Subtropical to temperate facies from a transition zone, mixed carbonate–siliciclastic system, Palaeogene, North Carolina, USA



Palaeogene passive margin sediments on the US mid-Atlantic coastal plain provide valuable insight into facies interaction and distribution on mixed carbonate–siliciclastic shelves. This study utilizes well cuttings, outcrop, core, and seismic data to document temporal and spatial variations in admixed bryozoan-rich skeletal carbonates and sandy siliciclastic units that were deposited on a humid passive margin located in the vicinity of a major marine transition zone. This zone was situated between north-flowing, warm waters of the ancestral Gulf Stream (carbonate dominated settings) and south-flowing, cold waters of the ancestral Labrador Current (siliciclastic dominated settings). Some degree of mixing of carbonates and siliciclastics occurs in all facies; however, siliciclastic-prone sediments predominate in nearshore settings, while carbonate-prone sediments are more common in more open marine settings of the inner shelf break and deep shelf. A distinctive dual-break shelf depositional profile originated following a major Late Cretaceous to Palaeocene transgression that drowned the earlier shallow platform. This profile was characterized by prominent mid-shelf break dividing the shallow shelf from the deep shelf and a major continental shelf/slope break. Incomplete filling of available accommodation space during successive buildup of the shallow shelf preserved the topographic break on this passive margin. Storm wave base also contributed to the preservation of the dual-break shelf geometry by beveling shallow shelf sediments and transporting them onto and seaward of the mid-shelf break. Sediment fines in deep shelf facies were produced in place, transported downdip from the shallow shelf by storm ebb currents and boundary currents, and reworked from adjacent areas of the deep shelf by strike-parallel boundary currents. Regional climate and boundary currents controlled whether carbonate or siliciclastic material was deposited on the shelf, with warmer waters and more humid climates favouring carbonate deposition and cooler, more arid conditions favouring glaucony and siliciclastic dominated deposition. Continuous wave and current sweeping of the shallow shelf favoured deposition of mud-lean facies across much of the shallow shelf. Skeletal components in much of the carbonate-rich strata formed in warm, nutrient-rich subtropical waters, as indicated by widespread occurrences of larger benthic foraminifera and molluscan assemblages. These indicators of warm water deposition within the bryozoan-mollusk-rich carbonate assemblage on this shelf provide an example of a warm water bryomol assemblage; such facies generally are associated with cooler water depositional settings.