John W. Snedden

Recommendations for a uniform chronostratigraphic designation system for Phanerozoic depositional sequences

Many published sequence-stratigraphic frameworks lack a systematic and consistent designation for both depositional sequences and key surfaces, despite the original goal to provide a fully integrated stratigraphic architecture, including diagnostic age information. Based on a system in use for more than 10 yr at ExxonMobil, we recommend methodologies for a chronostratigraphic designation system (CDS) using more uniform and robust sequence-stratigraphic designations. After objectively defining important physical stratigraphic surfaces, biostratigraphic and other age-constraining information is used to designate surfaces and the packages of rocks they bound. This leads to the establishment of a sequence chronostratigraphic framework for a local area of investigation (outcrop section, field, region, or basin). Only after demonstrating clear well-documented ties to Phanerozoic global coastal onlaps or cycle charts are these sequences and associated surfaces considered as “global” entities and designated as such. Higher frequency sequences and surfaces are also accommodated in this CDS. Alternative designations for areas with limited or poor quality chronostratigraphic information are also discussed. The CDS has proven to have great use in all Phanerozoic strata, different tectonic settings, and depositional environments, especially when chronostratigraphic age constraints are robust. We have used this system at regional and basinal scales in many geographic locations to help reduce uncertainty in identifying and correlating reservoirs, sources, and seal rocks. Predicting the local distribution and quality of reservoirs as well as seals within a producing field and near field wildcats is also facilitated by this system. This system has demonstrated use in the correlation of outcrop sections within a basin or between basins. Rigorous use of the CDS proposed here will permit meaningful regional and/or interbasinal correlation that is difficult to carry out with the diverse systems currently in use. This uniform designation scheme will also facilitate communication within a company and between institutions, as well as among academic investigators.

Deep crustal structure in the eastern Gulf of Mexico

We use air gun data recorded by ocean bottom seismometers to constrain the velocity structure along Gulf of Mexico Basin Opening Line 4, a profile extending from the northwestern Florida peninsula across the Florida Escarpment to the central Gulf of Mexico. Moderately thinned continental crust with a Moho depth of 32–33 km, average sediment thickness of 6 km, and an average crustal thickness of 27 km is interpreted on the northeast end of the profile offshore Florida. Thinned and intruded continental crust is identified over a horizontal distance of 225 km where the crustal layer thins from 25 km to 6–7 km; mean seismic velocities of the crust in this region increase from 6.55 km/s to 6.95 km/s from northeast to southwest and are evidence for increased magmatic input as rifting developed. Oceanic crust with an average thickness of 5.6–5.7 km is observed over a distance of 175 km on the southwest end of the profile, with an extinct spreading ridge with an axial valley morphology imaged on a coincident seismic reflection profile. Anomalously high upper oceanic crust velocities of 6.0–6.7 km/s are interpreted as massive basalt flows and could reflect increased temperatures during emplacement. Integrating well, seismic reflection and our seismic refraction data allow us to estimate a full-spreading rate of 2.2 cm/yr for seafloor spreading along the profile; this indicates that oceanic crust was emplaced at a slow-spreading center.

Deep crustal structure of the northeastern Gulf of Mexico: Implications for rift evolution and seafloor spreading

We image deep crustal structure using marine seismic refraction data recorded by a linear array of ocean-bottom seismometers in the Gulf of Mexico Basin Opening project (GUMBO Line 3) in order to provide new constraints on the nature of continental and oceanic crust in the northeastern Gulf of Mexico. GUMBO Line 3 extends ~524 km from the continental shelf offshore Pensacola, Florida, across the De Soto Canyon and into the central Gulf basin. Travel times from long offset, wide angle reflections and refractions resolve compressional seismic velocities and layer boundaries for sediment, crystalline crust, and upper mantle. We compare our results with coincident multichannel seismic reflection data. Our velocity model recovers shallow seismic velocities (~2.0–4.5 km/s) that we interpret as evaporites and clastic sediments. A Cretaceous carbonate platform is interpreted beneath the De Soto Canyon with seismic velocities >5.0 km/s. Crystalline continental crust thins seaward along GUMBO Line 3 from 23–10 km across the De Soto Canyon. High seismic velocity lower crust (>7.2 km/s) is interpreted as extensive syn-rift magmatism and possibly mafic underplating, common features at volcanic rift margins with high mantle potential temperatures. In the central Gulf basin we interpret thick oceanic crust (>8 km) emplaced at a slow full-spreading rate (~24 mm/yr). We suggest a sustained thermal anomaly during slow seafloor-spreading conditions led to voluminous basalt flows from a spreading ridge that overprinted seafloor magnetic anomalies in the northeastern Gulf of Mexico.

Early Miocene continental-scale sediment supply to the Gulf of Mexico Basin based on detrital zircon analysis

The early Miocene was a period of major continental margin progradation in the Gulf of Mexico Basin that accompanied prominent tectonic and climatic changes in North America. However, sediment pathways from continental upland sources to deep basinal sinks remain poorly constrained. This study presents 2192 new detrital zircon U-Pb analyses from 19 Lower Miocene samples spanning the entire northern Gulf of Mexico margin to elucidate early Miocene sediment provenance and paleodrainage systems. The U-Pb age patterns indicate that the Great Plains, southern Rocky Mountains, and mid-Cenozoic volcanic field were the major source terranes for the western-central Gulf of Mexico coast, whereas the Appalachian foreland basin and Appalachian Mountains mainly contributed sediment to the eastern Gulf of Mexico coast. Local source terranes included the Llano uplift and Edwards Plateau in central Texas and the Ouachita Mountains and foreland basin in Oklahoma and Arkansas. A comparison to previous detrital zircon studies around the Gulf of Mexico indicates that sediment recycling was important during the early Miocene. Sediment associated with major paleorivers, including the Rio Bravo, Rio Grande, Houston-Brazos, Red, Mississippi, Tombigbee, and Apalachicola Rivers, can be differentiated using the detrital zircon U-Pb analyses. These data help to better define the early Miocene source-to-sink system in the northern Gulf of Mexico, by relating the basin fill to hinterland tectonic and geological evolution. In comparison to the Paleocene−Eocene Wilcox drainage system, the early Miocene drainage system of the northern Gulf of Mexico was smaller and received less input from western Mexico arc terranes and Archean basement in Wyoming. This drainage area reduction, related to regional thermal uplift and Basin and Range−Rio Grande rifting, likely explains the reduced sediment volume of the Lower Miocene strata in the Gulf of Mexico relative to the Paleocene−Eocene Wilcox Group.

Channel-belt scaling relationship and application to early Miocene source-to-sink systems in the Gulf of Mexico basin

In past decades, numerous studies have focused on the alluvial sedimentary record of basin fill. Paleo–drainage basin characteristics, such as drainage area or axial river length, have received little attention, mostly because the paleo–drainage system underwent erosion or bypass, and its record is commonly modified and overprinted by subsequent tectonism or erosional processes. In this work, we estimate the drainage areas of early Miocene systems in the Gulf of Mexico basin by using scaling relationships between drainage area and river channel dimensions (e.g., depth) developed in source-to-sink studies. Channel-belt thickness was used to estimate channel depth and was measured from numerous geophysical well logs. Both lower channel-belt thickness and bankfull thickness were measured to estimate the paleo–water depth at low and bankfull stages. Previous paleogeographic reconstruction using detrital zircon and petrographic provenance analysis and continental geomorphic synthesis constrains independent estimates of drainage basin extent. Comparison of results generated by the two independent approaches indicates that drainage basin areas predicted from channel-belt thickness are reasonable and suggests that bankfull thickness correlates best with drainage basin area. The channel bankfull thickness also correlates with reconstructed submarine fan dimension. This work demonstrates application to the deep-time stratigraphic archive, where records of drainage basin characteristics are commonly modified or lost.

Detrital-zircon records of Cenomanian, Paleocene, and Oligocene Gulf of Mexico drainage integration and sediment routing: Implications for scales of basin-floor fans

This paper uses detrital zircon (DZ) provenance and geochronological data to reconstruct paleodrainage areas and lengths for sediment-routing systems that fed the Cenomanian Tuscaloosa-Woodbine, Paleocene Wilcox, and Oligo cene Vicksburg-Frio clastic wedges of the northern Gulf of Mexico (GoM) margin. During the Cenomanian, an ancestral Tennessee-Alabama River system with a distinctive Appalachian DZ signature was the largest system contributing water and sediment to the GoM, with a series of smaller systems draining the Ouachita Mountains and discharging sediment to the western GoM. By early Paleocene Wilcox deposition, drainage of the southern half of North America had reorganized such that GoM contributing areas stretched from the Western Cordillera to the Appalachians, and sediment was delivered to a primary depocenter in the northwestern GoM, the Rockdale depocenter fed by a paleo–Brazos-Colorado River system, as well as to the paleo–Mississippi River in southern Louisiana. By the Oligocene, the western drainage divide for the GoM had migrated east to the Laramide Rockies, with much of the Rockies now draining through the paleo–Red River and paleo– Arkansas River systems to join the paleo–Mississippi River in the southern Mississippi embayment. The paleo–Tennessee River had diverted to the north toward its present-day junction with the Ohio River by this time, thus becoming a tributary to the paleo-Mississippi within the northern Mississippi embayment. Hence, the paleo-Mississippi was the largest Oligocene system of the northern GoM margin. Drainage basin organization has had a profound impact on sediment delivery to the northern GoM margin. We use paleodrainage reconstructions to predict scales of associated basin-floor fans and test our predictions against measurements made from an extensive GoM database. We predict large fan systems for the Cenomanian paleo–Tennessee-Alabama, and especially for the two major depocenters of the early Paleocene paleo–Brazos-Colorado and late Paleocene–earliest Eocene paleo-Mississippi systems, and for the Oligocene paleo-Mississippi. With the notable exception of the Oligocene, measured fans reside within the range of our predictions, indicating that this approach can be exported to other basins that are less data rich.

The Cretaceous‐Paleogene boundary deposit in the Gulf of Mexico: Large‐scale oceanic basin response to the Chicxulub impact

Hydrocarbon exploration in the last decade has yielded sufficient data to evaluate the Gulf of Mexico basin response to the Chicxulub asteroid impact. Given its passive marine setting and proximity to the impact structure on the Yucatan Peninsula, the gulf is the premier locale in which to study the near-field geologic effect of a bolide impact. We mapped a thick (decimeter- to hectometer-scale) deposit of carbonate debris at the Cretaceous-Paleogene boundary that is ubiquitous in the gulf and readily identifiable on borehole and seismic data. We interpret deposits seen in seismic and borehole data in the deepwater gulf to be predominately muddy debrites with minor turbidites based on cores in the southeastern gulf. Mapping of the deposit in the northern Gulf of Mexico reveals that the impact redistributed roughly 1.05 × 105 km3 of sediment therein and over 1.98 × 105 km3 gulfwide. Deposit distribution suggests that the majority of sediment derived from coastal and shallow-water environments throughout the gulf via seismic and megatsunamic processes initiated by the impact. The Texas shelf and northern margin of the Florida Platform were significant sources of sediment, while the central and southern Florida Platform underwent more localized platform collapse. The crustal structure of the ancestral gulf influenced postimpact deposition both directly and indirectly through its control on salt distribution in the Louann Salt Basin. Nevertheless, impact-generated deposition overwhelmed virtually all topography and depositional systems at the start of the Cenozoic, blanketing the gulf with carbonate debris within days.

Validation of empirical source-to-sink scaling relationships in a continental-scale system: The Gulf of Mexico basin Cenozoic record

Empirical scaling relationships between known deepwater siliciclastic submarine fan systems and their linked drainage basins have previously been established for modern to submodern depositional systems and in a few ancient, small-scale basins. Comprehensive mapping in the subsurface Gulf of Mexico basin and geological mapping of the North American drainage network facilitates a more rigorous test of scaling relationships in a continental-size system with multiple mountain source terranes, rivers, deltas, slopes, and abyssal plain fan systems formed over 65 m.y. of geologic time. An immense database of drilled wells and high-quality industry seismic data in this prolific hydrocarbon basin provide the independent measure of deepwater fan distribution and dimensions necessary to test source-to-sink system scaling relationships. Analysis of over 40 documented deepwater fan and apron systems in the Gulf of Mexico, ranging in age from Paleocene to Pleistocene, reveals that submarine-fan system scales vary predictably with catchment length and area. All fan system run-out lengths, as measured from shelf margin to mapped fan termination, fall in a range of 10%–50% of the drainage basin length, and most are comparable in scale to large (Mississippi River–scale) systems although some smaller fans are present (e.g., Oligocene Rio Bravo system). For larger systems such as those of the Paleocene Wilcox depositional episodes, fan runout lengths generally fall in the range of 10%–25% of the longest river length. Submarine fan widths, mapped from both seismic reflection data and well control, appear to scale with fan run-out lengths, though with a lower correlation (R2 = 0.40) probably due to uncertainty in mapping fan width in some subsalt settings. Catchment area has a high correlation (R2 = 0.85) with river length, suggesting that fluvial discharge and sediment flux may be primary drivers of ancient fan size. Validation of these first-order source-to-sink scaling relationships provides a predictive tool in frontier basins with less data. Application to less-constrained early Eocene fan systems of the southern Gulf of Mexico demonstrates the utility for exploration as well as paleogeographic reconstructions of ancient drainage systems. This approach has considerable utility in estimating dimensions of known but poorly constrained submarine fans in the subsurface or exposed in outcrop. INTRODUCTION Source-to-sink analysis is a broad and rapidly evolving scientific approach to paleogeographic reconstructions, but one that also has practical applications relevant to the global search for hydrocarbon resources (Sømme et al., 2009a; Walsh et al., 2016; Helland-Hansen et al., 2016). Quantification of the scales of modern and Pleistocene systems suggests linkages within and between segments of sediment-dispersal systems that terrigenous clastics follow from highland source terranes toward the basinal sinks. This makes it possible to predict the unknown geomorphological dimensions of one segment from empirical measurements of another (Sømme et al., 2009a; Bhattacharya et al., 2016). For example, deepwater depositional systems can be linked in many cases to the rivers that carry the sediment, and thus should scale with fluvial system properties (Blum and Hattier-Womack, 2009). The possibility of linking drainage basin (catchment) characteristics with basinal deposits is intriguing for its potential utility in prediction for interpretation of Earth history, and as a predictive tool for subsurface exploration, particularly in areas where seismic reflection resolution is of poor quality, including areas of poor illumination due to thick salt canopy cover (e.g., Meyer et al., 2007). A first application of the source-to-sink approach for an ancient, deepwater subsurface system was in the hydrocarbon-bearing Maastrichtian–Danian Ormen Lange deepwater system of offshore Norway (Sømme et al., 2009b). This test of a small depositional system suggested great promise for first-order prediction of reservoir dimensions such as submarine fan length and width. Deepwater deposits at the termini of continental fluvial systems represent valuable sedimentary archives of climate and tectonic histories along the source-to-sink continuum (Covault et al., 2010, 2011; Barnes et al., 2013; Romans et al., 2016). Identifying the dimensions and therefore the spatial distribution of submarine fans in successions in the subsurface or even in poorly mapped outcrops allows that record to be accessed and illuminated. Firstorder morphological parameters estimated from the scaling relationships for large source-to-sink systems like the Gulf of Mexico might further be used to extend paleogeographic reconstructions to updip areas where the sedimentary record is absent due to erosional truncation. In Namibia, for example, uplift and erosion removed large portions of the onshore record of Early Cretaceous river systems and drainage basins (Green et al., 2009), challenging efforts at GEOSPHERE GEOSPHERE; v. 14, no. 2 doi:10.1130/GES01452.1 9 figures; 3 tables; 1 supplemental file CORRESPONDENCE: jsnedden@ ig .utexas .edu CITATION: Snedden, J.W., Galloway, W.E., Milliken, K.T., Xu, J., Whiteaker, T., and Blum, M.D., 2018, Validation of empirical source-to-sink scaling relationships in a continental-scale system: The Gulf of Mexico basin Cenozoic record: Geosphere, v. 14, no. 2, p. 768–784, doi:10.1130/GES01452.1. Science Editor: Raymond M. Russo Associate Editor: Brandon McElroy Received 3 November 2016 Revision received 3 November 2017 Accepted 5 January 2018 Published online 26 January 2018

The Northern Gulf of Mexico During OAE2 and the Relationship Between Water Depth and Black Shale Development

Despite their name, Oceanic Anoxic Events (OAEs) are not periods of uniform anoxia and black shale deposition in ancient oceans. Shelf environments account for the majority of productivity and organic carbon burial in the modern ocean, and this was likely true in the Cretaceous as well. However, it is unlikely that the mechanisms for such an increase were uniform across all shelf environments. Some, like the northwest margin of Africa, were characterized by strong upwelling, but what might drive enhanced productivity on shelves not geographically suited for upwelling? To address this, we use micropaleontology, carbon isotopes, and sedimentology to present the first record of Oceanic Anoxic Event 2 (OAE2) from the northern Gulf of Mexico shelf. Here OAE2 occurred during the deposition of the well-oxygenated, inner neritic/lower estuarine Lower Tuscaloosa Sandstone. The overlying organic-rich oxygen-poor Marine Tuscaloosa Shale is entirely Turonian in age. We trace organic matter enrichment from the Spinks Core into the deepwater Gulf of Mexico, where wireline log calculations and public geochemical data indicate organic enrichment and anoxia throughout the Cenomanian-Turonian boundary interval. Redox change and organic matter preservation across the Gulf of Mexico shelf were driven by sea level rise prior to the early Turonian highstand, which caused the advection of nutrient-rich, oxygen-poor waters onto the shelf. This results in organic matter mass accumulation rates 1–2 orders of magnitude lower than upwelling sites like the NW African margin, but it likely occurred over a much larger geographic area, suggesting that sea level rise was an important component of the overall increase in carbon burial during OAE2.

Upper Jurassic Tithonian-centered source mapping in the deepwater northern Gulf of Mexico

AbstractLong the subject of speculation, the origin, distribution, and quality of Mesozoic source beds in the deepwater Gulf of Mexico (GOM) are now open to analytical study and hypothesis. We have developed new maps and concepts for organic richness and lithofacies patterns of the primary Upper Jurassic oil-prone source rock interval spanning the Kimmeridgian to Lower Berriasian in the northern GOM. This interval, previously referred to as the Tithonian-centered source, includes the Haynesville and Bossier shales, which lie within supersequences representing second-order transgressive and high-stand systems tracts, respectively. A newly developed gulf-wide Cotton Valley-Bossier paleogeographic map based on a novel paleotectonic model for the Mesozoic provides the framework for this source mapping study. Organic richness averages up to 4.7% and 6.5% total organic carbon for the Kimmeridgian and Tithonian-Lower Berriasian supersequences, respectively, based on the log overlay Δ log R technique and increase...