Albert C. Hine

Platforms of the Nicaraguan Rise: Examples of the sensitivity of carbonate sedimentation to excess trophic resources

The Nicaraguan Rise is an active tectonic structure in the western Caribbean. Carbonate accumulation on its platforms has not kept pace with relative Holocene sea-level rise, despite a tropical location remote from terrigenous sedimentation. Trophic resources apparently exceed levels favoring coral-reef development because sponge-algal communities dominate the drowning western platforms, in contrast to mixed coral-algal benthos on Pedro Bank and well- developed coral reefs along the north coast of Jamaica. Concentrations of biotic pigments in sea-surface waters show a corresponding west-east gradient; oceanic waters flowing over the western banks carry nearly twice as much biotic pigment as oceanic waters north of Jamaica. Sources enriching the western Caribbean are terrestrial runoff, upwelling off northern South America, and topographic upwelling over the Nicaraguan Rise. That relatively modest levels of trophic resources can suppress coral-reef development holds important implications for understanding carbonate platform drownings in the geologic record.

Halimeda bioherms along an open seaway: Miskito Channel, Nicaraguan Rise, SW Caribbean Sea

A recent research cruise to examine small, detached carbonate platforms situated on the Nicaraguan Rise in the SW Caribbean Sea has revealed the presence of numerous Halimeda bioherms. Based upon interpretations from seismic reflection data some exceed 140 m in relief. This is the first documented occurrence of these green-algal buildups in the Caribbean/Bahama Bank region. The Halimeda bioherms form a nearly continuous band that borders the margins of the Miskito Channel—a shallow, open, 125 km long seaway. This 220 m deep channel bisects the Miskito Bank which is a major carbonate shelf. In seismic profile the bioherms appear acoustically “soft” and reveal a local relief of 20–30 m. Tops of these features lie in about 40–50 m of water. Samples from dredge hauls are coarse, poorly cemented packstones/grainstones which are dominated by largely unbroken, disarticulated Halimeda segments set in a poorly sorted sandy matrix. Exposed surfaces were stained brown. Very little living material was brought up in the dredges. The significance of these bioherms and their full extent in the Caribbean are not understood. Undoubtedly, further study will provide important answers concerning their role in the geologic development of Caribbean carbonate platforms.

Magnitude and timing of episodic sea-level rise during the last deglaciation

A succession of elevated ridge deposits on the south Florida margin was mapped using high-resolution seismic and side-scan sonar imaging in water depths ranging from 50 to 124 m. The ridges are interpreted to be subtidal shoal complexes and paleoshorelines (eolian dune or beach) formed during the last sea-level transgression. Oolitic and skeletal grainstones and mixed skeletal-peloidal-ooid packstones were recovered using a research submersible. All of the grains are of shallow-water or intertidal origin, and both marine and nonmarine cements were identified. Formation and preservation of these features are attributed to episodic and rapid changes in the rate of the deglacial sea-level rise at the onset of the termination 1A δ18O excursion. This high-resolution record of sea-level change appears to be related to deglacial processes operating on submillennial time scales and supports increasing evidence of rapid episodic fluctuations in ice volume, climate, and ocean-circulation patterns during glacial-interglacial transitions.

Coastal lithosome preservation: Evidence from the shoreface and inner continental shelf off Bogue Banks, North Carolina

Abstract Seismic and vibracore data from the continental shelf as well as borehole data from an adjacent barrier island indicate that the migrating shoreface, responding to rising sea level, has nearly completely removed the entire coastal sedimentary record in northern Onslow Bay, North Carolina. This process was active throughout the Quaternary and even the late Tertiary as middle Tertiary sediments and rocks directly crop out on the sea floor in vast areas of this shelf sector. Seaward of Bogue Banks (a Holocene barrier island located along northern Onslow Bay) seismic sequences have been truncated by the modern shoreface which extends to about 12 m depth. Correlations with borehole data on the island indicate that these sequences are Pleistocene and Holocene in age. The Holocene sequences contain numerous channels which are interpreted to be relict tidal inlets. However, none extends vertically lower than the shoreface indicating that landward translation of the shoreface would remove most of even the deepest Holocene coastal lithosomes, parts of the Pleistocene and even a portion of the Tertiary. The middle and inner portions of the continental shelf have been incised by numerous channels. Channel facies are an important component of shelf sedimentary and stratigraphic sequences and comprise about one-third of northern Onslow Bay. Similar seismic infilling facies and vibracore data as well as the poor preservation potential of the tidal inlet throats indicate that the channels were lower coastal plain streams. The infilling sediments are mostly muds with some sands and shells. Dates from shells via the amino acid racemization technique are Pleistocene in age. These channels, with some having multiple infilling events, assume greater importance when one recognizes that essentially they alone hold the vast majority of the Quaternary record of numerous sea-level fluctuations.

Quaternary bryozoan reef mounds in cool-water, upper slope environments: Great Australian Bight

Bryozoan reef mounds are common features in the geological record, occurring within mid-ramp, slope paleoenvironments, especially in Paleozoic carbonate successions, but until now have not been recorded from the modern ocean. Recent scientific drilling in the Great Australian Bight (Ocean Drilling Program Leg 182) has confirmed the existence of shallow subsurface bryozoan reef mounds in modern water depths of 200–350 m. These structures have as much as 65 m of synoptic relief, and occur both as single mounds and as mound complexes. They are unlithified, have a floatstone texture, and are rich in delicate branching, encrusting and/or nodular-arborescent, flat-robust branching, fenestrate, and articulated zooidal bryozoan growth forms. The muddy matrix is composed of foraminifers, serpulids, fecal pellets, irregular bioclasts, sponge spicules, and calcareous nannofossils. The 14C accelerator mass spectrometry dates of 26.6–35.1 ka indicate that the most recent mounds, the tops of which are 7–10 m below the modern seafloor, flourished during the last glacial lowstand but perished during transgressive sea-level rise. This history reflects changing oceanographic current patterns; strong upwelling during lowstands, and reduced upwelling and lowered trophic resources during highstands. Large specimens of benthic foraminifers restricted to the mounds confirm overall mesotrophic growth conditions. The mounds are similar in geometry, scale, general composition, and paleoenvironments to older structures, but lack obvious microbial influence and extensive synsedimentary cementation. Such differences reflect either short-term local conditions or long-term temporal changes in ocean chemistry and biology.

Megabreccia shedding from modern, low-relief carbonate platforms, Nicaraguan Rise

Single-channel seismic reflection data from the margins of lowrelief (150-250 m, measured from edge of bank to basin) carbonate platforms on the northern Nicaraguan Rise reveal complex seismic intervals consisting of mounded, chaotic seismic facies interspersed with discontinuous, parallel/laminated seismic facies. We interpret that these intervals contain megabreccias (chaotic facies) and sandy turbidites (parallel/laminated facies). One megabreccia is exposed on the sea floor displaying an overall fan shape having individual blocks measuring nearly 300 m across and >110 m high. The source area consists of a scalloped embayment with a headwall scarp 180 m high. Reflections within the platform are sharply truncated by this escarpment. This single megabreccia is ∼120 m thick and extends ∼27 km along slope and ∼16 km out into the basin. Other megabreccias within the basin have individual blocks measuring >400 m across. Rocks from dredge hauls are a mixture of shallow- and deep-water facies. Shallow-water facies consist of mixed, skeletal grain-stones and Halimeda packstones. Deep-water facies are massive chalks, chalks with shallow-water skeletal grains, and chalk-block breccias. This indicates that the megabreccias formed as a result of bank-margin collapse, during which the ensuing debris flow eroded into slope and basin facies, mixing rock types together. We speculate that bank-margin-collapse events, resulting in megabreccia formation, may have been seismically triggered, and we emphasize that these large-scale, mass-wasting events occurred along margins of low-relief carbonate platforms.

Bedrock controls on barrier island development: West-central Florida coast

Abstract Fifty vibracores and 130 km of high-resolution seismic profiles were taken in the lagoons and nearshore area of northern Pinellas County, Florida, to determine the role of antecedent topography upon barrier island location, origin and development. East—west seismic profiles show a terrace with 1–1.5 m of relief located along the seaward trend of the barrier islands between −4 and −6 m MSL. This antecedent topography is composed of a relatively erosion resistant Miocene limestone and the terrace is usually present as an increase in slope. A contour map of the pre-Holocene surface shows that this subsurface feature is continuous along the entire study area and is mimicked by the opposing orientations of Honeymoon and Caladesi Islands. The correspondence of the modern barrier system to the antecedent topography indicates that: (1) the islands formed on the terrace; or that (2) landward migration of the barriers was stopped by grounding on this feature. Stratigraphic analyses of lagoonal and nearshore environments were conducted in order to establish the overall response of the northern Pinellas coastal system to the late Holocene sea-level rise. This analysis indicates a basically transgressive Holocene sequence with lagoonal sediments occurring seaward of the islands. Underlying the lagoonal deposits in all areas is a 1 m thick relict Pleistocene unit that has been flooded, reworked and vegetated by intertidal to subtidal organisms. A model of development for these barriers is proposed which incorporates an early transgressive history followed by a recent progradational phase. The progradational phase was probably initiated by the late Holocene slowdown in sea-level rise and an increased sediment supply as the barrier islands migrated over a relict sediment source. The antecedent topography thus served as a stabilization point for landward barrier migration and seaward progradation. The antecedent topographic control of barrier island location described by this model is dependent upon the coincident decline in the rate of sea-level rise and an increased sediment supply.

The seismic stratigraphy of Okanagan Lake, British Columbia; a record of rapid deglaciation in a deep ‘fiord-lake’ basin

This paper presents the first detailed data regarding the newly discovered deep infill of Okanagan Lake. Okanagan Lake (50°00′N, 119°30′W) is 120 km long, ∼ 3–5 km wide and occupies a glacially overdeepened bedrock basin in the southern interior of British Columbia. This basin, and other elongate lakes of the region (e.g. Shuswap, Kootenay, Kalamalka, Canim and Mahood lakes), mark the site of westward flowing ice streams within successive Cordilleran ice sheets. An air gun seismic survey of Okanagan Lake shows that the bedrock floor is nearly 650 m below sea-level, more than 2000 m below the rim of the surrounding plateau. The maximum thickness of Pleistocene sediment in Okanagan Lake basin approaches 800 m. Forty-six seismic reflection traverses and an axial profile show a relatively simple stratigraphy composed of three seismic sequences argued to be no older than the last glacial cycle (< 30 ka). A discontinuous basal unit (sequence I) characterized by large-scale diffractions, and up to 460 m thick, infills the narrow, V-shaped bedrock floor of the basin and is interpreted as a boulder gravel deposited by subglacial meltwaters. Overlying seismic sequence II is composed of two sub-sequences. Sub-sequence IIa is a chaotic to massive facies up to 736 m thick. Lakeshore exposures close to where this unit reaches lake level show deformed and chaotically-bedded glaciolacustrine silts containing gravel lens and large ice-rafted boulders. The surface topography of this sub-sequence is irregular and in general mimics the form of the underlying bedrock as a result of compaction. This sequence passes laterally into stratified facies (sub-sequence IIb) at the northern end of the basin. Seismic sequence II appears to record rapid ice-proximal dumping of glaciolacustrine silt as the Okanagan glacier backwasted upvalley in a deep lake. A thin (60 m max.) laminated seismic sequence (III) drapes the hummocky surface of sequence II and represents postglacial sedimentation from fan-deltas. The extreme thickness of sequences I and II in Okanagan Lake reflects the focussing of large volumes of meltwater and sediment into the basin during deglaciation; pre-existing sediments that pre-date the last glacial cycle appear to have been completely eroded. Glaciological conditions during sedimentation may have been similar to marine-based outlet glaciers calving in deep water in fiord basins. In contrast to marine settings where ice bergs are free to disperse, large volumes of dead ice were trapped within the basin; structural evidence for sedimentation around dead ice blocks has been previously used to argue that the Cordilleran Ice Sheet downwasted in situ. We emphasize in contrast, the trapping of dead ice left behind by rapidly calving lake-based outlet glaciers.

Three-dimensional sedimentary framework of the carbonate ramp slope of central west Florida: A sequential seismic stratigraphic perspective

The sequential post-Early Cretaceous stratigraphic evolution of the carbonate ramp slope of central west Florida has been determined via the integration of ∼1,500 km of seismic reflection profiles with drillcores, piston cores, and rock dredge hauls. Our goal has been to gain insight into, and increase our knowledge and understanding of, carbonate ramps in general, as well as to develop a model for potential application to the rock record. Our results, which focus on the Neogene, indicate that the three-dimensional sedimentary framework of the west Florida ramp slope has developed in response to the interplay of a number of processes, including (1) regional tectonic subsidence, (2) long-term sea-level change, (3) short-term sea-level oscillations, (4) large-scale gravity collapse, (5) a major paleoceanographic event, (6) fluctuating intensity of oceanic circulation, (7) lateral migration of ocean currents, and (8) open-ocean upwellang and pelagic carbonate production. During the Early Cretaceous (seismic sequence VI), the continental margin of west Florida was a large shallow-water carbonate/evaporate platform that had persisted since late stages of continental rifting in the Jurassic. During the mid-Cretaceous (seismic sequence V), terrigenous sediment/nutrient influx terminated platform growth and deposited a layer of neritic marl. For the next 60 m.y., during a long-term eustatic highstand in the Late Cretaceous-Paleogene (seismic sequence IV), the present west Florida slope was an open-ocean, deep- water marginal plateau that accumulated pelagic carbonates. Following a eustatic lowstand in the late Oligocene (∼30 m.y. ago), the carbonate ramp slope of central west Florida began to take shape as a low-relief ramp margin built seaward into the early and middle Miocene (seismic sequences III and II). In the latest early Miocene, large-scale gravity collapse generated a slope basin that was rapidly filled by prograding clinoforms. In the middle Miocene, 12-15 m.y. ago, intensification of oceanic circulation truncated antecedent clinoforms, blocked off-shelf sediment transport, and stimulated pelagic carbonate production. This oceanographic event resulted in the succession of seaward-prograding clinoforms by a slope-front-fill pelagic depositional system that has survived numerous sea-level oscillations into the present (seismic sequence I).

Cay Sal Bank, Bahamas — A partially drowned carbonate platform

Abstract Recent high-resolution seismic reflection profiling, sediment sampling, SCUBA observations and Landsat imagery show that Cay Sal Bank (CSB) has very limited reef development, no active sand shoals, few islands, a thin to non-existent sedimentary cover and a relatively deep margin (20–30 m) and shelf lagoon system (10–20 m), Windward and leeward margins can be discerned, but their general development is poor when compared to the shallower, more active margins of Little Bahama Bank (LBB) and Great Bahama Bank (GBB). Windward margins (facing north and east) along CSB are generally deep, rocky, sediment-barren terraces supporting limited, low-relief, relict(?) reefs. Leeward margins have small sand bodies (maximum thickness 10 m) covering reef structures at the bank edge indicating that offbank transport of sands has occurred. However, these marginal sand bodies are limited in areal extent, suggesting that this transport system was not ubiquitous along the south- and west-facing margins. Seismic and grab sample data from the deep (200–500 m) slopes seaward of the leeward margins show a thin, discontinuous unit of periplatform sand containing Halimeda , molluscs, and non-skeletal components derived from shallow water. The limited basinward extent (no deeper than 300 m) of this unit, recognized by its reflection-free seismic facies, also indicates that sand production and transport off the bank were never prolific. This is in stark contrast to new seismic data from the leeward margins of GBB which clearly show thick (20 m) sand bodies covering high reefs 12–15 m along the outer margin and multiple reflection-free units extending to great depths (600 m) all along the adjacent slope. The apparent immature development of normal bank-top processes and facies, and the absence of key modern depositional environments on CSB may be related to the rate at which this platform was submerged. Due to its comparatively low elevation, the initial Holocene flooding occurred at approximately 8–10 ka when sea level rise was rapid (6 m ka −1 ). By comparison, the higher LBB/GBB were flooded later at a much slower rate (1.5 m ka −1 ). The relatively rapid flooding of CSB provided little time for the shallow depositional environments to start up. The continued rapid rate of sea-level rise after drowning, and offbank transport of sediment and the export of chilled waters formed during winter months, prevented the resulting facies from catching up. Consequently, CSB appears to be partially drowned, particularly when compared to the other “healthier” rimmed Bahamian platforms. Drowned carbonate banks are very common in the ancient record and potentially provide excellent stratigraphic traps for hydrocarbons. CSB provides a modern example of a bank that may be in the very early stages of termination. Final drowning of carbonate platforms may occur as a result of frequent higher order sea-level fluctuations superimposed on initial stage of a lower order sea-level cycle.