A Gulf of Mexico Basin connection to the Arctic Ocean enabled during the Cretaceous by development of the North American Western Interior Seaway (WIS) helps to explain an increase in surface productivity and the spread of dysoxic deep-water benthic foraminiferal biomes in the middle to late Cretaceous. An earlier study of GoMB benthic foraminifera identified two benthic foraminiferal assemblages representing two oxygen-restricted (dysoxic) lithobiofacies biomes (Fillon 2009). The first is defined as a dysoxic proximal prodeltaic Ecozone 3/Lithobiofacies B1 (“E3/B1 dysoxite biome”) and the second is defined as a dysoxic distal prodeltaic Ecozone 4/Lithobiofacies B2 (“E4/B2 sapropelite biome”). These dysoxic biomes occupy areas of seafloor near enough to river deltas to receive high organic matter and dissolved nutrient fluxes carried by river discharge plumes. Bacterial oxidation of the river-borne organic matter and the dissolved nutrient fed phytoplankton fluxes support development of a Gulf of Mexico Hypoxic Zone on inner to middle portions of the shelf and a typical Gulf of Mexico oceanic oxygen minimum zone (OMZ) on upper to middle portions of the continental slope. Dysoxic waters of the OMZ intersect the Gulf of Mexico seabed between outer shelf depths of ca. 100 m and mid to lower slope depths of ca. 1300 m. These dysoxic biomes are observed to expand during the mid to late Cretaceous interval of greenhouse warmth and global oceanic anoxia, especially during periodic regressive events when the OMZ expands down-slope and contract during periodic transgressions when the OMZ shallows.

The sedimentary architecture and provenance of the early Mesozoic incipient northern Gulf of Mexico basin remains controversial due to both lack of outcrop exposure and sample scarcity across the southern United States with subcrop depths approaching 6 km. The Eagle Mills Formation and coeval deposition across the northern Gulf of Mexico provides both a stratigraphic foundation for some similar to 15-km-thick overlying Mesozoic and Cenozoic deposits, and a coeval pre-salt equivalent for southern synrift deposits, in one of the most economically significant hydrocarbon basins in the world. This study presents more than 3200 new detrital zircon U-Pb analyses from sixteen Late Triassic pre-salt, siliciclastic, subcrop well samples, and combines over 14,000 linear kilometers of 2-D multi-channel seismic reflection data, 1511 geophysical well logs, and biostratigraphic data from 2478 wells to construct basin-scale pre-salt isochore and structure maps spanning the northern Gulf of Mexico margin from Florida to the USA-Mexican border. The data show that incipient Gulf of Mexico paleodrainage pathways held individual distinctions between basement sources and tectonic controls in three primary regions across the northern Gulf of Mexico: (1) The western Gulf of Mexico paleodrainage extended from the Central Texas uplift highlands to the submarine Potosi Fan on the western margin of Laurentia with local tributary sources from the East Mexico Arc, Yucatan/Maya, and Marathon-Ouachita provinces as evidenced by inverse Monte Carlo unmixing of peri-Gondwanan (ca. 700-500 Ma), Appalachian/Ouachita (500-280 Ma), Grenville (1250-950 Ma), and Mid-Continent/Granite-Rhyolite Province (1500-1300 Ma) detrital zircon ages. Isochore and associated geophysical well and seismic data suggest that by Early Jurassic time this depocenter had shifted into the present-day western Gulf of Mexico as East Mexico Arc development continued. (2) Southerly drainage in the north-central Gulf of Mexico region bifurcated around the Sabine and Monroe uplifted terranes with southwestern flow characterized by peri-Gondwanan detrital zircon ages from late Paleozoic accreted basement or discrete flexural successor basins, and southeastern fluvial networks distinguished by traditional North American basement province sources including Grenville, Mid-Continent, and Yavapai-Mazatzal. (3) Eastern Gulf of Mexico regional paleodrainage, with regional southern flow dictated by the brittle extensional tectonics of the South Georgia Rift as well as the regional southern flexure of the South Florida Basin, resulted in almost all pre-salt detrital zircon siliciclastic ages from this region to be dominated by local Gondwanan/peri-Gondwanan aged sources including the proximal Suwannee terrane and Osceola Granite complex. These regional, synrift sediment provenance models provide the first critical allochthonous evidence of Late Triassic-Early Jurassic paleodrainage stemming from the Appalachian-Ouachita hinterlands into the incipient northern Gulf of Mexico basin with critical implications for pre-salt hydrocarbon exploration and carbon sequestration reservoir potential.

To date, quantification of individual components that contribute to shallow and deep-seated subsidence in passive margin deltas worldwide has proven problematic. A new, regional gridded chronostratigraphic dataset for the Lower Mississippi Delta region, derived from 80,928 well reports across the northern Gulf of Mexico (GOM), has bridged the disparity between geodetic mean rates measuring total land surface subsidence across annual-to-decadal timescales and the deep-seated stratigraphic subsidence rates that record isostatic response over timescales of >10(4) years. Through a quantitative assessment of gridded chronostratigraphic surfaces, sections, and subsidence rates extending from the Middle Pleistocene (0.58 Ma) to the Late Pliocene (3.85 Ma), we identify both temporal and spatial variability in deep-seated subsidence across the northern GOM. Targeted deep-seated subsidence data extracted across prior GOM Holocene sea-level sample locations have revealed more than an order of magnitude greater rates of isostatic compensation in the Mississippi depocentre versus similar GOM sea-level control sites in Florida and Alabama, casting doubt on efforts towards a representative Holocene sea-level curve. Spatial variability in subsidence was also assessed locally in both the strike and dip directions to assess the contributions of growth faults. Fault throw displacement magnitude was discovered to decrease with depth, accounting for less than half of the total deep-seated subsidence record of the Middle Pleistocene. Temporal subsidence complexities were also revealed including a direct, inverse logarithmic relationship between subsidence rate and time indicating variable subsidence component controls across different timescales. Despite the spatial and temporal complexities, this dataset serves as the first regional baseline for deep-seated subsidence rates across the northern GOM.

Knowing how to recognize the seaward limits of ancient shelf systems, i.e., the paleo-shelf edge has long been critical in considering where to drill. Nevertheless, framing a precise universal definition of the continental shelf edge is virtually impossible because the character of the boundary between the continental shelf and continental slope varies dramatically from one geologic province to another. Even within a relatively small ocean basin like the Gulf of Mexico shelf edges vary considerably in character depending on a variety factors, including sediment type, grain-size, eustasy, isostasy, the competency of underlying strata, exposed relict morphology, and regional tectonism. Today, northern Gulf of Mexico shelf edges are of two main types. From Texas to the Florida Panhandle shelf edges record the gradual break in slope where deltaic clinoform beds deposited during the last glacial low sea level stand (at ca. −120 m) gradually increase their seaward dip. These relict features are cloaked by a thin (typically <2 m thick) drape of sediment deposited during the Holocene transgression and current highstand. If we expand our view of the margin, and extend it deeper into the subsurface (Fig. 2), it is apparent that the ~19 ka MIS–2 shelf-edge delta (Fig. 1) reflects a landward shift of about 2 km in ~251 ka of the location of the shelf edge from an earlier ~270 ka MIS–8 shelf-edge delta. Due to a combination of growth fault activity and sediment loading the upper surface of the MIS–8 delta is now situated at about −230 m below present sea level. These high-resolution seismic views of late Quaternary shelf edges and associated shelf-edge deltas are correlated with detailed bio-, litho-, oxygen isotope, and 14C stratigraphic data established in four shelf and upper slope core holes acquired as part of the Gulf of Mexico Shelf Slope Research Consortium (Roberts et al., 2004). In the northern Gulf of Mexico, we learned is that it is relatively simple to identify shelf edge positions in high-resolution seismic records, but also that shelf edge deltaic clinoform sets are typically less than 100 m thick (~75 ms two-way travel time) and therefore much more difficult to recognize on conventional industry seismic data. Moreover, while ~100 m thick sandy deltaic section is readily apparent in conventional well logs, sandy deltaic systems cannot always be distinguished from other sand-rich depositional systems like submarine fans based on well logs alone. A multidisciplinary approach that includes biostratigraphy is critical in distinguishing ancient shelf edge systems. One of the oldest biostratigraphic methods used in identifying the location of ancient shelf edges is based on understanding the water depth dependent paleoecology of benthic foraminiferal assemblages. Seminal paleontological work in the petroleum industry was focused on this issue and oil and gas companies sponsored a great deal of basic paleontological research on the water depth distributions of modern foraminiferal taxa. This led to the recognition of a number of water depth dependent benthic environments based on different key benthic foraminifera. Subsequently, paleontologists working for different companies subdivided the neritic, bathyal, and abyssal paleoenvironments into 5 to 7 different numbered ecozones, for example, (1) inner neritic (~inner shelf), (2) middle neritic (~middle shelf), (3) outer neritic (~outer shelf), (4) upper bathyal (~upper slope), (5) lower bathyal (~lower slope and continental rise), and (6) abyssal (abyssal plain). Such ecozone assignments were made subjectively, using proprietary ecozonation schemes incorporating lists of diagnostic benthic foraminifera correlated to paleobathymetry (Fig. 3). The value of benthic foraminiferal paleobathymetric ecozonation in identifying the location of paleo shelf edges in boreholes is clearly demonstrated in Figure 3. Reflecting the radically different paleoecologies in which they lived, fossil benthic foraminiferal assemblages in sediments overlying the delta differ significantly from those in the deltaic clinoform beds. While the curves in Figure 3 are based on Texaco’s index taxa and ecozonation scheme, other company’s schemes based on different criteria (Fig. 4) also can perform well. In Figure 4, Texaco’s paleobathymetric ecozonation is seen to be based on a significant number of foraminiferal taxa (18) that in the lithobiofacies model proposed by Fillon (2009) are representative of proximal deltaic sequences (Fig. 4C). Shell’s ecozonation (Fig. 4D) also incorporates a fairly high number (8) of proximal deltaic lithobiofacies indicator species. With respect to deltas and shelf edges, such ecozonation schemes are late-highstand-lowstand weighted, capturing periods of extensive delta progradation at the shelf edge. In contrast, Paleo-Data’s ecozonation (Fig. 4A) incorporates a relatively low number (4) of proximal deltaic lithobiofacies indicator species compared to the overall number of lithobiofacies index taxa charted. Culver’s (1988) ecozonation (Fig. 4B) incorporates fewer lithobiofacies taxa overall so the 4 deltaic lithobiofacies taxa included represent a significant proximal deltaic influence in his scheme. The differences in proportional abundance of proximal deltaic lithobiofacies indicator species charted for these four zonation schemes suggest that we should look to other lithobiofacies indices to gain new insight into the relationships between different ecozonation schemes and shelf edges. The key to locating the shelf edge in the ecozonation schemes charted in Figure 4 appears to lie in the distribution of distal prodeltaic organic rich hemipelagic shelf slope and rise lithobiofacies index taxa (orange diamonds in Figure 4). As distal prodeltaic wedges are thickest on the upper slope immediately seaward of shelf edge deltas (e.g., Fig. 2) it stands to reason that within each of the 4 charted paleobathymetric ecozonation schemes the shelf edge should be located just proximal of the ecozone containing the highest concentration of distal prodeltaic lithobiofacies index taxa (vertical red bars in Figure 4). The distribution of calcareous bank lithobiofacies index taxa which are generally restricted to living within the photic zone and in warmer offshore waters (Fillon et al., 1998), though much less common than prodeltaic index taxa in the 4 charted zonation schemes, also offers insight into the location of the shelf edge (Fig. 4). In a further nod to shelf edge complexity, it is important to consider that while prodeltaic strata were deposited concurrently with shelf edge deltaic deposition during a period of lowered sea level, in most of the northern Gulf of Mexico, calcareous bank taxa occupy the shelf edge niche only after drowning of the delta surface during an ensuing sea level rise. In contrast, the tropical Gulf Loop Current in the northeastern corner of the Gulf and along the Florida margin provides sufficient winter warmth for mixed siliciclastic/calciclastic shelf systems to develop. In those areas calcareous bank taxa may be included in the prograding clinoforms comprising shelf edge deltas and in thin sedimentary units draping relict delta surfaces (e.g., the MP288 area in Figure 2). The utility of looking at both paleobathymetric ecozones and lithobiofacies is perhaps best illustrated along the western margin of the Florida Platform. There the modern shelf edge reflects a shallowly buried, locally faulted Cretaceous barrier reef that has also been subtly modified by late Paleogene-Neogene sediment loading. The paleoenvironmental affinities of benthic foraminifera in the underlying carbonate reef section contrast strongly both in age and paleo environmental affinities with those in the overlying mixed siliciclastic/calciclastic sediment drape as well as with dominantly siliciclastic shelf edge systems in the western Gulf. This complexity makes defining paleo-shelf edges very challenging. In a companion paper (Fillon and Waterman, 2014, this volume), new tools that we are developing to meet challenges such as reliably defining shelf edges in paleoecozone maps and in accumulation rate and isostatically adjusted depth maps are introduced. In that paper, we examine the shelf edge problem in the light of new map data, and introduce several strategies that may help to unravel the paleoecologic and geomorphologic complexities of modern and ancient shelf edges with the goal of better characterizing plays and assessing reservoir risk.

Stacked and laterally offset clinoform packages associated with lowstand deltas make up the Gulf of Mexico shelf edge, from the modern Mississippi River delta to De-Soto Canyon. Offshore Mississippi-Alabama, within the Lagniappe delta complex, thick sandy clinoforms display impedance contrasts recorded on high resolution seismic profiles that indicate significant gas charging. Carbon-14 dating places the Lagniappe delta at the shelf-slope transition only about 1,000 yrs before the latest Pleistocene glacial maximum. Thin heterolithic clinoform toes extend downslope to a channel-levee system that feeds by-passed sediment to a deep-slope fan. These laminated sand, silt, and clay units create effective capillary seals that inhibit vertical hydrocarbon migration while allowing lateral and updip transport. Clinoform toes extend downslope into the gas hydrate stability zone which acts as a regulator of updip hydrocarbon migration. During the approximately 100,000-yr glacioeustatic cycles typical of the Pleistocene epoch, concomitant reduction in hydrostatic pressure and increase in water temperature periodically occur at the top of the gas hydrate stability zone causing hydrate instability. Under rising-to-high sea level conditions, Loop Current intrusions raise bottom water temperatures on the upper slope (<1,000 m) causing near surface gas hydrates to decompose, making gas available for updip transport. Gas observed seeping from truncated clinoforms, and 13C-depleted authigenic carbonates found in clinoform cores, strongly suggest that hydrocarbon migration is an on-going process. When sea levels fall, gas hydrates decompose, releasing gas into surrounding sediments where capillary seals encourage updip migration. An abrupt sea-level fall causes rapid gas hydrate decomposition and slope failures, mobilizing large volumes of sediment for transport to deep-water depositional sites.

PreviousNext No AccessSEG Technical Program Expanded Abstracts 2006Deep imaging of Mesozoic section and crustal features in the North central Gulf of MexicoAuthors: Theodore StieglitzRichard FillonTheodore StieglitzGX TechnologySearch for more papers by this author and Richard FillonEarth Studies Group and GX TechnologySearch for more papers by this authorhttps://doi.org/10.1190/1.2370318 SectionsAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail Abstract Regional understanding of Gulf of Mexico (GOM) tectonic history is predicated upon a union of advanced seismic imaging and geological interpretation. Historical deep imaging of the GOM has been limited to Tertiary objectives. In a general sense, the accuracy of geo‐tectonic models are limited by our ability to “see” and image the Earth's stratigraphy.We will present examples from a regional 2D composite ocean bottom cable (OBC) and streamer seismic line designed to image the deep North central Gulf of Mexico (data courtesy of GX Technology). The test line location is coincident with the GulfSpan program line 1600 (GS 1600) and proximal to a vintage line interpreted by Peel et al. (1995) showing an extended unbroken section of Jurassic and Cretaceous sediments from craton to basin.Our examples will lend insight into understanding the deeper section including Mesozoic and basement fabric. We will employ an integrated work flow which includes a suite of advanced seismic tools in coordination with passive geophysical methods and an openness to creatively explore geological possibilities.Permalink: https://doi.org/10.1190/1.2370318FiguresReferencesRelatedDetailsCited byGulf of Mexico deep seismic imaging and Mesozoic interpretation initiativeR. H. Fillon, T. Stieglitz, T. Matava, and M. G. Dinkelman6 October 2006 SEG Technical Program Expanded Abstracts 2006ISSN (print):1052-3812 ISSN (online):1949-4645Copyright: 2006 Pages: 3541 publication data© 2006 Copyright © 2006 Society of Exploration GeophysicistsPublisher:Society of Exploration Geophysicists HistoryPublished Online: 06 Oct 2006 CITATION INFORMATION Theodore Stieglitz and Richard Fillon, (2006), "Deep imaging of Mesozoic section and crustal features in the North central Gulf of Mexico," SEG Technical Program Expanded Abstracts : 543-547. https://doi.org/10.1190/1.2370318 Plain-Language Summary PDF DownloadLoading ...

Sequence accumulation rate maps based on graphic correlation of reported stratigraphic events in >200,000 industry wells and published studies provide a detailed view of Paleocene-Eocene deposystems in the Gulf of Mexico Basin that is chronostratigraphically consistent across the entire region. At the end of the Mesozoic, epeiric carbonates, basin margin banks, steep continental slopes, and deepening eastern and western basin floors establish a morphologic framework that begins receiving large volumes of siliciclastic detritus from the developing Laramide uplift. This sediment flood initiates the Paleocene-Eocene depositional cycle. Paleocene-Eocene depositional cycle accumulation patterns reveal accommodation dominated primarily by the K/T calcareous to siliceous basin transition and secondarily by mobile salt. Primary Paleocene-Eocene depositional cycle architecture consists of expanded shelf margin depocenters, the La Salle, Calvert, and Holly Springs delta systems. Shelf-edge delta lobes associated with these systems supply sediment to robust Texas–Campeche Basin fan systems, the Chincontepec fed from southeastern Mexico, the La Salle and Yoakum Canyon systems fed from South Texas, and the Western Holly Springs system fed from Southeast Texas. Eastern lobes of the Holly Springs delta system in South Louisiana feed a weaker salt related fan system in the north central Gulf via the St. Landry Canyon, the Eastern Holly Springs fan system, and a more distal Chicxulub fan in the deep eastern Gulf. Inclusions of Eocene sediments in salt bodies within the Neogene salt canopy and higher accumulation rate subsalt Paleocene-Eocene section support Paleocene-Eocene depositional cycle salt displacement during deposition of the Eastern Holly Springs fan. Slope aprons play a role in the transfer of sediments from shelf to basin during parts of the Paleocene-Eocene depositional cycle, but for most of the interval sediments are delivered directly to fans, bypassing the lower slope. The end of the Paleocene-Eocene depositional cycle coincides with stabilization of Rocky Mountain foreland basins and uplift of the Colorado Plateau and the Rio Grande Rift. A sediment-starved western Gulf Basin in the late Eocene limits optimal reservoir development in the Western Gulf to the early Eocene (ca. 55.2-47.7 Ma). Although Paleocene-Eocene depositional cycle fans in the eastern Gulf are thinner they may contain significant quantities of sand. The period of optimal reservoir accumulation in the eastern Gulf may last somewhat longer, extending into the earliest Oligocene (ca. 55.2-32.5 Ma).