Sea Level and the Paleoenvironmental Interpretation of the middle to late Holocene Hanna Bay Limestone, San Salvador Island, Bahamas: a High Foreshore Setting without a Eustatic Highstand : 無料・フリー素材/写真
Sea Level and the Paleoenvironmental Interpretation of the middle to late Holocene Hanna Bay Limestone, San Salvador Island, Bahamas: a High Foreshore Setting without a Eustatic Highstand / James St. John
| ライセンス | クリエイティブ・コモンズ 表示 2.1 |
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| 説明 | These are my personal notes taken during a geology presentation at the 15th Symposium on the Geology of the Bahamas and Other Carbonate Regions, which was held at the Gerace Research Centre on San Salvador Island, Bahamas in June 2010. I give the notes here because they may be of some interest. Do not expect them to always be in complete sentences, etc.-----------------------------------Sea Level and the Paleoenvironmental Interpretation of the middle to late Holocene Hanna Bay Limestone, San Salvador Island, Bahamas: a High Foreshore Setting without a Eustatic HighstandPresented by: Michael Savarese (Department of Marine & Ecological Sciences, Florida Gulf Coast University, Forty Myers, Florida, USA) (www.fgcu.edu/CAS/Departments/MES/1165.asp)20 June 2010----------Looking at the middle to late Holocene Hanna Bay Limestone on San Salvador Island, Bahamas. (www.flickr.com/photos/jsjgeology/15618557584)A mid-Holocene highstand doesn’t exist in Florida. Looked at the Hanna Bay Limestone on San Salvador to check for it.The Western Atlantic sea level curve, from 10,000 years ago to now, is a relatively smooth curve, with a couple slope changes, but it is otherwise rising. This information is based on mangrove peats and Acropora coral occurrences.Florida oyster reefs show a 3,000 years ago-to-now gradual rise in sea level with no blips.The northern Gulf of Mexico shows a few blips on the Holocene sea level curve based on data from there. A middle to late Holocene highstand has been reported globally - Gulf of Mexico, Australia, China, etc.If this was eustatic, it suggests short period sea level fluctuations cause abrupt changes over decadal/centennial scales. Abrupt ice sheet collapse in Antarctica or Greenland may be a factor behind these changes. This has implications for our future and sea level rise. The Hanna Bay Member in the Bahamas consists of foreshore rocks sitting +2 meters above present sea level. Do Hanna Bay rocks really represent foreshore rocks? Hanna Bay Member limestones are mostly eolian + backshore deposits + foreshore deposits. The foreshore deposits have been questioned. But they are there.Hanna Bay rocks are prolific around San Salvador Island - the Hanna Bay type locality and Grotto Beach have good exposures. Hanna Bay rocks at Grotto Beach are at +2 m above mean sea level.(www.flickr.com/photos/jsjgeology/16240138842)(www.flickr.com/photos/jsjgeology/16240137992) Hanna Bay limestones have tabular, cross-laminated beds with shallow seaward dips (7° to 8°) - this is in the normal range of features for foreshore beach deposits. Also present: some graded bedding and some shallow angular unconformities from the erosional influence of younger beds. Fenestral porosity is present, as are swash lines & rill lineations - both of these are hard to preserve, but they are there. Rill lineations are returning water lineations. This is all consistent with a foreshore interpretation. An excellent swash line with fenestral fabric on its seaward side is preserved on a bedding plane exposure at Grotto Beach.(www.flickr.com/photos/jsjgeology/16215474986) Modern beaches on the leeward coast of San Salvador have fine-grained to coarse-grained sands - lumps and bioclasts are common allochems.Hanna Bay limestones are composed of medium- to coarse-grained sands with lumps + bioclasts as allochems. Slightly coarser materials show up in Hanna Bay thin sections. Otherwise it’s a good correlation. Ichnofauna in Hanna Bay limestones - vegemorphs. This is contradictory information. Skolithos and Diplocraterion are common in foreshore deposits. Psilonichnus is a ghost crab burrow - it’s a backshore trace fossil. Vegemorphs indicate a backshore or dune setting. Petrology - allochems are consistent with a wave-swept origin rather than a wind-transported origin.Cement - freshwater phreatic + vadose cements predominate.Exception: isopachous, acicular rim cements in intergrain pores & at basal parts of the section exposed at Grotto Beach. Acicular rim cements are consistent with marine phreatic cement origin.Allochems - principally bioclasts and lumps.Intergrain cements - blocky, isopachous to meniscus (not marine phreatic in origin).Low in the section at Grotto Beach is intergranular acicular cement (aragonite) - can get that with sea spray (also seen on the eastern side of North Point Peninsula, for example). The Hanna Bay & Grotto Beach sections of the Hanna Bay Limestone give different dates (= whole rock radiocarbon dates).Stratigraphically disordered dates (three dates for each section) (all of these are calendar years before 1950):Grotto Beach locality:780 years (top of section)1230 years1000 years (bottom of section)Hanna Bay locality:4060 years (top of section)4520 years3260 years (bottom of section)This disordering is understandable given that the whole rock dates are averages of the ages of various allochems and cements.The two sections are different enough in dates to indicate two separate highstands.Hanna Bay limestones do represent foreshore deposits.Cement petrography, vegemorphs,and ichnofossils suggest a very short-lived sea level high - the outcrop got quickly transferred to a supratidal position.Hanna Bay foreshore deposits are diachronous across their geographic range. Sediments can get deposited in foreshore settings in short-duration, high-frequency sea level highs (high berms).Grotto Beach has modern foreshore deposits near the Cockburn Town Fossil Reef wall - a recent high foreshore. The same is seen at Little Exuma Island. Do you need a sea level highstand, then?Storm tides possibly explain this (the sediments don’t indicate storm activity, though).Perigean spring tides possibly explain this.Another possible explanation is regional shift in oceanic circulation (e.g., Florida Current perturbations; North Atlantic Oscillations). Can get local sea level highs from a combination of normal oceanographic phenomena.A eustatic highstand is not supported based on evidence from San Salvador. Each case of a purported mid-Holocene highstand needs to be evaluated on a case by case basis.There are ephemeral and non-ephemeral indicators for eustatic sea level highs.Ephemeral indicators - foreshore deposits from a perigean or spring tide.Non-ephemeral indicators - Acropora reefs.-----------------Jim Carew comment: We got 2800 & 3200 years before present [1950] dates at Grotto Beach for Hanna Bay limestones.Mike Savarese response: It depends on where you sample the cliffs. Foreshore deposits can be readily left behind by short-term events.------------------ |
| 撮影日 | 2015-09-07 14:25:41 |
| 撮影者 | James St. John |
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