sediment--the opposite of siliciclastic systems. Drowning unconformities are another example for the significance of lithologic change in sequence stratigraphy. Flanks of carbonate platforms are generally steeper than siliciclastic slopes. When carbonate platforms are drowned and buried by siliciclastics, an unconformity ensues because the clastics are unable to assume the steep carbonate slope angle or because they are shed from other directions than the carbonates. Examples of drowning unconformities include the Lower Cretaceous platforms off West Africa and off eastern North America as well as the middle Cretaceous unconformity in the Gulf of Mexico. Highstand shedding and drowning unconformities of platforms illustrate that not all depositional systems respond alike to changes in sea level and that sequence boundaries may be caused by lithologic change. These lithologic turning points need not be related to sea level. In a very general way, sequence boundaries can be viewed as changes in the pattern of sediment input and dispersal in a basin. Sea level fluctuations are one way to induce such changes, but tectonic movements and environmental change represent important alternatives, demonstrated by the seismic stratigraphy of the deep Gulf of Mexico. End_of_Article - Last_Page 1522------------

Platform/Foreslope Facies and Buildup Geometry Resulting from Short-Term and Long-Term Eustatic Sea Level Fluctuations: Latemar Buildup (Middle Triassic), Dolomites, Northern Italy: ABSTRACT

Superimposed short-term and long-term eustatic sea level fluctuations directly controlled Latemar platform stratigraphy and indirectly influenced the deeper water facies and overall buildup geometry. Deeper water facies, the foreslope and toe of slope, are a function of platform submergence (highstand shedding) and subaerial exposure (lowstand lithification and erosion) and thus only indirectly reflect eustatic fluctuations. The Latemar consists of a platform core (3-4 km wide, 700 m thick) with a narrow margin, flanked by foreslope (30-35/sup 0/ dips), toe of slope, and basin deposits. The shallowing-upward platform sequence records a long-term (about 10 m.y.) eustatic sea level oscillation with an amplitude of about 150 m. The lower 250 m marks an initial catch-up phase (subtidal carbonates); the upper 450 m marks the sequential keep-up phase (meter-scale cyclic carbonates). These cycles record platform submergence and exposure caused by short-term (10/sup 4/-10/sup 5/ years) Milankovitch eustatic oscillations superimposed on the long-term trend. Platform submergence and exposure conditions result in contrasting foreslope deposits. During highstands, platform-derived sands bypass the foreslope, accumulating as toe-of-slope graded beds and basin turbidites. During lowstands, sand supply ceases, producing basin hard-grounds. Foreslope megabreccias contain margin-derived boundstone clasts, with only minor platform-derived sands (highstands) and lithified clasts (lowstands). Asmore » the platform margin/foreslope contact is nearly vertical, a progressively increasing volume of foreslope megabreccia was needed to maintain the depositional geometry. This coincides with the most commonly exposed platform interval, suggesting that platform exposure determines buildup flank geometry by controlling megabreccia clast production.« less

Glacial versus interglacial sedimentation rates and turbidite frequency in the Bahamas

The southern Tongue of the Ocean is a 1300-m-deep, flat-floored basin in the Bahamas that receives large amounts of sediment from the carbonate platforms surrounding it on three sides. We have examined five 8–13-m-long piston cores and determined bulk sedimentation rates, turbidite frequency, and turbidite accumulation rates for the past two glacial and interglacial periods. The mean of bulk sedimentation rates is four to six times higher in interglacial periods; average accumulation rates of recognizable turbidites are higher by a factor of 21 to 45, and interglacial turbidite frequency is higher by a factor of 6 to 14. Sediment composition indicates that increased interglacial rates are due to higher accumulation of platform-derived material. Additional data from other Bahamian basins as well as published material from the Caribbean strongly suggest that highstand shedding is a general trend in pure carbonate depositional systems. Carbonate platforms without a siliciclastic component export more material during highstands of sea level when the platform tops are flooded and produce sediment. The response of carbonate platforms to Quaternary sea-level cycles is opposed to that of siliciclastic ocean margins, where sediment is stored on the inner shelf during highstands and passed on to continental rises and abyssal plains during lowstands of sea level.