Sandy and gravelly carbonate sediments off Kikai-jima, southern Japan, a coral reef-related island shelf, represent the northernmost sub-tropical, carbonate deposits in the Central Ryukyu Islands (Ryukyus). On the Kikai-jima shelf, at water depths of 61–105 m, these sediments are characterized by macroid pavements. Since the abundance of very small and of exceptionally large macroids may indicate specific hydrodynamic controls regarding constraints on growth and taphonomy, the detailed analysis of Recent and fossil macroid pavements is ecologically and environmentally meaningful. Macroids, ranging in size from ca. 25 to 130 mm in diameter, are spheroidal and sub-spheroidal in shape and consist mainly of the encrusting foraminifer Acervulina inhaerens and subordinate thin encrusting and lumpy coralline algae. Accessory components include bryozoans, serpulids and, to a lesser amount, encrusting arborescent foraminifera (Homotrema, Miniacina). Sizes, shapes and growth-forms of the studied macroids, the Entobia-Gastrochaenolites-Trypanites-Maeandropolydora ichnocoenosis and the Bioerosion Index for coated grains (introduced, herein) are indicative of low sedimentation rates and occasional movement due to current action. The deep-water tidally induced current energy was sufficient to maintain multidirectional growth (spheroidal shapes) of the larger macroids and to initiate macroid growth using the diverse biogenic remnants as nuclei. The asymmetric inner arrangement suggests possible periods of stability for the macroids. The residence time of the coated grain in its original environment determines the size and morphology of the macroid and the selection of coating organisms. The composition of the coating community is mainly a consequence of component growth rates in relation to turnover time and residence time.
Recent macroids on the Kikai-jima shelf, Central Ryukyu Islands, Japan
BASSI, Davide;
2012
Abstract
Sandy and gravelly carbonate sediments off Kikai-jima, southern Japan, a coral reef-related island shelf, represent the northernmost sub-tropical, carbonate deposits in the Central Ryukyu Islands (Ryukyus). On the Kikai-jima shelf, at water depths of 61–105 m, these sediments are characterized by macroid pavements. Since the abundance of very small and of exceptionally large macroids may indicate specific hydrodynamic controls regarding constraints on growth and taphonomy, the detailed analysis of Recent and fossil macroid pavements is ecologically and environmentally meaningful. Macroids, ranging in size from ca. 25 to 130 mm in diameter, are spheroidal and sub-spheroidal in shape and consist mainly of the encrusting foraminifer Acervulina inhaerens and subordinate thin encrusting and lumpy coralline algae. Accessory components include bryozoans, serpulids and, to a lesser amount, encrusting arborescent foraminifera (Homotrema, Miniacina). Sizes, shapes and growth-forms of the studied macroids, the Entobia-Gastrochaenolites-Trypanites-Maeandropolydora ichnocoenosis and the Bioerosion Index for coated grains (introduced, herein) are indicative of low sedimentation rates and occasional movement due to current action. The deep-water tidally induced current energy was sufficient to maintain multidirectional growth (spheroidal shapes) of the larger macroids and to initiate macroid growth using the diverse biogenic remnants as nuclei. The asymmetric inner arrangement suggests possible periods of stability for the macroids. The residence time of the coated grain in its original environment determines the size and morphology of the macroid and the selection of coating organisms. The composition of the coating community is mainly a consequence of component growth rates in relation to turnover time and residence time.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
