Mississippi Fan Research Articles

Generation of high-frequency topographic Rossby waves in the Gulf of Mexico

The Loop Current Eddy (LCE) separation cycle energizes deep circulation in the eastern Gulf of Mexico, transferring energy from the surface intensified Loop Current (LC) to the typically quiescent lower layers. To document the generation and radiation of deep energy during this cycle, an array of 24 current and pressure recording inverted echo sounders (CPIES) is deployed in the region 89°W to 86°W, 25°N to 27.5°N with the intent to capture circulation near bathymetric features thought to be important for current-topographic interactions: Campeche Bank, Mississippi Fan, and West Florida Shelf. During the nearly two-year deployment, June 2019 to May 2021, three LCE separation events are observed, during which energy injected into the deep Gulf organizes into two distinct frequency bands (1/100 – 1/20 days–1 and 1/20 – 1/10 days–1). High-frequency variability dominates the array’s northwest corner in the vicinity of the Mississippi Fan. Wave properties are consistent with topographic Rossby Waves (TRWs) with wavelengths of 150 – 300 km. Their generation coincides with each LCE separation and is attributed to an upper-lower layer resonant coupling between surface meanders and the sloping topography of the Mississippi Fan. TRWs captured by the CPIES array will likely intensify as wavelengths shorten in steeper topography along propagation pathways towards the Sigsbee Escarpment, generating hazardous currents with the potential to disrupt oil and gas operations in the region.

Seismic geomorphology of the Chandeleur submarine landslide in the northern Gulf of Mexico

Abstract The Chandeleur Submarine Landslide Complex occurs on the upper Mississippi Fan of the Gulf of Mexico in approximately 1100 m of water, 200 km SE of New Orleans, Louisiana. This part of the Mississippi Fan received high sedimentation throughout the Pleistocene, causing high pore fluid pressure and abundant slope failures, though few as large as the Chandeleur. Given its large size, proximity to major coastal cities and seafloor infrastructures, we examine the Chandeleur Slide to understand what led to the initial slope failure and decipher its post-failure transport behaviour using 2D and 3D multichannel seismic surveys, high-resolution bathymetric data, and well logs. We find a large sediment mass with a translational-rotational behaviour that was displaced to the south/SE up to 40 km from the source area. The Chandeleur Slide includes extensional faulting in the headscarp area and compressional structures in the toe where confined by a natural ramp-like structure. Beneath the Chandeleur Slide, we identify a regional sand-rich unit (called the Blue Unit) that is known to be overpressured. Beneath the Blue Unit we observe an upward-migrating salt diapir. We suggest one possible scenario for the origin of the Chandeleur Slide is the combined effects of an upward-migrating salt diapir impinging on an already overpressured Blue Unit, leading to the initial failure. The initial failure was followed by retrogressive headwall retreat northward, which created the prominent scarp on the seafloor. In total, the Chandeleur Slide complex covers an area of about 1000 km 2 and contains about 300 km 3 of sediment.

Integrated characterization and failure mechanism for a mid-Pleistocene mass transport complex-dominant interval in the Mars Ursa Basin, northern Gulf of Mexico, USA

The economic and operational risks associated with mass transport complexes (MTCs) in deepwater hydrocarbon exploration act as a principal motivation to investigate their depositional elements using industry data. There is a lack of extensive seismic and well data coverage that limits the understanding of the processes associated with the evolution of MTCs within deepwater sedimentary basins. This study leverages a unique integrated data set to evaluate the depositional character and potential failure mechanisms of seven identified MTCs preserved in a synkinematic mid-Pleistocene MTC-dominant interval that spans the hydrocarbon-bearing Mars Ursa Basin in the northern Gulf of Mexico. Through seismic interpretation and attribute extraction methods using a 3D prestack depthmigrated seismic survey, we have determined the kinematic indicators and preserved the morphodomain geometries of the identified MTCs. The MTC-dominant interval covers an area of 631 km2, a volume of 392 km3, and a maximum thickness of 549 m in minibasin centers. The interval is penetrated by 15 boreholes that provide stratigraphic and lithologic calibration of the morphometric analyses. The lithologic composition of the MTC-rich interval is claystone/mudstone-dominant with a few interbedded, thin sandstones. The identified kinematic indicators and geometric extent of the identified MTCs are a function of the local salt tectonics extrabasinal controls. The stratigraphic framework presented in this study constrains the timing of failures to a period of high sediment deposition related to a major increase of glacial input into the Quaternary Mississippi Fan. This study offers borehole-calibrated MTC morphometrics preserved in an MTC-dominant interval whose failure is triggered by local salt inflation, but ultimately it is a consequence of loading following increased sediment supply into the basin. The results from this robust data set build upon past integrated seismic-well studies that strive to improve the understanding of MTC processes and their implications in hydrocarbon exploration across salt sedimentary basins.

Interpretation of Gas Hydrate Province Supported by Petrophysical Analyses: Mississippi Fan, Gulf of Mexico

AbstractA vast accumulation of high‐saturation gas hydrates has been hypothesized to be present in the central Gulf of Mexico; the interpretation was based on seismic travel time deficits underlying high velocity Pleistocene coarse‐grained turbidites. While these deposits could represent one of the largest‐known gas hydrate reservoirs, some workers have concluded that lithologic contrasts are instead the primary cause of observed velocity pull‐ups. Here, using cuttings and core, well logs, and seismic data from the Mississippi Fan, we demonstrate that lithologic differences can account for a maximum of ~6–33% of the total observed travel time deficits. This conclusion supports the previous interpretation of a gas hydrate province, provides a basis for distinguishing the cause of velocity pull‐ups, and suggests that the future of gas hydrate exploration should continue to include the coarsest‐grained component of the Earth's largest near‐seafloor submarine fans.

Stratigraphic aliasing and the transient nature of deep-water depositional sequences: Revisiting the Mississippi Fan

Stratigraphic aliasing and the transient nature of deep-water depositional sequences: Revisiting the Mississippi Fan

Gas hydrates in coarse-grained reservoirs interpreted from velocity pull up: Mississippi Fan, Gulf of Mexico: REPLY

Gas hydrates in coarse-grained reservoirs interpreted from velocity pull up: Mississippi Fan, Gulf of Mexico: REPLY

Gas hydrates in coarse-grained reservoirs interpreted from velocity pull up: Mississippi Fan, Gulf of Mexico: COMMENT

Gas hydrates in coarse-grained reservoirs interpreted from velocity pull up: Mississippi Fan, Gulf of Mexico: COMMENT

Late Pleistocene glacial transitions in North America altered major river drainages, as revealed by deep-sea sediment

Sediment eroded from continents during ice ages can be rapidly (<104 years) transferred via rivers to the deep-sea and preserved in submarine fans, becoming a viable record of landscape evolution. We applied chemical weathering proxies and zircon geo-thermo-chronometry to late Pleistocene sediment recovered from the deep-sea Mississippi fan, revealing interactions between the Laurentide ice sheet (LIS) and broader Mississippi–Missouri catchment between ca. 70,000 and 10,000 years ago (70 to 10 ka). Sediment contribution from the Missouri catchment to the Mississippi fan was low between 70 and 30 ka but roughly doubled after the Last Glacial Maximum (LGM). Therefore, pre-LGM glacial advance profoundly altered the vast Missouri drainage through ice dams and/or re-routing of the river, thereby controlling the transfer of continental debris and freshwater toward southern outlets.

Gas hydrates in coarse-grained reservoirs interpreted from velocity pull up: Mississippi Fan, Gulf of Mexico

Gas hydrates in coarse-grained reservoirs interpreted from velocity pull up: Mississippi Fan, Gulf of Mexico

Muddy sand and sandy mud on the distal Mississippi fan: Implications for lobe depositional processes

We used laser particle size analysis (LPSA) to quantitatively analyze grain-size characteristics from distal Mississippi submarine fan deposits (Gulf of Mexico) and relate them to established depositional models along the spectrum of sediment gravity flows. One hundred and seventy-nine (179) sediment samples from 22 beds were obtained from cores of Deep Sea Drilling Project Leg 96 Sites 614, 615, and 621. The sediment from Sites 614 and 615 was deposited in lobes ∼500 km downstream from the Mississippi canyon head. Most samples were described as sand from visual inspection of cores, containing >75% volume of sand grains, with clay content <5% based on LPSA. A muddy sand definition was given to sands comprising >20% silt and finer grains, with generally higher clay content (3%–16%). However, LPSA data show that facies assignments from visual core description overestimated the proportion of sand grains: 15% of samples initially interpreted as sand contain <75% sand grains; 40% of muddy sand samples have <50% sand. Samples from lobes at Sites 614 and 615 were compared to samples from Site 621, where beds are sandier with a relatively narrow distribution of grain sizes deposited in a more proximal channel setting, <250 km from the canyon head. We interpret the large proportion of silt and finer grains in the distal lobes to reflect diminished turbulence within sediment gravity flows as they expand from confined to unconfined settings downstream of channel conduits. Entrainment of finer-grained sediment at the channel-to-lobe transition could have also contributed to larger proportions of silt and finer grains in distal lobes.

Gulf of Mexico Loop Current path variability

Loop Current, LC, path variability exhibits a continuum of spatial and temporal scales, all are called meanders in this work. They arise from a variety of processes, including short and long waves, frontal eddies with or without closed cores and developing baroclinic instability. They have been extensively studied with satellite sea surface temperature SST, and height, SSH. Yet, these systems provide an incomplete view into LC meandering: SST measurements are hampered by cloud coverage and low thermal contrast in summer months and SSH measurements by altimeter temporal and spatial resolution. In an effort to resolve LC meander temporal and spatial scales, they are investigated using a mesoscale resolving in situ array deployed in the Gulf of Mexico. The array, which consisted of twenty-five inverted echo sounders with pressure gauges, PIES, and current meter moorings, was deployed April 2009 and recovered in October–November 2011. The broad extent of the array, nominally 89°W to 85°W, 25°N to 27°N, enabled quantitative mapping of the regional circulation. LC meander properties are characterized as a function of spatial distribution of energy, frequency, wavenumber, and phase speed. Dispersion characteristics and meander scales are comparable to those found in the Gulf Stream. Phase speeds increase with frequency and range from 8 to 50kmd−1. Wavelengths associated with each band are as follows: 460km for the 100 to 40d band, 350km for the 40 to 20d band, 270km for the 20 to 10d band and 230km for the 10 to 3d band. The strongest variability is in the 100 to 40d band. Spatially the 100 to 40d variability is concentrated to east of the Mississippi Fan, growing and propagating downstream along the eastern portion of the LC. Meanders between 40 and 20d propagate along the full encompassed length of the LC. Their temporal amplitudes peak at the time of LC eddy detachment and separation. Meanders with shorter periods than 20d do not always propagate along the full encompassed length of the LC, perhaps due to the location of the LC.

Cyclonic activity in the eastern Gulf of Mexico: Characterization from along-track altimetry and in situ drifter trajectories

The shedding sequence of the Loop Current (LC) inside the Gulf of Mexico (GoM) is strongly influenced by cyclonic frontal eddies around its edge. Along-track altimetry data, analyzed based on a wavelet decomposition to provide estimates of individual cyclones’ diameter, amplitude and relative vorticity, and in situ surface drifter data from the Global Drifter Program, are used to investigate the cyclonic activity in the eastern Gulf of Mexico, where the LC extends and retracts. By analyzing this ∼20year long (1992–2011) combined set of observation data records, we were able to complement previous findings, to confirm results from modeling studies and to provide new insights on the LC frontal dynamics.Drifter data indicate, for the first time, that Loop Current Frontal Eddies (LCFEs) are in solid-body rotation close to their core. This property makes relative vorticity the most robust diagnostic from along-track altimetry for characterizing LCFEs in the eastern GoM, based on consistent comparisons with drifter data. Both data sets are complementary for describing the LCFEs’ regional variability.LCFEs observed in the deep southeastern GoM show intense relative vorticity, but they are not frequently observed. The study of an unprecedented, long drifter trajectory suggests that they are not intensified locally. This implies that, among LCFEs coming from the northern GoM, only intense ones reach the deep southeastern GoM.The observation datasets provide, for the first time, quantitative evidences of processes so far only identified with models: LCFEs are intensified when they are advected over the Mississippi Fan in the northern GoM; a small area north of Campeche Bank shows intense LCFE activity.The altimetry and drifter data confirm and complement results from more limited datasets: LCFEs forming the Tortugas Eddies, at the entrance of the Straits of Florida, are the most intensely observed LCFEs in altimetry. Coming from the GoM interior, they can be modified, in size and intensity, just before or during their stay in the Dry Tortugas area, whereas they were long considered to be modified only after they were advected in the Straits of Florida. A single drifter trajectory illustrates in detail the erosion of a LCFE in the southeastern GoM, presumably under the influence of the LC, which leads to the re-arrangement of vorticity and the formation of a distinct LCFE, just upstream the Dry Tortugas area.The study also confirms that LCFEs have diameters of 80–120km, generally larger east and north of the LC. They tend to stay for long periods in the northeastern GoM, and thus may undergo vortex merging with incoming eddies, as previously noted on individual episodes. Altimetry reveals that the largest LCFEs reside in the central eastern GoM, in a location where they are known to block the LC extension; however, our long altimetry dataset shows that these large eddies are not always involved in such LC blocking.Finally, this study led to improvements in data treatment, for both along-track altimetry and drifter trajectories. The wavelet approach used to derive eddy characteristics from along-track altimetry allows robust estimates of the eddy relative vorticity. The drifter dataset has been extended to include data from drifters even after they lost their drogue; the latter data treatment can be applied in areas of moderate winds.

Simulating the dynamics and intensification of cyclonic Loop Current Frontal Eddies in the Gulf of Mexico

The dynamics associated with the Loop Current (LC) variability in the Gulf of Mexico (GoM) are studied using a 5‐year, free‐running numerical simulation with the Hybrid Coordinate Ocean Model (HYCOM). The dynamics of major GoM circulation features are represented: the extension of the LC and the associated anticyclonic, warm core Loop Current Eddies (LCEs) and cyclonic Loop Current Frontal Eddies (LCFEs). The study focuses on the dynamics of the LCFEs and their role during the LCEs shedding, which dramatically affects the GoM circulation. We analyze several characteristics of the LC frontal dynamics. Modeled LCFEs have a coherent vertical structure, which extends to the deep layers of the GoM. They may split in two separate upper and lower layer eddies. Deep and surface remnants from different frontal eddies are able to align to form new, coherent structures. LCFEs intensify along the extended LC northern edge when flowing over the deep northern GoM shelf slope that forms the Mississippi Fan, through a “promontory effect” in which the incoming cyclone aggregates positive potential vorticity anomalies in lower layers, leading to the intensification of the whole vortex structure. LCFEs may also expand further along the LC path by horizontal vortex merging, when they are blocked between the LC and the northeast corner of the continental shelf in the GoM. The intensification and merging due to topographic effects explain the enlarged frontal eddies observed on the eastern side of the Loop Current. These larger eddies further migrate along the LC front and may play a role in the shedding sequence.

Threshold of borehole failure: Breaking in before breaking out, Mississippi fan, Gulf of Mexico

Resistivity images from Integrated Ocean Drilling Program (IODP) Site U1322 on the Mississippi fan (Gulf of Mexico) show borehole failure as (1) low-resistivity bands interpreted as breakouts and (2) high-resistivity bands. Both features occur as opposing pairs on opposite sides of the borehole, and have similar azimuthal orientations and widths. Failures occur at depths of 90–216 m in sediments very rich in expansive (smectite-illite) clays of 40%–50% porosity that are younger than 65 ka. The low-resistivity breakouts resemble similar features in other IODP boreholes from southwest Japan and offshore Oregon. The high-resistivity features are unknown in other boreholes. Estimates of stress magnitudes based on the overburden stress and the extensional tectonic environment in the Gulf of Mexico predict that the borehole was at failure. Experiments were conducted on cores with lithologies equivalent to those of the borehole failure localities from IODP Site U1322 and adjacent Site U1324. These experiments suggest an elastic-plastic deformation with strains of 10%–15% before reaching a plastic yielding. In the experiments, strain softening during plastic deformation ranges from 0% to 20%. Physically the experimental samples show a combination of lateral bulging and discrete conjugate shears. These experiments suggest that the resistive areas in the borehole are an initial state of bulging, or extrusion, into the borehole. We call these extrusive failures “breakins” to distinguish them from traditional breakouts. Extrusion into borehole decreases the amount of conductive borehole fluid between the bulging sediment and the resistivity tool, increasing the resistivity signal. The high residual strength of the sediment prevents disaggregation and spalling. Where spalling has developed, breakouts occur. This analysis is the first documentation of this incipient stage of borehole failure.

How Did Thin Submarine Debris Flows Carry Boulder-Sized Intraclasts for Remarkable Distances Across Low Gradients to the Far Reaches of the Mississippi Fan?

Submarine density flows dominate sediment transport into many parts of the ocean and form submarine fans, which are some of the largest sediment accumulations on Earth. Previous studies often assumed that the distal fringes of submarine fans would be dominated by extensive sheet-like deposits from dilute and expanded turbidity currents, because only turbidity currents would transport large volumes of sediment for long distances across such low gradients. Deposits with a remarkable frondlike shape occur at the furthest fringe of the Mississippi submarine fan. Understanding the emplacement process of these deposits is important because their frondlike shape suggests deposition from debris flows rather than turbidity currents. Previous analyses concluded that these deposits comprise a complex arrangement of thin interbedded turbidity current and debris flow deposits. Here we propose a different internal geometry for the frondlike deposits. Intervals previously described as in place turbidites are interpreted to be clasts within a single debrite interval that is ~ 1 to 2 m thick. This debrite interval is underlain by clean sand. The debrite interval could have been emplaced by multiple debris flows, or one event comprising multiple pulses. Clasts in the debrite interval are much bigger than previously thought, including boulders with diameters that can exceed 50 cm. We show that thin (~ 2 m) and extremely fluidal debris flows could have carried these boulders across gradients of only 0.06° to the fringes of one of the world's largest submarine fans, without hydroplaning. This long transport distance could occur if the density of the clasts was less than that of the surrounding debris flow. Emplacement of the basal clean sand layer appears closely linked to debris flow deposition because the clean sand layer pinches out in a location similar to that of the overlying debrite. We therefore suggest that the basal clean sand most likely settled out from the overlying debris flows, and that the clean sand is not a fore running turbidity current deposit. Shearing at the base of a mud-rich debris flow would most likely produce a muddy basal sand interval, rather than the clean sand that is observed. This basal sand interval is therefore less likely to be a record of a lubricating layer with high-pore-fluid-pressure on which the debris flows moved. This study shows how thin and highly mobile debris flows can redistribute large volumes of sediment including bouldersized clasts long distances into the deep ocean across remarkably low gradients.

Geological structure, evolution stages, and petroliferous complexes of the Gulf of Mexico basin

The Gulf of Mexico basin occupies a vast region encompassing the southern continental margin of North America, a considerable part of the Greater Antilles, and the intervening Sigsbee Deep with the oceanic crust. In the north, the basin is contiguous with spurs of the Hercynian Appalachians, the Mississippi Interior and Permian basins. The Mississippi Fan, one of the largest in the world, governs the bottom topography and structure in the eastern Gulf of Mexico. The abyssal basin is surrounded in many areas by steep continental slopes passing in places into escarpments: Sigsbee, Campeche, and others. It is only in the Yucatan Peninsula region that the continental slope merges with a wide shelf. The Cuban-North Haiti meganticlinorium frames the basin on the Cuba Island side.

A jökulhlaup from a laurentian captured ice shelf to the gulf of mexico could have caused the bølling warming

.Since the rapid rate of global warming at the onset of the Bølling interstadial became evident, its cause has been under debate. It coincides closely in time with a strong global transgression called meltwater pulse 1a. One attempt at solution says that a meltwater pulse of Antarctic origin could cause an increase in North Atlantic Deep Water formation, and thus give rise to the Bølling interstadial. However, others have disputed that Antarctic meltwater would have that effect, and furthermore, the start of the Bølling interstadial is not even associated with an increase in North Atlantic Deep Water. A controversial hypothesis says that some Laurentian meltwater came from a jökulhlaup (sub‐glacial outburst flood), but no study has yet shown unequivocally that sufficient amounts of water could be stored under the ice. Furthermore, according to all available data a melt‐water pulse from the Laurentian ice would give rise to strong cooling, not warming. Nevertheless, meg‐afloods appear instrumental in accumulating the Mississippi Fan, created entirely during the Quaternary period, and dramatic climate changes are characteristic of this period. This paper presents a hypothetical chain of events, building on the published literature and simple calculations, to investigate whether the order of magnitude is reasonable. The hypothesis is that a jökulhlaup from a Laurentian captured ice shelf flowed out through the Mississippi, boosted the Gulf Stream, reinvigorated the North Atlantic circulation, and as a result triggered the Bølling warm phase.

Sea-level history of the Gulf of Mexico since the Last Glacial Maximum with implications for the melting history of the Laurentide Ice Sheet

Sea-level records from the Gulf of Mexico at the Last Glacial Maximum, 20 ka, are up to 35 m higher than time-equivalent sea-level records from equatorial regions. The most popular hypothesis for explaining this disparity has been uplift due to the forebulge created by loading from Mississippi River sediments. Using over 50 new radiocarbon dates as well as existing published data obtained from shallow-marine deposits within the northern Gulf of Mexico and numerical models simulating the impact of loading due to the Mississippi Fan and glacio-hydro-isostasy, we test several possible explanations for this sea-level disparity. We find that neither a large radiocarbon reservoir, sedimentary loading due to the Mississippi Fan, nor large-scale regional uplift can explain this disparity. We do find that with an appropriate model for the Laurentide Ice Sheet, the observations from the Gulf of Mexico can be explained by the process of glacio-hydro-isostasy. Our analysis suggests that in order to explain this disparity one must consider a Laurentide Ice Sheet reconstruction with less ice from 15 ka to its disappearance 6 ka and more ice from the Last Glacial Maximum to 15 ka than some earlier models have suggested. This supports a Laurentide contribution to meltwater pulse 1-A, which could not have come entirely from its southern sector.

Sedimentological history of Bryant Canyon area, northwest Gulf of Mexico, during the last 135 kyr (Marine Isotope Stages 1–6): A proxy record of Mississippi River discharge

Bryant and Eastern Canyons are located in northwest Gulf of Mexico, and are characterized by a complex sedimentological history related to glacioeustatic cycles, river discharges, and interactions between depositional and halokinetic processes. This study is based on detailed sedimentological analysis from forty-eight long cores from these two canyons. This paper determines the evolutionary history of the canyons and assesses the response of sedimentary processes to morphological, climatic, hydrological, and sea-level changes. During the last glaciation, the upper and middle continental slope was supplied with sediments by low density turbidity currents derived from the depositional segregation (deposition of the coarsest material in the most proximal locations) of large turbidity currents initiated on the outer shelf. The lower continental slope was supplied with sediment by westward flowing bottom currents, originated from the entrainment of the most diluted wash-load and tails of turbidity currents from the Mississippi Fan. Bryant and Eastern Canyon systems were active during the penultimate glaciation, Marine Isotope Stage (MIS) 6, and were supplied with sediments by an ancestral shelf-margin Mississippi River delta. Gravity flows transported enormous amounts of sediment to the continental slope and abyssal plain of the northwest Gulf of Mexico. The sea-level rise at MIS 5 led to confinement of river-sourced sediments to the widespread continental shelf of the northwest Gulf of Mexico, and consequently to the cessation of gravity flows. During the first 40 kyr of MIS 5, salt diapirs transformed the canyons into a network of intraslope basins. The sea level dropped to the mid-shelf during MIS 3 and 4, but never reached the shelf-break, and therefore, river-sourced sediments remained largely confined to the shelf. However, seaward sediment transportation was achieved occasionally through turbidity currents related to sediment failures, storms, and high-river discharges. Four high river discharge events have been identified during this period. The first three were centred at 37, 45, and 53 cal ka BP. The last high river discharge occurred at the end of MIS 3 (29.4-33.2 cal ka BP), and resulted in the deposition of closely-spaced, mud turbidites over the entire continental slope. The Laurentide Ice Sheet (LIS) was restricted north of the upper Mississippi River valley during 60 to ∼ 30 cal ka BP and therefore, the high river discharge events are interpreted as melt-water events, related to brief southward advancements of the LIS, which resulted in the flooding of Mississippi River. The extensive lowering of sea level during the last glacial maximum (MIS 2) resulted in the almost direct discharge of Mississippi River sediments to the upper continental slope leading to the development of abundant turbidity currents. Eleven wet–dry cycles during this period are defined; they probably originated from episodic subglacial melt-water floods, released from southern parts of the LIS. The last deglaciation event is characterized by the development of a major melt water event at 16.5–13 cal ka BP that resulted in the deposition of distinct, organic-rich sediments. At about 13 cal ka BP, the melt water discharges of the LIS in North America switched from the Mississippi River to either the St. Lawrence or Mackenzie River valleys, causing the domination of hemipelagic sedimentation on the continental slope of the northwest Gulf of Mexico. Isotopic data indicate that melt-water discharges returned to the Mississippi River Valley at ∼ 11.4 cal ka BP. The absence of any sedimentological indication on the continental slope of the northwest Gulf of Mexico of the return of the melt-water discharges to the Mississippi River is attributed to the confinement of river-sourced sediments on the continental shelf due to the rise of the sea level.

Geothermal heat flow in the northeast margin of the Gulf of Mexico

Eighty-two seafloor heat-flow measurements were recently obtained across the Mississippi Fan region in the deepwater northeastern Gulf of Mexico. These data display an abrupt transition in heat flow between an area near the center of Pleistocene deposition (∼20 mW/m 2 ) and the eastern margin of the fan (∼40 mW/m 2 ). Although deposition of fan sediments has very likely suppressed the shallow subseafloor thermal regime, causing lower seafloor heat-flow values near the center, the magnitude and abruptness of the heat-flow contrast cannot be fully accounted for by the mechanisms related to sedimentary deposition, which include radiogenic heat production in sediments, pore-fluid migration, and presence of salt structures. The most plausible explanation for the sharp heat-flow contrast is that the heat released from the igneous basement is significantly greater in the eastern margin of the fan. The zone of contrasting heat flow lies along a previously suggested boundary between the oceanic crust and the thin transitional crust in the northeastern Gulf of Mexico. The area of higher heat flow coincides with the suggested zone of transitional crust, which, because of its granitic origin, generates greater amounts of radiogenic heat than oceanic crust. This finding opens up the possibility that heat-flow data may be used in delineating crustal lithologic boundaries along continental margins.