Abstract Palaeogeographical reconstruction of the Oxfordian-age (Jurassic) depositional systems in the southern Gulf of Mexico is supported by detailed sedimentological analyses, detrital zircon geothermochronology and plate tectonic restorations. This integrated approach departs from prior local studies by placing the Bacab Sandstone, a major reservoir in offshore Mexico, in a larger basin- to regional-scale context. Sedimentary characteristics derived from detailed core description show how remarkably similar the Bacab Sandstone is to coeval sandstones of the Norphlet Formation of the northern Gulf of Mexico. A comparison of lithofacies associations suggests similar depositional processes and palaeoclimate regimes, with aeolian dunes forming near base level with adequate wind, sediment supply and arid climatic conditions promoting development of dunes and adjacent sabhkas. Construction of prominent ergs (aeolian sand seas) with lateral transitions to updip fluvial–wadi systems is envisioned in Mexico, comparable to the Norphlet of the northern Gulf of Mexico. Reservoir quality over the southern Mexican offshore area is variable, as numerous well penetrations over several decades have demonstrated. However, in the core area of the Ek-Balam Field, the best reservoirs have surprisingly good porosity and permeability for their age (Jurassic) and present-day depth of burial (>4000 m). However, published information is not definitive on the factors mitigating subsurface reservoir quality destruction. It is possible that similar processes preserving or enhancing porosity under deep burial conditions for the Norphlet are likely to have operated in the Ek and Balam well locations. Available petrographical data on framework grains are similar for Oxfordian sandstones in both the northern and southern Gulf of Mexico. Detrital zircon U–Pb geochronology, however, indicates that different source terranes supplied clastics to the Oxfordian Bacab and Norphlet sandstones. Detrital zircon geothermochronology age spectra indicate that the Mayan (Yucatán) Block was the primary terrane for the Bacab Sandstone. This is separate and distinct from Norphlet source areas that vary from Appalachian (Laurentian) to Pan-African (Gondwanan and peri-Gondwanan) terranes. While it possible that the Bacab and Norphlet sandstones were continuous and connected across the Yucatán margin that lies between the two areas, detrital zircon provenance results do not show any indication of common source terranes. Dimensional considerations, such as the contrasting ratios of Bacab and Norphlet source areas to their respective documented areas of deposition, also support the notion of separated aeolian palaeoenvironments.

Extended Abstract Although the final stage of formation of the Gulf of Mexico is fairly well constrained, earlier evolution is still debated. The final stage was rotation of Yucatan about a Florida Straits Euler pole that created most of the observed oceanic crust ( Pindell and Dewey, 1982 ). From observations of salt overlying seaward-dipping reflectors (diagnostic of volcanism during the rift to drift transition) in the northeast Gulf of Mexico, we suggest that salt was deposited at the onset of sea floor spreading, which coincides with initiation of the rotational motion of Yucatan. It is important to understand Yucatan motion that preceded this rotation because delineating any presalt play that might exist would be dependent on understanding of depositional systems developed during this early motion of Yucatan. Very little is known about the nature of presalt deposition in the northern Gulf of Mexico. Salt is Callovian or earliest Oxfordian in age, and the next oldest rocks known from the northern Gulf of Mexico are Late Triassic red beds found in what are generally regarded as proximal grabens formed during early rifting. This gap in knowledge, what we refer to as the “50 million year gap,” can potentially be bridged by incorporating analogs with known systems in Mexico and northern South America. There are uncertainties here, however, mostly based on how Mexico and northern South America are palinspastically restored and the fact that these rocks are in a proximal location. In particular, we note that there was a long-lived continental margin arc in Mexico that lasted from the Permian through the Middle Jurassic ( Barboza-Gudino et al. , 2012 ). A lot of the rocks of this age seen in Mexico that are linked to Gulf of Mexico rifting are in fact associated with this arc. In this presentation, we will review reconstructions of the region and develop a tectonic model that forms the basis for further understanding of rifting in the Gulf of Mexico.

Published estimates for the original volume of Mid-Jurassic Louann evaporites found throughout the entire Gulf of Mexico Basin vary widely. Volume totals derived from both map data and actual volume numbers, range from about 10,500 km 3 (2,500 mi 3 ) to 839,000 km 3 (200,000 mi 3 ), an 80 fold variation. Little new information has been published during the past twenty-five years to address this disparity. But gaining knowledge of the present day volume of salt would be an important metric if debates concerning the origin of the salt and the nature of the Gulf of Mexico Basin during salt deposition are to be reconciled. A methodology now exists to estimate more accurately and quantitatively the volume of salt present in a given area. Multiple, recent generations of 3-D seismic depth volumes in the offshore Gulf of Mexico require that salt velocities be inserted. This vital processing step includes a systematic picking and interpretation of the tops and bases of all salt bodies encountered. The resulting models of salt velocity allow salt volume in the 3-D data sets to be calculated. Combining the salt volumes calculated from multiple seismic surveys offers new stratigraphic insights across large portions of the original salt basin. A comparison of salt volumes derived from seismic data cubes and volumes derived from published maps can now be made. The comparisons should give some suggestion as to the accuracy of the map data. By extrapolation it should also give a more accurate and quantitative estimation of the original salt volume deposited in the Gulf of Mexico basin.

Abstract The Oxfordian Smackover Formation is generally acknowledged to be a hydrocarbon source for numerous reservoirs in the Gulf of Mexico, both onshore and offshore. More than 25 wells in the eastern Gulf of Mexico have penetrated the Smackover since 2003. Offshore, the Smackover consists predominantly of limestone and shale containing thin organic layers. Immediately above the lower Smackover is a widespread shale marker. This thin shale is correlated as the base of the upper Smackover Formation, which consists of interbedded shale and limestone. This study will demonstrate that the lower Smackover Formation in the eastern Gulf of Mexico (Mississippi Canyon and De Soto Canyon offshore areas) is composed of a series of seven units that occur in the same sequence in virtually every well in which the lower Smackover has been encountered. Although the seven individual units can be resolved readily with the proper wireline suite, each has a sub-seismic thickness. The overall thickness of the lower Smackover is about 300 +/-100 feet. Unlike the lower Smackover, the surrounding Mesozoic formations, from Cotton Valley to Norphlet, vary greatly in thickness in the eastern Gulf. The initial correlations of the units in the lower Smackover were made by comparing the gamma ray, resistivity, and density log patterns with the computed mineralogy of Elemental Capture Spectroscopy (ECS) wireline logs. It was immediately obvious that the same sequence of beds/units was present in the lower Smackover in well after well. Within the lower Smackover Formation is a conspicuous zone characterized by iron-bearing minerals having a matrix density in excess of 3.0 g/cm 3 throughout. However, X-Ray Diffraction (XRD) data from rotary sidewall cores was necessary to validate the mineralogy. Because the mineralogy of the ECS log is a model-based calculation from the elemental concentrations of iron, calcium, aluminum, etc,. rather than a direct measurement, the modeled mineralogy can be inaccurate as was the case in the bottom two units. Mineralogy of the seven units has been verified by XRD analyses, albeit from a limited number of rotary sidewall cores obtained in only five wells. The top three units are limestones which vary in carbonate, clay, and pyrite content. The fourth and fifth units contain significant amounts of high density minerals, particularly siderite and pyrite. The sixth zone is dominated by anhydrite. The seventh unit is a hematite-rich shale and its base is an unconformity. Although wireline data are plentiful, analysis of the seven units within the lower Smackover is hampered by the limited amount of rock data and the complete lack of whole core. Many depositional and geochemical questions suggested by the unusual mineralogy and sequence of beds remain unanswered.

Abstract The central Gulf of Mexico Basin formed by Callovian rifting followed by oceanic spreading during anticlockwise rotation of Yucatan Block between Oxfordian (160Ma) and Valanginian (140Ma). The rotation pole was in the Straits of Florida, resulting in a wiper-blade motion having greatest sweep along a transform paralleling the coast offshore Tampico-Misantla Basin. Oceanic crust progressively separated the 160-140Ma stratigraphy, creating bands of ocean crust lacking Oxfordian, Kimmeridgian, Tithonian and, near the ridge, even Berriasian deposition: present day, these stages appear progressively absent towards the spreading center. After a basin-wide 140Ma (Valanginian) unconformity on the rimming shelves at the end of spreading, thermal subsidence resumed and water deepened further, in the basin. However, an arch separating the Gulf of Mexico and paleo-Caribbean basins across the Florida Straits remained a bathymetric sill to deep ocean circulation post-140Ma as before. The impact of this tectonic/bathymetric evolution on the presence and ultimate expellable potential (UEP) of source rocks in the basin center is profound. Using a scheme of basin-wide correlated organic matter depositional acmes (recently developed by Petroleum Systems LLC, where the acme age is expressed in Ma), candidate source rocks in the Gulf of Mexico are: Acme A157 (late Oxfordian); A154 (Kimmeridgian); A148 (Tithonian); A144 (Berriasian/Portlandian). All four acmes have zones of nondeposition due to oceanic spreading, and older acmes are absent over a greater area. Some workers have proposed the main deep-water basin source rock in the United States is Tithonian; however, new work (so far limited to eastern deep water Mississippi Canyon, Atwater Valley, and Desoto Canyon) indicates that A157 ( i.e. , the oldest, most susceptible to nondeposition) is the most effective source rock. Risk on Acme 157-144 non-deposition can be mitigated if source rocks younger than 140Ma, having significant ultimate expellable potential, were deposited in the deep basin. Based on recent reappraisal of data in DSDP Leg 77, Cuba and rafted sediment blocks in U.S. waters, significant ultimate expellable potential does exist in five younger Early to mid-Cretaceous acmes: A138 (Valanginian); A120 (early Aptian); A110 (early Albian); A101 (late Albian); and A98 (early Cenomanian). A138 may result from restricted circulation, reinforced during the 140Ma uplift of rimming shelves, while A120, A110, and A101 can be correlated with oceanic anoxic events OAE1a, b, and d.

Abstract We cannot hope to predict Mesozoic depositional processes and sediment properties well enough to plan effective regional exploration strategies without considering the big picture of Gulf of Mexico deposystem evolution. The two critical big picture elements are the kinematics and timing of the Yucatan Block's detachment and separation from North America and the various major expansions and contractions and the ultimate disappearance of the Western Interior Seaway. Although a number of authors, including this one, have speculated on the timing of separation of Yucatan from North America ( Fillon, 2007a ), no definitive evidence exists: i.e. , drilled samples of the ocean crust and the sediments directly overlying it. Without that unambiguous information we must infer the paleogeographic evolution of the early Gulf of Mexico Basin from deposystem architecture by asking questions such as when do Gulf of Mexico deposystems transition from architectures consistent with deposition in a youthful blockfaulted basin underlain by actively attenuating continental crust to deposition in a mature basin having stable margins surrounding a central region underlain by subsiding ocean crust. An understanding of the paleogeography and paleoceanography of the Gulf of Mexico Basin derived from deposystem architecture can help provide answers to crustal kinematic questions and to more exploration focused questions such as: where, and in section of what age should we look to find facies similar to the organic rich, generative Haynesville Shale facies of eastern Texas and western Louisiana. Although we all know something about the Western Interior Seaway, most of us working on the Mesozoic of the Gulf of Mexico Basin have not spent much time considering what effects it might have had on the prospectivity of Gulf of Mexico deposystems. Through much of Albian and Late Cretaceous time the Western Interior Seaway connected the Gulf of Mexico Basin with the Arctic Ocean Basin. The effects of the establishment and intermittent blocking of this major seaway connecting arctic and tropical water masses on global paleoceanography, on global paleoenvironments, and locally on onshore and offshore Gulf Basin deposystems cannot be ignored in our quest to understand the Mesozoic of the Gulf Rim. This paper is a “big picture” review of Gulf of Mexico Basin deposystem evolution within the Late Jurassic (Oxfordian)–Late Cretaceous (Maastrichtian) interval. Seventeen Mesozoic chronosequences are defined therein based on chronostratigraphic data garnered from over 130,000 industry well and pseudowell penetrations of Mesozoic section in the Gulf of Mexico Basin region. Examination of the collected data suggests that grouping the seventeen Gulf of Mexico Mesozoic chronosequences into seven super-chronose-quences optimally distinguishes key phases of deposystem and basin evolution. The oldest super-chronosequence defined in this study, dubbed “MG,” encompasses ca. 16.45 Ma of Norphlet through lowermost Cotton Valley Late Jurassic deposition. Sediment distribution and accumulation rates within the MG interval clearly define the rectilinear configuration of the earliest Gulf of Mexico Basin. This early basin geometry is consistent with fault controlled attenuation and foundering of North American continental crust, associated flooding, and rapid depositional infill concurrent with the earliest detachment of the Mayan (Yucatan) crustal block from North America. The Yucatan block, although showing an affinity with South American (Amazonian) terranes ( Martens, 2009 ), was left attached to the North American plate when North America began pulling away from Gondwana during the initial breakup of Pangea ( Fillon, 2007a ). The next younger super-chronosequence, “MF,” contains a. ca. 13.47 Ma record of Cotton Valley, Bossier, Knowles limestone., Late Tithonian through mid Hauterivian, deposition. The “MF” interval reflects the same rectilinear outline as the “MG,” but is marked by decreased accumulation rates, suggesting that the fault bounded crustal attenuation, rapid sediment infill phase had markedly slowed. The ca. 9.4 Ma of Hosston, Sligo, Sunniland limestone, James limestone, mid-Hauterivian through Early Aptian section contained within the succeeding “ME” super-chronosequence records modification of the early rectilinear basin outline by a temporary reactivation of attenuation and foundering in the western portion of the Gulf of Mexico Basin. “ME” sediment distribution patterns also indicate development of a depositional continental margin and accumulation of true continental margin type deltaic and reef systems. These observations suggest that during this interval a deep continental basin, probably floored by ocean crust, was beginning to form outboard of the attenuated continental crust. Sediment distribution and accumulation rates within the ca. 23.5 Ma Rodessa through lower Washita, Early Aptian through Early Cenomanian “MD” super-chronosequence reflect growth of the Wisconsin interior seaway and a stable phase of relatively low accumulation rates throughout the entire Gulf of Mexico Basin deposystem. During this interval, deposition was very likely influenced by a vigorous tidal and thermohaline current circulation driven by strong temperature contrasts within the Gulf of Mexico–Wisconsin interior seaway–Arctic Ocean connection. The next younger super-chronosequence, “MC,” contains a ca. ca. 16.0 Ma record of Dantzler, Washita, Lower Pine Key, Eutaw, Woodbine, Eagle Ford, Austin, and Early Cenomanian through Late Santonian (Late Cretaceous) deposition. During this phase, there is a marked reduction of accumulation rates in the north-western portion of the basin, attributable perhaps to expansion of the Western interior seaway and continued subsidence of the old Gulf of Mexico Basin margin. Associated small, perhaps tidal submarine delta-like depopods developed, perhaps in response to the regional Western interior seaway transgression ( Blakey, 2014 ). These delta-like depocenters appear to define a new basin margin presaging the modern curved shape of western Gulf of Mexico so familiar to us today. Here also we see the first unambiguous evidence of abyssal deposition in the deepest portion of the Gulf of Mexico Basin underlain by ocean crust. The succeeding ca. 12.82 Ma interval of Late Santonian through Early Maastrichtian upper Pine Key, upper Selma, upper Austin, Taylor, Olmos, Saratoga, and low accumulation rate mainly chalk and marl deposition contained within the “MB” super-chronosequence provides evidence of transgressive onlap associated with an expanding and deepening interior seaway during “MB” time. “MB” onlap has the effect of temporarily reemphasizing structural trends inherited from crustal attenuation that took place during “ME” time. Finally, the ca. 5.4 Ma long terminal Mesozoic “MA” super-chronosequence consists of Maastrichtian, Navarro equivalent, low accumulation rate marls deposited along the basin margin. These low accumulation rate basin rim sediments and low accumulation rate slope sediments are punctuated by high accumulation rate canyon fill and lobe-shaped slope depopods which are probably attributable to sediment reworking, transport and deposition by transitional Cretaceous-Paleogene (K/P) interval mega-tsunami backwash flows immediately following the Chicxulub impact. Higher accumulation rates in the deeper parts of the basin underlain by ocean crust are also consistent with high volume backwash flows.

Abstract The Mesozoic Gulf of Mexico Basin is considered in this discussion as the set of contiguous, Triassic and Jurassic sub-basins directly involved in the counterclockwise rotation of the Yucatan Block from North America in the Late Jurassic. The rifting and seafloor spreading history of the basin is less well understood than analogous salt basins of the Atlantic margins, largely because the base salt surface is significantly deeper and has hereto widely been considered acoustic basement. In 2012, 17,000 km of 2D PSDM reflection seismic data ( SuperCache ) were acquired across the deep water of the U.S. Gulf of Mexico. The unique acquisition configuration of long-offset, powerful source, and deep-tow of both source and receivers was designed to optimize the imaging of the presalt architecture of the basin to a depth of 40 km. On these seismic data, the base of the salt and its correlative unconformity, continental and oceanic basement, and the Moho are evident. In combination with vintage, reflection seismic data, shipboard and regional gravity data, and regional refraction profiles, a crustal interpretation has been extended to the greater Gulf of Mexico Basin. The continental crustal architecture is described in terms of crustal thinning: from low (<30%) to transitional (>70%). Synkinematic sequences are recognized within the Late Triassic to the Middle Oxfordian (~70 my). The final break-up phase occurred within 15 my, ending with a basin-wide open marine transgression and initial emplacement of oceanic crust at 160±1 Ma; continued extension may have occurred in the eastern part of the basin in the latest Jurassic. The basin margins are considered to be intermediate between magmapoor and volcanic end-members. The ocean crust tapers from a maximum width of 700 km in the west, where it is anomalously thin, to anomalously thick as it approaches the pole of rotation in the Straits of Florida. The architecture of extinct spreading valleys and fracture zones is analogous to the modern, slow spreading mid-Atlantic ocean, suggesting that spreading continued until the latest Jurassic (~142 Ma), possibly as late as within the early Cretaceous (~132 Ma).

Abstract We present a reduced-to-pole, total magnetic intensity map derived from merged aeromagnetic surveys in and around the Gulf of Mexico. Most of the deep central Gulf crust has a magnetic pattern of orthogonally intersecting features similar to, and interpreted as, fracture zones and ridge segments of oceanic crust formed by seafloor spreading. This spreading or drift phase occurred after the primary synrift phase of continental stretching across the greater Gulf of Mexico region, and thus the ocean crust rests within a broader zone of stretched continental crust with Yucatán, western Florida, the southern USA, and eastern Mexico forming the surrounding continental margins. We identify three regional magnetic anomaly trends that can be used to constrain the Gulf of Mexico’s Late Jurassic through earliest Cretaceous spreading history. A central magnetic anomaly trend is interpreted to accord with the later increments and cessation of seafloor spreading, for which a stage pole of rotation is estimated. Two flanking magnetic anomaly trends to the north and south of the central one, respectively, occur just basin-ward from the inferred depositional limits of autochthonous Callovian-Early Oxfordian salt. These anomalies appear to define the landward limits of oceanic crust in the northern and southern Gulf, and probably lie in crust that is medial or Late Oxfordian in age. They have similar mapped patterns that can be reconstructed onto one another and hence are probably genetically related but separated by spreading. These landward anomalies are best fit around a different stage pole than the central anomaly; thus the rotation pole appears to have jumped during spreading in the Gulf. Seismic reflection data show that the two outer anomalies occur at the basement “step ups” to the oceanic crust or the basinward shoulders of the “outer marginal troughs.” Until specific magnetic source modelling is done on the outer anomaly pair, we favor an edge-effect interpretation caused by the intrusive interface between Oxfordian oceanic crust and serpentinized and exhumed subcontinental mantle, the latter of which we infer forms the step ups to the oceanic crust. In addition, the aeromagnetic map shows a north-south trending “Campeche Magnetic Anomaly” downslope from the western shelf edge of Yucatán that we argue helps to constrain the reconstruction of Yucatán along Texas at the start of the synrift stage. Thus, the aeromagnetic map provides vital insights into the kinematics of all three stages of the basin's development, namely the synrift, the drift, and the interpreted intervening transitional phase of crustal hyperextension/mantle exhumation along the Gulf’s magma-poor continent-ocean transitions.