Donald F. McNeill

Magnetostratigraphic dating of shallow-water carbonates from San Salvador, Bahamas

Magnetostratigraphic results are reported here from a sequence of late Neogene-Quaternary shallow-water carbonate sediments from a continuous core drilled on the island of San Salvador, Bahamas. On the basis of the remanent magnetism of 136 samples from a 91-m measured section of core, the polarity sequence can be correlated with the magnetic polarity time scale from the Gilbert chron (early Pliocene) through the late Brunhes chron (late Pleistocene-Holocene). Magnetic polarities were determined on the basis of relative up-down direction in the unoriented core. Extraction studies of the magnetic particles reveal the presence of single-domain crystals of magnetite resembling those produced by the magnetotactic bacteria and algae. The sequence of reversals provides a minimum of six new major chronostratigraphic markers for the Pliocene-Pleistocene of the Bahamas; it confirms and refines the local timing of both the lithologic change from skeletal to nonskeletal sediments and the disappearance of coral and molluscan species from the Bahamas as upper late Pliocene (between 2.6 and 2.7 Ma). That the primary magnetic remanence is preserved in shallow-water carbonates, including replacement dolomites, suggests that this technique could be used to date similar Tertiary and possibly even older carbonate sequences. The establishment of a reliable magnetostratigraphy provides refined dating of shallow-water carbonates and regional faunal appearances or disappearances, sediment accumulation rates, subsidence, and depositional events.

Integrated paleontologic and paleomagnetic stratigraphy of the upper Neogene deposits around Limon, Costa Rica: A coastal emergence record of the Central American Isthmus

A chronostratigraphic study of mixed carbonate and siliciclastic sediments (Limon Group) along the Caribbean side of the Central American Isthmus was conducted to provide refined depositional ages on the uplifted, nearshore marine sequence exposed near Limon, Costa Rica. These upper Neogene sediments provide a rich faunal history that spanned the closure of the isthmus and cessation of marine circulation between the Pacific Ocean and Caribbean Sea. This faunal archive provides a critical link in regional assessment of evolutionary changes that resulted from the development of the Central American Isthmus. Results from this study have shown that integrated biostratigraphic and magnetostratigraphic techniques can be successfully combined to provide refined age dating in shallow-marine sediments, even where some microfossil reworking has occurred. We propose the new Quebrada Chocolate Formation to represent latest early–early late Pliocene mixed reefal and siliciclastic deposits that overlie the early Pliocene Rio Banano Formation. The superjacent Moin Formation is expanded to include a second reefal interval deposited near the late Pliocene-Pleistocene boundary as well as fine-grained, mollusk-rich deposits in the forereef and lagoon. The style of mixed-system reef geometry between the two units is distinct. The Quebrada Chocolate Formation reefs consist of a lower interval of alternating reefal units buried by siliciclastic sediment and an upper unit comprised of stacked, reefal buildups with relatively minor siliciclastic matrix. These stacked reefs formed on a rising sea level. In contrast, the Moin Formation reefs are isolated corals and patch reefs within a siliciclastic matrix that were deposited during the peak (maximum flooding) of the sea-level highstand. Our refined age model for the Limon Group sediments allows reassessment of Pleistocene uplift rates. We calculate a rate of about 50 m/m.y., slightly less than previous rate estimates, attributed to the shallow subduction in the Pacific of the Cocos Ridge beneath the Central American island-arc system. Development of similar high-resolution age models from other marine basins should provide the chronostratigraphic control necessary to assess environmental events and evolutionary trends in shallow-marine faunas separated and isolated by the Central American Isthmus.

Early dolomitization of platform carbonates and the preservation of magnetic polarity

Results from a combination of techniques are presented to evaluate the nature of magnetization in shallow-water platform carbonates which have undergone recrystallization during early calcification and dolomitization. Magnetic grain separates, coercivity spectra, modified Lowrie-Fuller tests, magnetization efficiency, and magnetostratigraphic constraints indicate that the ultrafine-grained magnetite is preserved during early burial geochemical regimes, inversion from aragonite/high-magnesium calcite to low-magnesium calcite, and even pervasive dolomitization. These single-domain crystals are thought to occur as interacting multigrain clusters, some of which may exceed 1 μm in diameter. These large clusters may help prohibit magnetic reorientation during diagenesis. Furthermore, during both fabric preserving and fabric destructive dolomitization, the ultrafine-scale replacement process restricts reorientation of the clusters, thus preserving depositional or early postdeposition magnetic orientation. This early dolomitization (matrix stabilization) may even help protect and extend the subsurface lifespan of the original polarity.

New Tertiary stratigraphy for the Florida Keys and southern peninsula of Florida

Seven lithologic formations, ranging in age from Oligocene to Pleistocene, were recently penetrated by core holes in southernmost Florida. From bottom to top, they are the early Oligocene Suwannee Limestone; late-early Oligocene-to-Miocene Arcadia Formation, basal Hawthorn Group; late Miocene Peace River Formation, upper Hawthorn Group; newly proposed late Miocene-to-Pliocene Long Key and Stock Island Formations; and Pleistocene Key Largo and Miami Limestones. The rocks of the Suwannee Limestone form a third-order sequence. Although the entire thickness was not penetrated, 96 m of Suwannee core from one well contains at least 50 vertically stacked, exposure-capped limestone cycles, presumably related to rapid eustatic fluctuations while experiencing tropical to subtropical conditions. The Arcadia Formation is a composite sequence containing four high-frequency sequences composed of multiple vertically stacked carbonate cycles. Most cycles do not show evidence of subaerial exposure and were deposited under more temperate conditions, relative to the Suwannee Limestone. The Arcadia Formation in southernmost Florida is bounded by regional unconformities representing third-order sequence boundaries. Post-Arcadia transgression produced a major backstepping of sediment accumulation above the upper sequence boundary of the Arcadia Formation. The Peace River Formation, composed of diatomaceous mudstones, has been identified only beneath the Florida peninsula and is not present beneath the Florida Keys. Deposition occurred during marine transgressive to high-stand conditions and a local phosphatization event (recorded in northeast Florida). The transgression is possibly related to a global rise in sea level, which resulted in upwelling of relatively cooler, relatively nutrient-rich water masses onto the Florida Platform. It is proposed that the absence of Peace River sediments beneath the Keys is due to sediment bypass of the upper surface of the Arcadia, a result of sediment sweeping by an ancestral Florida current. During late Miocene to Pliocene time in the Florida Keys, siliciclastics of the Long Key Formation and fine-grained carbonates of the Stock Island Formation prograded toward the southern edge of the Florida Platform and downlapped onto the regional unconformity at the top of the Arcadia. Shallow-marine Pleistocene limestones (Key Largo and Miami Limestones), deposited during tropical to subtropical conditions, drape over accretionary successions of the Long Key and Stock Island Formations.

The Structural and Sedimentological Controls on the Reoccupation of Quaternary Incised Valleys, Belize Southern Lagoon

A grid of high-resolution single-channel seismic data from the Belize southern lagoon has documented the development of several generations of stacked late Pleistocene (400 ka-Holocene) incised-valley fills. A feedback mechanism between tectonics, reefal buildups, and incised-valley location is apparent in the data. Incised valleys are recognized as medium- to high-amplitude reflections that truncate reflections produced by older strata on the seismic data. The Holocene valley fills in the Belize southern lagoon contain three seismic facies units (from bottom to top): (1) a basal unit characterized by moderately high-amplitude, chaotic reflections, or progradational patterns, that downlap onto the valley floor; (2) a middle unit characterized by continuous, horizontal, near-transparent reflections that drape the basal unit and onlap the valley walls; and (3) an upper unit that consists predominantly of transparent to low-amplitude, low-angle downlapping reflections. The middle and upper units are separated by an erosional surface. Based on core evidence and seismic facies analysis, the basal, middle, and upper seismic and sedimentary facies units are interpreted as fluvial and carbonate sands, estuarine muds, and marine sands and muds, respectively. The erosional surface separating the upper and middle units is interpreted to be a ravinement surface that marks the transition from brackish estuarine conditions to open-marine conditions. Faulting decreases and reefal buildups increase upsection. The oldest valleys mimic structural trends and flow through areas currently occupied by extensive carbonate reefs. Rapid, high-amplitude late Pleistocene sea level rises may have led to wides pread reef development, creating a template that subsequent incisions (reoccupation valleys) followed, regardless of structural trends. The intimate relationship among tectonics, sea level change, carbonate reef buildups, and incised-valley reoccupation serves as a model for understanding ancient, tropical mixed-carbonate-siliciclastic systems. Accelerator mass spectrometer 14C dates of sediment core samples (marine gastropods, marine mollusks, barnacles, worm tubes, peat, and oysters) indicate all the sediment cored (1-4 m) in this study is Holocene in age (between 4740 ±60 and 11,230 ±90 14C age). This radiocarbon dating consistently links the erosional surface between the upper and middle units with the intersection of the regional sea level curve (nonmarine/marine boundary).

Pliocene/Pleistocene platform facies transition recorded in calciturbidites (Exuma Sound, Bahamas)

Abstract The composition of Pliocene-Pleistocene calciturbidites of ODP Hole 632A (Exuma Sound, Bahamas) has been determined by point-count analysis of thin-sections in order to correlate basin/platform events which influenced sedimentation. Strontium isotope chronostratigraphy has been used to correlate basin turbidite compositional variations to the evolution of the platform, as documented by magnetostratigraphy. Magnetostratigraphy of Great Bahama Bank cores shows that between 3.4 and 2.5 Ma (Late Pliocene) ooids and peloids became the dominant sediments across the platform. After this period the platform has been subjected to a series of subaerial exposure events. This led to slow accumulation on the platform and starvation of the basin. Around 1 Ma the platform was reflooded, the transgression being followed by an increased production of ooids. There is a change in calciturbidite composition in the earliest Pleistocene coinciding with a regional unconformity. Pleistocene turbidites, containing abundant skeletal material and ooids, overlie Pliocene/Early Pleistocene turbidites, rich in mud and skeletal material. The duration of the hiatus, represented by the unconformity, based on 87 Sr/ 86 Sr dating is 1.0-0.7 Ma. Prior to this hiatus sedimentation rates in the basin are an order of magnitude lower than present-day accumulation rates. We conclude that reflooding of the platform around 0.8 Ma led to shedding of both non-skeletal and skeletal sediments. The change of Great Bahama Bank from a Pliocene reef-rimmed skeletal platform to the present-day flat-topped bank, dominated by non-skeletal sediments, can be correlated with the observed change in turbidite composition.

Earlier (late Pliocene) first appearance of the Caribbean reef-building coral Acropora palmata: Stratigraphic and evolutionary implications

An integrated stratigraphic study of reefal deposits on the Caribbean side of the Isthmus of Panama (Limon, Costa Rica) has discovered a significantly earlier first appearance of the major reef-building coral Acropora palmata . A. palmata is here reported from the early late Pliocene, constrained in age to within the Gauss chron (ca. 3.6–2.6 Ma). This coral was previously thought to have originated in the earliest Pleistocene and has subsequently been used as a Quaternary marker throughout the Caribbean and the Bahamas. An earlier appearance in the southern Caribbean implies a diachronous first appearance datum relative to the northern Caribbean. This older age also places A. palmata well within the transition phase of a Pliocene (4–1 Ma) faunal turnover that was marked by widespread extinction and origination of Caribbean coral species. An early late Pliocene origination is coincident with formation of the Isthmus, climate reorganization, and frequent sea-level changes associated with onset of Northern Hemisphere glaciations. The rapid growth and accumulation rates that characterize A. palmata may therefore be adaptive to these fluctuating environmental conditions, enabling its success during the subsequent Pleistocene glacial cycles.

Rise and fall of Pliocene free-living corals in the Caribbean

Climate change is currently having an impact on shallow-water corals, and global circulation models predict that levels of p CO 2 and temperature will rise within the next century above anything recorded for at least the past 650 k.y. The Pliocene Epoch is a recent, albeit imperfect, geologic analog for such conditions in the Caribbean. Diverse communities of free-living solitary and flabelo-meandroid (FSFM) corals inhabited shallow nearshore to deeper oligophotic habitats of the Pliocene. FSFM corals were well suited to the low-angle depositional profiles, increased productivity, increased sedimentation, and warmer temperatures of the Pliocene. Origination rates of FSFM coral species between 8 and 4 Ma are roughly double other zooxanthellate corals. FSFM corals underwent abrupt extinction between 2 and 1 Ma, as environmental conditions changed and suitable habitat was eliminated. The evolutionary bottleneck of Pliocene–Pleistocene extinctions and relic steep-margined Pleistocene topography may leave modern faunas vulnerable as we return to Pliocene-like conditions.

Applications of Paleomagnetism to Sedimentary Geology

Applications of Paleomagnetism to Sedimentary Geology - Based on the 1991 SEPM Research Symposium, the results are directed towards bringing the disciplines of Paleomagnetism and sedimentary geology closer together. Advances in the field of sedimentary geology will likely result from continued development of new ideas, questioning of old dogma, and, most importantly, providing means for testing these new hypotheses. It is hoped that the union of these two disciplines will help address many fundamental geological questions, such as the perennial problems of precise age-dating, stratigraphic correlation and geometries, understanding the timing and nature of post-depositional diagenetic fabrics, and the intriguing relationship between hydrocarbons and magnetization. The reader will find an unusual diversity of research topics presented in this volume. This diversity serves as a testimony to the potential applications awaiting the sedimentary geologist willing to explore these new paleomagnetic tools.

Magnetostratigraphic dating of the uplifted atoll of Maré: Geodynamics of the Loyalty Ridge, SW Pacific

The Loyalty Islands (SW Pacific) are uplifted as they are progressively affected by the lithospheric flexure of the Australian plate, before its subduction under the New Hebrides Arc. These geodynamic changes are constrained by magnetostratigraphically dating two sections from Mare Island, where mineral extractions coupled with rock-magnetic experiments suggest that the magnetic remanence is mostly carried by a mixture of single-domain to multidomain magnetite/maghemite. With the help of faunal determinations and radiometric dating, the sequences of polarity reversals, correlated to the geomagnetic polarity timescale, range from the top of Chron C4n (late Miocene) to the top of the Gauss Chron (late Pliocene). This new chronostratigraphy refines the timing of two distinctive carbonate units (rhodolith platform/coral reefs) separated by a hardground whose transition is known to coincide approximately with a regional event around the Miocene/Pliocene boundary. The magnetostratigraphic dating indicates that the hardground represents about a 1.9 m.y. hiatus and suggests variable sedimentation rates ranging from 4.7 to 65.4 m/m.y. during the atoll construction. The lithospheric bulge seems to have influenced the evolution of Mare Island some 3.1 m.y. ago, leading to a diachronous emersion of the northeast and southwest rim of the atoll with a mean uplift rate of the order of 4 cm/kyr.