Calcarenite (Hanna Bay Member, Rice Bay Formation, Holocene; Grotto Bay, San Salvador Island, Bahama by James St. John

Hanna Bay Member of the upper Rice Bay Formation at Grotto Beach. This is the youngest bedrock unit on San Salvador Island. These well-sorted limestones consist of sand-sized grains of aragonite (CaCO3). On the continents, many quartz sandstones are technically called quartz arenites. Because the sand grains making up these Bahamian rocks are calcareous (composed of calcium carbonate), the limestones are called calcarenites. When examined microscopically, the calcareous sand grains can be seen touching each other - the rock is grain-supported. This results in an alternative name for these Bahamian limestones - grainstones. “Calcarenite” seems to be a more useful, more thoroughly descriptive term for these particular rocks, so I use that, versus “grainstone” (although “calcarenitic grainstone” could be used as well). The little-used petrologic term aragonitite could also be applied to these aragonitic limestones. Hanna Bay Member limestones are more or less planar-bedded, and gently dip toward the modern ocean. The seaward dip, the sorting, the occurrence of more coarse-grained, finely fragmented seashell horizons, plus preserved sedimentary structures on some bedding planes such as bubble porosity and swash lines, indicate that these Hanna Bay Member rocks represent beach deposits. At other localities, the Hanna Bay Member includes back beach dune facies. At the locality shown above (Grotto Beach), the lithified beach deposits are exposed two meters above current, mean sea level. Some geologists have taken this as evidence for a mid-Holocene highstand, and that modern sea level is lower than it was during portions of the mid-Holocene. A similar conclusion has been reached based on geologic evidence from elsewhere in the New World and the Old World. Recent work on this very Hanna Bay Member outcrop has thrown doubt on the validity of the mid-Holocene highstand interpretation (Savarese &amp; Hoeflein, 2012). The aragonite sand grains in the Hanna Bay Member are principally bioclasts (worn mollusc shell fragments &amp; coral skeleton fragments &amp; calcareous algae fragments, etc.) and peloids (tiny, pellet-shaped masses composed of micrite/very fine-grained carbonate - some are likely microcoprolites, others are of uncertain origin). Age: Holocene (MIS 1) Locality: shoreline outcrop along the southwestern margin of Grotto Bay, southwestern San Salvador Island, eastern Bahamas ------------------ Reference cited: Savarese, M. &amp; F.J. Hoeflein. 2012. Sea level and the paleoenvironmental interpretation of the middle to upper Holocene Hanna Bay Limestone, San Salvador, Bahamas: a high foreshore setting without a higher-than-present eustatic highstand. pp. 163-183 in Proceedings of the 15th Symposium on the Geology of the Bahamas and Other Carbonate Regions. --------------------------------------- The surface bedrock geology of San Salvador consists entirely of Pleistocene and Holocene limestones. Thick and relatively unforgiving vegetation covers most of the island’s interior (apart from inland lakes). Because of this, the most easily-accessible rock outcrops are along the island’s shorelines. ------------------------------ Stratigraphic Succession in the Bahamas: Rice Bay Formation (Holocene, &lt;10 ka), subdivided into two members (Hanna Bay Member over North Point Member) -------------------- Grotto Beach Formation (lower Upper Pleistocene, 119-131 ka), subdivided into two members (Cockburn Town Member over French Bay Member) -------------------- Owl's Hole Formation (Middle Pleistocene, ~215-220 ka &amp; ~327-333 ka &amp; ~398-410 ka &amp; older) ------------------------------ San Salvador’s surface bedrock can be divided into two broad lithologic categories: 1) LIMESTONES 2) PALEOSOLS The limestones were deposited during sea level highstands (actually, only during the highest of the highstands). During such highstands (for example, right now), the San Salvador carbonate platform is partly flooded by ocean water. At such times, the “carbonate factory” is on, and abundant carbonate sediment grains are generated by shallow-water organisms living on the platform. The abundance of carbonate sediment means there will be abundant carbonate sedimentary rock formed after burial and cementation (diagenesis). These sea level highstands correspond with the climatically warm interglacials during the Pleistocene Ice Age. Based on geochronologic dating on various Bahamas islands, and based on a modern understanding of the history of Pleistocene-Holocene global sea level changes, surficial limestones in the Bahamas are known to have been deposited at the following times (expressed in terms of marine isotope stages, “MIS” - these are the glacial-interglacial climatic cycles determined from δ18O analysis): 1) MIS 1 - the Holocene, &lt;10 k.y. This is the current sea level highstand. 2) MIS 5e - during the Sangamonian Interglacial, in the early Late Pleistocene, from 119 to 131 k.y. (sea level peaked at ~125 k.y.) 3) MIS 7 - ~215 to 220 k.y. - late Middle Pleistocene 4) MIS 9 - ~327-333 k.y. - late Middle Pleistocene 5) MIS 11 - ~398-410 k.y. - late Middle Pleistocene Bahamian limestones deposited during MIS 1 are called the Rice Bay Formation. Limestones deposited during MIS 5e are called the Grotto Beach Formation. Limestones deposited during MIS 7, 9, 11, and perhaps as old as MIS 13 and 15, are called the Owl’s Hole Formation. These stratigraphic units were first established on San Salvador Island (the type sections are there), but geologic work elsewhere has shown that the same stratigraphic succession also applies to the rest of the Bahamas. During times of lowstands (= times of climatically cold glacial intervals of the Pleistocene Ice Age), weathering and pedogenesis results in the development of soils. With burial and diagenesis, these soils become paleosols. The most common paleosol type in the Bahamas is calcrete (a.k.a. caliche; a.k.a. terra rosa). Calcrete horizons cap all Pleistocene-aged stratigraphic units in the Bahamas, except where erosion has removed them. Calcretes separate all major stratigraphic units. Sometimes, calcrete-looking horizons are encountered in the field that are not true paleosols. ---------------------------- Subsurface Stratigraphy of San Salvador Island: The island’s stratigraphy below the Owl’s Hole Formation was revealed by a core drilled down ~168 meters (~550-feet) below the surface (for details, see Supko, 1977). The well site was at 3 meters above sea level near Graham’s Harbour beach, between Line Hole Settlement and Singer Bar Point (northern margin of San Salvador Island). The first 37 meters were limestones. Below that, dolostones dominate, alternating with some mixed dolostone-limestone intervals. Reddish-brown calcretes separate major units. Supko (1977) infers that the lowest rocks in the core are Upper Miocene to Lower Pliocene, based on known Bahamas Platform subsidence rates. In light of the successful island-to-island correlations of Middle Pleistocene, Upper Pleistocene, and Holocene units throughout the Bahamas (see the Bahamas geologic literature), it seems reasonable to conclude that San Salvador’s subsurface dolostones may correlate well with sub-Pleistocene dolostone units exposed in the far-southeastern portions of the Bahamas Platform. Recent field work on Mayaguana Island has resulted in the identification of Miocene, Pliocene, and Lower Pleistocene surface outcrops (see: <a href="http://www2.newark.ohio-state.edu/facultystaff/personal/jstjohn/Documents/Geology-talks/Kindler-talk.htm" rel="nofollow">www2.newark.ohio-state.edu/facultystaff/personal/jstjohn/...</a>). On Mayaguana, the worked-out stratigraphy is: - Rice Bay Formation (Holocene) - Grotto Beach Formation (Upper Pleistocene) - Owl’s Hole Formation (Middle Pleistocene) - Misery Point Formation (Lower Pleistocene) - Timber Bay Formation (Pliocene) - Little Bay Formation (Upper Miocene) - Mayaguana Formation (Lower Miocene) The Timber Bay Fm. and Little Bay Fm. are completely dolomitized. The Mayaguana Fm. is ~5% dolomitized. The Misery Point Fm. is nondolomitized, but the original aragonite mineralogy is absent. ---------------------------- The stratigraphic information presented here is synthesized from the Bahamian geologic literature. ---------------------------- Supko, P.R. 1977. Subsurface dolomites, San Salvador, Bahamas. Journal of Sedimentary Petrology 47: 1063-1077. Bowman, P.A. &amp; J.W. Teeter. 1982. The distribution of living and fossil Foraminifera and their use in the interpretation of the post-Pleistocene history of Little Lake, San Salvador, Bahamas. 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