Stratigraphy and Sedimentology of Pleistocene and Holocene Carbonate Eolianites, Kaua'i, Hawai'i, U.S.A.
Charles T. Blay, Mark W. Longman, 2001. "Stratigraphy and Sedimentology of Pleistocene and Holocene Carbonate Eolianites, Kaua'i, Hawai'i, U.S.A.", Modern and Ancient Carbonate Eolianites: Sedimentology, Sequence Stratigraphy, and Diagenesis, F. E. (Rick) Abegg, David B. Loope, Paul M. (Mitch) Harris
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An extensive complex of Pleistocene and Holocene carbonate eolian sandstone units and unconsolidated sand dunes along the southeast coast of Kaua'i is described, informally named, and interpreted. These well-exposed deposits, which have received little previous geologic study, include Recent beach and unconsolidated eolian sand bodies, and four lithified eolianite units that are assigned member status in the Māhā'ulepū formation (new informal name). The eolianites formed during the past half million years as carbonate sand grains, derived from shallow-marine organisms such as coralline red algae, corals, molluscs, echinoderms, and foraminifers. were blown by prevailing trade winds on and along shore from localized highstand beach deposits.
Modern eolian deposits of the Pā'ā-Mābā'ulepū area are sand-body complexes of stacked and amalgamated elongate sand lobes up to 10 m thick. They typically extend for about a kilometer downwind from small “pocket” beaches.
The laterally discontinuous lithified carbonate eolianites are delineated and mappedon the basis of superposition, separation vertically by distinct red paleosols and/or caliche crusts, lateral distribution, and diagenetic textures. All eolianites are composed of moderately-sorted to well-sorted, fine- to medium-grained carbonate sand characterized by large-scale cross-bedding with bed sets commonly 2 to 5 m thick. Root mottling, rhizoliths, and land-crab burrows are common and increase in abundance upward to the capping paleosol and caliche units.
The Pā'ā member (new informal name), the youngest unit of the Māhā'ulepū formation, consists of up to 20 m of very weakly lithified eolianite. Grains have retained their original mineralogy, minor cement (only 3–5%) composed of v-adose equant calcite with meniscus fabric locally binds the grains, and 30 to 40% primary intergranular porosity is present. The underlying Makawehi member (new name) is characterized by moderately well-indurated eolianite up to 12 m thick, a basal shoreline deposit 1 m thick, and a capping paleosol and caliche unit 1 to 2 m thick. Grains are only partly neomorpfiosed and leached. Cements are mainly equant calcite with a vadose fabric and form less than 10% of the rock.
The two oldest eolianite members in the Māhā'ulepū formation are thePā'ō'ō member (new informal name), which is up to 14 m thick, and the underlying wel I-indurated Punahoa member (new informai name), which is up to 25 m thick and the most widespread of all the eolianite deposits. All grains in these members have stabilized to calcite, and equant calcite cement is common. Rocks of the Pā'ō'ō member retain 15 to 30% primary intergranular porosity and have well-preserved intact grains. In contrast, rocks of the Punahoa member show the most diagenetic alteration and contain common secondary moldic and vuggy porosity. The Punahoa member laps on to basalt ridges over 150 m above sea level and extends more than 1.5 km from its shoreline sediment source area. In addition to a karsted upper surface and cap of reddish paleosol, the Punahoa member displays large-scale cavernous porosity, one large sinkhole, and collapse breccias.
Both the modem sand bodies and the older eolianites consist of sediment derived from isolated beaches and transported to the southwest and west by the region's strong prevailing trade winds. The thick bodies of stacked and laterally amalgamated elongate sand lobes display paleowind rose diagrams with bimodal patterns. The eolianite successions represent interglacial highstand deposits; the interbedded red paleosols and light-gray caliche units developed during periods of eustatically lowered sea level. Much of the soil most likely was derived from local late-stage explosive volcanisrn and weathering of nearby volcanics.
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Carbonate eolianites had long been considered to be limited to the Quaternary, but a number of Mesozoic and Paleozoic examples have been documented in the past 15 years. Thus, an increased awareness of carbonate eolianites is required to properly interpret the rock record and to assess their spatial and temporal distribution. The papers of this volume will help geologists to: (1) recognize carbonate eolianites and understand their preservation potential—recognitional criteria for most carbonate environments are common knowledge, but this is less true for carbonate eolianites; (2) understand their sedimentologic and diagenetic variability—diagenesis of carbonate eolianites has important economic considerations. Whereas Quaternary eolian limestones are commonly porous, Paleozoic and Mesozoic examples are typically tight owing to compaction; (3) understand the Psilionichnus (marginal marine) and Scoyenia (nonmarine) Ichnofacies—carbonate eolianites are not devoid of trace fossils; (4) interpret them in a sequence stratigraphic framework—interpretations of relative sea level during eolian deposition can be difficult, as differences between transgressive, regressive, and deflationsourced eolianites are subtle. Thus, the placement of sequence boundaries within interbedded eolian and subtidal carbonate successions is not entirely straightforward.