Wings, mushrooms, and Christmas trees: The carbonate seismic geomorphology of Central Luconia, Miocene–present, offshore Sarawak, northwest Borneo

Abstract

Central Luconia is a geological province of the Sarawak Basin, offshore northwest Borneo, characterized by extensive development of Miocene to Holocene carbonate buildups. The buildups are up to 2-km (1-mi) thick and reveal complex seismic geomorphologies including out-building, in-building, carbonate-siliciclastic intercalations, and coalescence of neighboring carbonate bodies. Intermittent carbonate and siliciclastic deposition is interpreted to have been governed by frequently oscillating sea level and variable siliciclastic input. Location of the buildups with respect to deltaic facies and the seismic expressions of marginal carbonate strata, locally known as wings, are used here to classify carbonate seismic geomorphologies. Buildups encompassed by deltaic topsets are said to reside onshelf and are flanked by marginal carbonate deposits conformable with the onlapping siliciclastics and interpreted as thin carbonate platforms deposited in situ over prominent erosional unconformities and flooding surfaces. Such marginal facies are described on the basis of their appearance as open wings. In contrast, buildups downlapped by prodelta foresets or bottomsets, or those draped by pelagic sediments, are said to reside offshelf and are characterized by high relief, steeply dipping marginal carbonate strata discordant with respect to the relatively flat-lying platform carbonates as well as the onlapping siliciclastics. These marginal strata have an appearance of closed wings and are interpreted as slope aprons composed of carbonate material derived from the productive platform tops and deposited off-platform in deeper water. Carbonate geometries range from contracting-upward pinnacles to expanding-upward platforms in onshelf settings, whereas buildups located offshelf have mostly contracting-upward or aggrading morphologies. Buildups deposited onshelf have generally maintained a similar height of depositional slopes as they evolved, reflecting rates of background siliciclastic deposition approximating those of carbonate growth. In contrast, depositional slopes of offshelf buildups have become progressively higher, owing to limited siliciclastic input. Observations presented here on carbonate geomorphology and adjacent siliciclastic stratigraphy provide important clues for the understanding of the geology of the province. Because carbonates and siliciclastics intercalate frequently, siliciclastic biostratigraphy can be used to precisely date and correlate geographically remote carbonate-reservoir intervals and intrareservoir depositional and diagenetic events such as marine transgressive or exposure surfaces or karst networks. Conversely, basin paleobathymetry can be measured by the size of carbonate clinoforms and the thickness of buildups established over flooded siliciclastic surfaces. Deposition of low-relief carbonate platforms over siliciclastic substrates indicates shallow-marine conditions, as opposed to the prevalent notion of kilometer-scale paleobathymetry. These interpretations have important implications for understanding of the sedimentology and lithology of the ambient basinal siliciclastics, with fundamental applications to hydrocarbon exploration.