PHYSICAL OCEANOGRAPHIC INFLUENCES ON SEDIMENTOLOGY OF REEF SAND APRONS:HOLOCENE OF ARANUKA ATOLL (KIRIBATI), EQUATORIAL PACIFIC

摘要

Depositional facies are the net product of a complex set of processes that impact sediment supply and transport in geomorphic systems. Although the general facies motifs of many isolated platforms throughout the geologic record are well documented, the details of geomorphological and sedimentological patterns, and the physical oceanographical processes that can impact sedimentological differentiation, are less well constrained. On these platforms, accumulations of reef-associated debris in platform-top reef sand aprons can form expansive geomorphic elements, hosting prolific hydrocarbon reserves in geologic analogs. To better understand the nature and scale of reef-sand-apron accumulations, this project integrates remotesensing, field, petrographical, and granulometrical characterization of surficial Holocene sediments with physical oceanographical observations and modeling of Aranuka Atoll, Republic of Kiribati, in the western equatorial Pacific. Results illustrate trends in hydrodynamics, geomorphology, and sedimentology from the platform margin to the platform interior. Current-meter data and modeling illustrate how the tides (2.5 m spring-tide amplitude) modulate wave energy (openocean annual average swell height of ~ 2 m; distal swell can be larger) to produce dominant on-platform flow (speeds up to 90 cm/s) on the northern reef sand apron. The strongest currents there appear to be impacted by southward-propagating waves related to fronts that pass thousands of kilometers north of the atoll, not local, wind-generated waves. These hydrodynamical influences are interpreted to have led to the development of the expansive northern reef sand apron (> 2000 m wide); the southeastern apron, with currents that reverse with the tides, includes a distinct, narrower sand apron (~ 1500 m wide). Concomitantly, the hydrodynamical patterns and platformward decrease in energy across the reef sand apron, coupled with changes in biota, are interpreted to control variability in sedimentary structures, bottom types, and sediment attributes. Sediment near the margin on the reef sand apron contains coral and red algal-rich coarse sand and gravel, passing to foraminifera-rich, medium to coarse sand toward the lagoon. The lagoon includes even finer sediment, chiefly fine and very fine sand, and patch reefs. Collectively, the results of this study illustrate that selective winnowing, differentiation of sediment size, type, and sorting, and nature and size of geomorphic elements ultimately are linked to the hydrodynamical patterns across the platform. The sedimentological (depositional porosity and permeability) and geomorphic (facies bodies) variability and the processes active on this reef sand apron provide a general conceptual model that may be applicable to predict trends in ancient reservoir analogs.