ABSTRACT This field guide reviews 19 sites providing insight to four Cenozoic deformational phases of the Cascadia forearc basin that onlaps Siletzia, an oceanic basaltic terrane accreted onto the North American plate at 51–49 Ma. The field stops visit disrupted slope facies, prodelta-slope channel complexes, shoreface successions, and highly fossiliferous estuarine sandstones. New detrital zircon U-Pb age calibration of the Cenozoic formations in the Coos Bay area and the Tyee basin at-large, affirm most previous biostratigraphic correlations and support that some of the upper-middle Eocene to Oligocene strata of the Coos Bay stratigraphic record represents what was differentially eroded off the Coast Range crest during ca. 30–25 Ma and younger deformations. This suggests that the strata along Cape Arago are a western “remnant” of the Paleogene Tyee basin. Zircon ages and biostratigraphic data encourages the extension of the Paleogene Coos Bay and Tyee forearc basin westward beyond the Fulmar fault and offshore Pan American and Fulmar wells. Integration of outcrop paleocurrents with anisotropy of magnetic susceptibility data from the middle Eocene Coaledo Formation affirms south-southeast to north-northwest sediment transport in current geographic orientation. Preliminary detrital remanent magnetism data show antipodal directions that are rotated clockwise with respect to the expected Eocene field direction. The data suggest the Eocene paleo-shoreline was relatively north-south similar to the modern shoreline, and that middle Eocene sediment transport was to the west in the area of present-day Coos Bay. A new hypothesis is reviewed that links the geographic isolation of the Coos Bay area from rivers draining the ancestral Cascades arc to the onset of uplift of the southern Oregon Coast Range during the late Oligocene to early Miocene.

Abstract: Worldwide cycle charts that inventory depositional sequences are widely used as a correlation model for comparison with local depositional cycles. These integrated charts provide a data set for considering eustatic cycles in response to tectonics and glaciology. Biostratigraphers occasionally find that the local cycle dates determined from recovered fossils do not precisely correlate with dates on some global charts. This paper attempts to answer this correlation concern by addressing the fundamental concept of cycle chart construction. To construct a cycle chart, each cycle, subsurface or outcrop, is identified by stratal geometries and depositional facies, and it is dated biostratigraphically using recovered fossils. The biochronozones assigned to each sequence-mapping surface are then used to correlate the local depositional sequence to a global timescale and are compared with the derived sequence onlap and eustatic curves. The reality of global chart construction is that many local variations resulting from tectonic deformation, crustal loading and unloading, rates of basin subsidence, environmental (biofacies) changes, and variations in sediment flux are averaged out in favor of assumed synchronicity. New studies of these local factors, especially where the time value of the hiatus is defined, may result in dates for primary stratal surfaces that will not necessarily be synchronous with the modal averages compiled into the global eustatic cycle charts. When the locally recognized biochronozones and inferred ages do not agree with the global sequence boundary ages, the biostratigrapher must decide if the discrepancy results from imprecision of the local fossil record or a lack of synchronicity between cycles. Many cycle sequence boundaries do correlate, suggesting that there is likely a global eustatic component. However, specific driving forces must be considered, which requires recognition and analysis of local variability. Each geoscientist needs to honor their local cyclic patterns with locally constrained time-stratigraphic data, most often biostratigraphic, and clearly state the timescale used for local-to-regional cycle correlation based on interpreted biochronozones. When local cycles are correlated to a global cycle chart, it must be affirmed that the same magnetobiochronostratigraphic timescale is used in each case. Using the clearly defined local biochronozone assignments, a best-fit correlation with a global cycle chart can be suggested, and global forcing factors can be considered.

Abstract Lithologic, biostratigraphic and isotopic data from cuttings provides calibration of three sigmoidal, clinoform packages separated by regionally continuous, parallel seismic facies. This depositional geometry is interpreted as a shelf-margin, deltaic wedge deposited within the East Breaks 160-161 minibasin. A chaotic seismic facies package extending over more than 84 mi 2 (218 km 2 ) occurs between two clinothem packages of Ericson Zone Y (71 ka BP to 12 ka BP). The chaotic package is interpreted to be a mass-transport complex that failed during the accelerated rate of sea level fall during late Oxygen Isotope Stage 3 (approximately 30 to 20 Ka). Clinothem foresets, bottom sets and the mass-transport complex are predominantly clay with minor siltstone. Sands are restricted to proximal topsets and fluvial channels. The mass-transport complex consists of three subfacies: rotated-block, hummocky-mounded, and disrupted. Distribution and volume of these facies suggests that only the rotated-block subfacies has been significantly transported, and that the hummocky-mounded and disrupted subfacies result from different degrees of disruption of ponded, clay-prone layered sediments by compression and dewatering triggered by the submarine slide of rotated blocks.

Abstract In many ways, sequence stratigraphy’s effect on stratigraphic interpretation is comparable to that of plate tectonics on structural geology. These are markers in the history of geology upon which talented minds can build the next advances. However, concepts that seem self-evident to today’s students faced painful periods of ridicule and resistance when first systematized by Peter Vail in the 1960s. The history of this slow acceptance is marked by a gradual evolution of concepts that bring basin tectonics, sea-level fluctuations, and sediment supply into an integrated stratigraphic solution. One basic idea is the time-significance of stratal surfaces and surfaces of discontinuity, and of the seismic reflections generated by them. Another concept is that cyclic sedimentary sequences form in response to varying rates of eustatic changes, tectonic subsidence, and sedimentary supply. The sequence model is highly variable but astonishingly robust in predicting facies and environments in a wide variety of basin and tectonic settings. Applications in industry and academia are widespread, especially in predicting deep-marine sands, seals, and source rocks in offshore settings. Other specific applications include sequence stratigraphy of carbonates, estuarine sands, incised valleys, forced regressions, and well-log and outcrop analysis. Development of eustatic cycle charts needs high-quality biostratigraphy for dating and environmental analysis. Advent of 3-D seismic data opens a myriad of uses involving attributes of the seismic signal. No matter how specialized, however, the good interpreter always starts with a rigorously defined chronostratigraphic framework of sequences and systems tracts for proper interpretation.

Abstract The depositional sequence was defined in 1977 as “a stratigraphic unit composed of a relatively conformable succession of genetically related strata and bounded at its top and base by unconformities or their correlative conformities.” This definition has proven singularly robust in any number of sedimentary and tectonic settings. The depositional sequence model evolves naturally from recognition of stratal surfaces in rocks as geological time lines and from the time-significance of unconformities and their correlative conformities as sequence boundaries. Seismic reflections follow stratal boundaries, and within limits of seismic resolution, represent time lines. Sequence boundaries have been found to be of the same age in basins worldwide, and it has been postulated that global sea level changes are a major control on the stratigraphic record, along with local basin tectonics and rates of sediment supply. Eustatic cycle charts plot the ages and magnitudes of global eustatic cycles. High-resolution biostratigraphy is essential to dating sequences and determining paleo-environments. The cyclic sequences bounded by these unconformities have a basic pattern of deposition which results from a relative fall and rise of sea level. This pattern can vary widely from basin to basin, depending on variations in basin tectonics and sediment supply. The accommodation model consists of lowstand, transgressive, and highstand systems tracts formed in response to various stages of the sea-level cycle. The maximum flooding surface marks the maximum transgression of marine facies on the shelf, and is an important correlation point. It is not a sequence boundary in our model because strata bounded by maximum flooding surfaces would include two disparate units separated by a significant unconformity, which is our sequence boundary. Several levels of sea level cyclicity may occur in a hierarchy that allows higher frequency cycles to be superposed or stacked into lower frequency cycles. The stacking of parasequences (fourth- or fifth-order cycles) into third-order (sequence) cycles is an example. Examples of stratigraphic sections from the Paleozoic of the Arabian plate and the northern Gulf of Mexico show applications of sequence stratigraphy to both extremes of the Phanerozoic spectrum. They also contrast characteristics of the best-known models of sedimentary response to cyclic depositional control.

Abstract Subregional 3-D seismic volumes and wireline logs permitted definition of second- to fifth-order (~10 my–10 ky) Frio and Anahuac (Oligocene) sequences, systems tracts, and associated syntectonics. Third- and most fourth-order sequences were correlated within several subregional wireline-log and seismic networks. Vicksburg and Miocene sequences were of secondary interest. Composite sequence logs ( Figs. 1 and 2 ) characterized principal fields. Sequence analysis identified and correlated all key surfaces: type 1 unconformities, maximum-flooding surfaces, and transgressive surfaces bounding systems tracts. Although microfossil occurrences are not necessarily required for sequence analysis, limited data were integrated with the final sequence frameworks, providing secondary verification of assigned ages.

Abstract In the twenty-five years since the landmark publication of AAPG Memoir 26 and later, SEPM Special Publication 42, the concepts of sequence stratigraphy have evolved rapidly. This discipline, an outgrowth of seismic stratigraphy, has spread far beyond applications to 2-D seismic data alone. Sequence stratigraphy has seen applications embracing data sets ranging from biostratigraphic to geochemical to physical oceanographic, and from borehole to outcrop, and finally, coming full cycle, to 3-D seismic data. Initially the domain of industry geoscientists, sequence stratigraphy has gained widespread acceptance among geoscientists in all professions, having been recognized as an approach that facilitates integration of a broad range of disciplines. The evolution of sequence stratigraphic concepts is far from complete. In particular, recent increased availability of high-quality 3-D seismic coverage promises to provide insights that will lead to further fine tuning of sequence concepts. In addition to enhanced 2-D profiles, 3-D seismic data afford exceptional plan views of the subsurface that in the past could only be inferred. These plan view images now comprise a fundamental starting point from which geologic analyses and interpretation can begin. Such images depict paleo-landscapes, which can be analyzed using time-honored principles of geomorphology, leading to the development of the discipline of seismic geomorphology . When used in conjunction with seismic stratigraphy, seismic geomorphology can significantly enhance sequence stratigraphic interpretations. The identification of depositional elements such as channels, valleys, shore faces, shelf ridges, etc. , in plan view, can be integrated with seismic stratigraphic analyses of associated seismic profiles to calibrate profile reflection patterns and refine analyses of basin fill histories. Systematic seismic geomorphologic analysis of 3-D seismic volumes can bring to light spatial and temporal relationships of successive depositional systems. Moreover, recognition of these systems and analyses of their succession can help in the identification of possible missing facies tracts. This approach, coupled with direct and indirect recognition of unconformities, comprises an integral aspect of sequence stratigraphic interpretation.

Abstract For high-frequency (fourth-order) depositional sequences, seismic-stratigraphic interpretation of vertical seismic sections commonly generates equivocal sequence boundaries and systems tracts because of limited vertical seismic resolution. Extending well-based, high-frequency sequence stratigraphy into a 3-D seismic survey area consequently proves to be a major challenge. We show that critical to such extension is recognition and interpretation of plano-form geomorphology of depositional systems. Emphasis should be shifted from interpreting vertical seismic data to developing new tools capable of extracting more horizontal, seismic-sedimentologic information. This case study of the Vermilion Block 50-Tiger Shoal field area, offshore Louisiana, shows that proportional, stratal slicing between Miocene flooding surfaces provides sequential and accurate seismic imagery of depositional systems. This imagery in turn serves as a basis for recognizing and mapping high-frequency systems tracts, sequence boundaries, and sequences in a geologic-time domain. In the Miocene interval, all of the fourth-order sequences or sequence sets from study wells can be seismically mapped at a resolution equivalent to 10 m in thickness, which is necessary for accurate reconstruction of the high-frequency sequence-stratigraphic framework in the region of seismic coverage outside well control.

Abstract Recent major discoveries at Mad Dog, Atlantis, and Neptune have opened a major new hydrocarbon province in the Western Atwater Foldbelt of the ultra-deepwater Gulf of Mexico. The hydrocarbons are reservoired in good quality Lower and Middle Miocene turbidite sandstones. There is a clear stratigraphic signature expressed in the ultra-deep water, with the principal reservoir sands strongly partitioned into discrete, high net/gross, laterally extensive bodies, at or close to interpreted sequence boundaries. The upper part of the sequence is dominated by thin-bedded turbidites and mudstones. Good quality biostratigraphic data allow us to correlate third and fourth order strati-graphic boundaries through all the wells across the region. Seismic data, south of the Sigsbee salt, for these deeper, older reservoirs provides resolution of the third order sequence boundaries. Compensation cycles have been mapped between the third order sequence boundaries, but it is not certain that these directly correspond to the fourth order sequences. The observed stratigraphic hierarchy starts at second order cyclicity, demonstrated by a systematic change in the distribution of net sand from one third order sequence to the next. The highest resolution sequences that we have been able to map are fifth order. As expected, there is a systematic relationship between the stratigraphic hierarchy and the lateral correlation length of stratigraphic surfaces; ie. condensed sections associated with third order sequences can be correlated over greater lengths than the flood surfaces associated with higher order sequences. Third order sequence boundaries are mapped over a distance of in excess of a hundred miles. Fourth order boundaries have been correlated with confidence over a distance of at least 30 miles, and the fifth order correlation surfaces can only be mapped confidently within a single field, a distance of a few miles. The nature of the flood surfaces changes with the position in the stratigraphic hierarchy and with the location relative to the sand feeder systems. In the core of the depositional system, we do not observe the foraminiferal marls that are characteristic of significant condensed sections elsewhere in the Gulf of Mexico. Our interpretation is that sediment is being supplied continuously to the basin floor in the form of sediment gravity flows and suspended sediment fall out from turbidity currents. As a result, third order flooding surfaces are characterized by high gamma shales, fourth order flooding surfaces are commonly associated with intervals of thin bedded turbidites; and fifth order flood surfaces have a variety of log signatures, and a ranges of facies association. Away from the focus of sediment input, foraminiferal marls are observed and some are interpreted to represent condensed sections. The detailed high frequency sequence stratigraphy that has been carried through the three principal discoveries in the Western Atwater Foldbelt has impacted the exploration and development strategy for the area. For example, we have been able to demonstrate the presence of significant paleotopography that affected the lowermost Miocene reservoir distribution. We have proven that the thick sand bodies associated with the third order sequence boundaries are laterally extensive and so have reduced the risk of finding good quality reservoir sands in appraisal wells and nearby exploration wells. We have also demonstrated that not all flood surfaces are created equal and are therefore likely to have different transmis-sibility properties. This could have a significant impact on fluid flow within the reservoir and therefore medium and long-term field development.

Abstract Examination of exploration drilling histories for many different global basins indicates a counter-intuitive temporal and spatial pattern in the way hydrocarbons are sometimes discovered. Conventional wisdom holds that for any given basin or play, a plot of cumulative discovered hydrocarbon volumes versus time or number of wells drilled generally show a steep curve (rapidly increasing volumes) early in the play history and a later plateau or terrace (slowly increasing volumes). Such a plot is called a creaming curve, as early success in a play is thought to inevitably give way to later failure as the play or basin is drilled-up. It is commonly thought that the “cream of the crop” of any play or basin is found early in the drilling history. By examining plays or basins with sufficiently long drilling histories and range of reservoir paleoenvironment and trap types, one actually finds two or three “terraces” to the creaming curve. The first string of successes in a given basin generally corresponds to exploitation of the highstand systems tract or sequence set reservoirs developed in updip structural traps. These reservoirs are typically marginal to shallow marine “shelfal” deposits, laterally continuous but lacking internal sealing facies and are seldom self-sourcing. The second or third terrace in the creaming curve usually involves the lowstand reservoir component (systems tract or sequence set), which is often developed in downdip deepwater or slope paleoenvironments. Transgressive (systems tract or sequence set) reservoirs, typically shallow marine shelfal sandstones that are sometimes self-sourced, are variably developed and may or may not occupy the second terrace of the creaming curve. These trends hold true for both second-order (3–10 my) and/or third-order (1–3 my) stratigraphic cycles, depending upon the scale of the basin or play. This analysis fits well with the definition of an exploration play provided by Magoon and Sanchez (1995) : a fully developed play is the simple volume difference between the petroleum system capability and the current discovered hydrocarbon volumes (commercial or not). Where the difference is large, either the petroleum system has significant leakage problems ( e.g ., Barents Sea Mesozoic play) or the lowstand systems tract or sequence set has not been fully exploited. Examples supporting these ideas are drawn from several global basins (Gulf of Mexico Miocene, Norway Upper Jurassic, Mahakam Delta, and Texas Wilcox). Case studies demonstrate how critical elements of exploration risk shift from trap and seal in highstand plays to reservoir and source in lowstand components of these plays.

Abstract Sequence stratigraphic application has emphasized the recognition and use of subaerial (fluvial entrenchment) or shallow marine/shoreface (regressive ravinement) surfaces as critical boundaries for defining sequences. These surfaces are variously objectively or conceptually associated with times of onset, maximum rate, and/or lowest position of relative sea level fall. However, well-dated Quaternary analogues demonstrate that the fluvial entrenchment surface is neither inherently synchronous nor regional, and that low-stand facies associations and their bounding surfaces are highly dependent upon the vagaries of paleogeography and sediment supply. Furthermore, some basin fills display stratigraphy in which demonstrable subaerial or ravinement surfaces correlative to fall events are poorly preserved or entirely lacking, but in which sequences can be defined by use of various combinations of transgressive ravinement, marine deflation, and marine starvation surfaces. These surfaces may not and need not correspond to a relative fall (or rise) of sea level. Selection of stratigraphic surfaces as sequence boundaries and interpretation of sequence systems tract compositions and relationships both require understanding of the overall depositional systems tract and of the full array of regime variables: sediment supply, sediment composition, base level change, and energy regime. Functional, reproducible, and chronostratigraphic “… genetically related successions of strata bounded by unconformities or their correlative conformities…” can be defined, correlated, mapped, dated, and interpreted through the use of a variety of regional stratigraphic surfaces of non-deposition and erosion.

Abstract The late Miocene–early Pliocene sediments of the northern greater Mars-Ursa intraslope basin in the Gulf of Mexico record high-resolution (ca. 0.5 Ma) cyclic deposition of couplets of deep water fan lobes (sheet sands) and channelized or amalgamated systems bounded by local to regional condensed sections. Internally, the sheet sand and channelized systems are separated by a transitional surface of bypass, avulsion, and/or erosion. These fourth-order cycles are the building blocks that make up third-order seismic facies assemblages, which in turn record the progressive fill and spill sedimentary dynamics in salt-withdrawal minibasins. Third-order sequences in the greater Mars-Ursa intraslope basin reflect regional variations in accommodation and sediment supply. The rate of deposition of an older (10.0–7.0 Ma) third-order assemblage is significantly greater than that of a younger (7.0–4.2 Ma) assemblage. The older assemblage has a greater abundance of thick, laterally extensive sheet sands and a higher composite net-to-gross ratio than the younger assemblage, which contains more amalgamated and bypass channels and associated over bank facies, and has a lower composite net-to-gross. Fourth-order cycles display compensational stacking patterns in which the overlying channel system is best developed where the sheet sand system is thin or not present. The fourth-order stratigraphy of the basin is controlled by high frequency variations in accommodation and sediment supply. Eustatic changes affect the supply of sediment from the shelf to the slope, but they are not a primary control in the formation of fourth-order deepwater sequences. Condensed sections that bound the fourth-order cycles are deep marine (pelagic) mudstones that drape topography. They occur in association with faunal abundance and diversity peaks and often correspond to abrupt changes in incremental overpressure. Because they represent periods of minimum relative sedimentation rates and maximum relative rates of accommodation creation, basin margins have their highest relief at the ends of these periods. Several of the condensed sections are interpreted to correlate to maximum flooding events on the shelf. Thick, high net-to-gross fan lobes that occur above the condensed sections have been deposited as sedimentation rates in the basin increased. They fill paleo-topographic lows and onlap abruptly against high-relief basin margins. Deep water faunal abundances in these sections are at relative minima due to the influx of sediment, and in some instances, the sands have a biofacies signature indicative of reworking. However, a lack of corresponding increases in either terrigenous content or reworked Cretaceous material suggests a local source for these reworked sands. Subtle transitional surfaces separating the sheets and channels are identified by the onset of apparent paleobathymetric deepening at the bases of faunal abundance and diversity peaks. Typically, these surfaces document intra-basinal avulsion or erosion and sediment bypass, caused by infill of available accommodation by sheet sands. The channelized systems or amalgamated channel/sheet systems that overlie transitional surfaces are associated with decreasing rates of sedimentation and low rates of creation of accommodation. Thin, laterally restricted amalgamated channels and sheet deposits may accumulate in the limited accommodation. If present, erosional channels and bypass scours are filled by sand.

Abstract Allostratigraphy is the only means available for formally naming stratigraphic units defined on the basis of observed bounding discontinuities. Sequence stratigraphy represents a powerful way of interpreting allostratigraphic units in the context of cyclic changes in accommodation and accumulation. Lack of consistency in definition and usage of sequence terms, however, shows that it is premature as a means of formal naming. Sequence stratigraphic terms are inconsistent in that they may convey either positional or temporal concepts. For example, many sequence stratigraphers’ reveal their temporal bias by referring to early and late subdivision within “systems tracts,” despite claims that “systems tracts” are defined purely physically. Sequence stratigraphic terminology also emphasizes that units are genetically related by a particular process. For example, sequence boundaries are defined as unconformities, and their correlative conformities, that show evidence of subaerial exposure. Marine discontinuities, although highly useful for allostratigraphic correlations, do not satisfy this strict definition of a sequence boundary. In addition, there are significant practical problems in defining a sequence boundary where it is expressed wholly as a correlative conformity. This potentially limits the ability to define sequences in areas where evidence for an unconformity may be cryptic or absent. The bias towards subaerial exposure has resulted in confusion in the interpretation of tectonically produced marine erosion surfaces in the Cretaceous Seaway of North America. Marine erosion may remove evidence of prior subaerial exposure, which must then be inferred rather than observed. Allostratigraphy is inherently more practical in that it emphasizes mappable, observable discontinuities, rather than inferred exposure surfaces. Until the terminological confusion in sequence stratigraphy has been mopped-up, it must remain a highly valuable tool for interpretation but a less valuable tool for formally naming rock units.

Abstract A sequence, as originally defined by Sloss and colleagues, was a stratigraphic unit bounded by subaerial unconformities. Such a stratigraphic unit proved to be of limited value because, in most instances, sequences could be recognized only on the margins of a basin where subaerial unconformities were present. Vail and colleagues greatly expanded the utility of sequences for basin analysis when they redefined the term as a unit bounded by unconformities or correlative conformities. The addition of correlative conformities allowed a sequence to potentially be recognized over an entire basin. This revised definition has led to the formulation of four different types of sequences, each having a different set of bounding surfaces. Vail and colleagues have defined two types: a type 1 depositional sequence and a type 2 depositional sequence. A type 1 depositional sequence utilizes a subaerial unconformity as the unconformable portion of the boundary and a time line equivalent to the start of base level fall for the correlative conformity. Because the subaerial unconformity migrates basinward during base level fall, much of it is therefore included within such a sequence rather than being on the boundary. Also it is impossible to objectively recognize a time line that corresponds to the start of base level fall. For these reasons a type 1 depositional sequence has little practical value. A type 2 depositional sequence also uses the subaerial unconformity as the unconformable portion of the boundary but uses a time line equivalent to the end, rather than the start, of base level fall for the correlative conformity. This resolves the problem of including a portion of the unconformity inside the sequence. However, it is essentially impossible to objectively recognize a time line that corresponds with the end of base level fall (start of base level rise) and thus this type of sequence also has no practical value. Galloway proposed the use of maximum flooding surfaces as sequence boundaries and named such a unit a genetic stratigraphic sequence. This alleviated the problem of major subjectivity in boundary recognition because maximum flooding surfaces can be determined by objective scientific analysis. However, this sequence type founders on the problem that the subaerial unconformity occurs within the sequence and thus it lacks genetic coherency on the basin margins. To overcome these major deficiencies in sequence definition, Embry and Johannessen have defined a fourth type of sequence that they term a T-R sequence. This sequence uses the subaerial unconformity as the unconformable portion of the boundary and the maximum regressive surface as the correlative conformity. This methodology keeps the subaerial unconformity on the boundary and also provides for a correlative conformity that can be objectively determined. It thus avoids the fatal flaws of previously defined types. A T-R sequence can be divided into a transgressive systems tract below and a regressive systems tract above by using the maximum flooding surface as a mutual boundary. T-R sequence stratigraphy, unlike the other proposed methodologies, has maximum practical utility with a minimum of stultifying jargon.

Abstract Stratigraphic analysis of sedimentary basins is critical for correlation in a basin, for reconstructing the geohistory of a basin, and for developing a successful petroleum exploration strategy for a basin. In studying only the shelfal areas of basins that are characterized by carbonate or mixed carbonate and siliciclastic deposition and in which stratal patterns are driven by low-frequency, tectonic-eustatic events, a stratigraphic analysis based on the cyclicity recorded in the strata (transgressive-regressive cycles) has utility for correlation, for geohistory interpretation, and in formulating petroleum exploration strategies. This is the case for the Cretaceous section in basins of the northeastern Gulf of Mexico. In utilizing the concept of transgressive-regressive (T-R) cycles, eight T-R cycles are recognized in Cretaceous (upper Valanginian to lower upper Maastrichtian) strata of the northern Gulf of Mexico. The cycles consist of a transgressive (aggrading and backstepping) phase and a regressive (infilling) phase. These T-R cycles are useful for intrabasin correlation of Cretaceous strata in the Mississippi interior salt basin and for interbasin correlation of Cretaceous strata in the Mississippi interior salt basin and the East Texas salt basin. Six regional Cretaceous unconformities and associated hiatuses (late Valanginian, middle Cenomanian, late Turonian to middle Coniacian, middle Campanian, late Campanian to early Maastrichtian, and late Maastrichtian) and nine regional transgressive events (early Aptian, early Albian, middle Albian, early Cenomanian, late Cenomanian to early Turonian, late Santonian, early middle Campanian, early late Campanian, and early Maastrichtian) have been identified as major events in the geohistory of these basins. Hydrocarbon production from Cretaceous reservoirs in the northeastern Gulf of Mexico can be categorized as to the phase of T-R cycles. Sandstone reservoirs associated with the early transgressive aggrading phase have accounted for 42% of the 7.437 TCF of natural gas produced from Cretaceous reservoirs in this region. Sandstone reservoirs associated with the late transgressive backstepping and regressive infilling phases have accounted for 63% of the 2 BBO produced from Cretaceous reservoirs in this region. These findings indicate that the primary natural gas exploration target in the northeastern Gulf of Mexico should be Cretaceous sandstone reservoirs of the transgressive aggrading phase of T-R cycles and that the principal oil exploration target in this region should be Cretaceous sandstone reservoirs of the transgressive backstepping and regressive infilling phases of T-R cycles. These exploration targets are associated with structural traps related to salt movement.