Transgressive-regressive cycles in Lower Cretaceous strata, Mississippi Interior Salt Basin area of the northeastern Gulf of Mexico, USA

Cretaceous Research (2002) 23, 409–438 doi:10.1006/cres.2002.1012, available online at http://www.idealibrary.com on

Transgressive-regressive cycles in Lower Cretaceous strata, Mississippi Interior Salt Basin area of the northeastern Gulf of Mexico, USA Ernest A. Mancini and T. Markham Puckett Center for Sedimentary Basin Studies and Department of Geological Sciences, University of Alabama, Tuscaloosa, Alabama 35487, USA Revised manuscript accepted 14 June 2002

Four transgressive-regressive (T-R) cycles and five T-R subcycles have been recognized in Lower Cretaceous strata of the northeastern Gulf of Mexico. These T-R cycles are the LKEGR-TR 1 (Lower Cretaceous, Eastern Gulf Region) (upper Valanginian–upper Aptian), the LKEGR-TR 2 (upper Aptian–middle Albian), the LKEGR-TR 3 (middle–upper Albian), and the LKEGR-TR 4 (upper Albian–lower Cenomanian) cycles. The LKEGR-TR 1 Cycle consists of three subcycles: LKEGR-TR 1–1 (upper Valanginian–lower Aptian), LKEGR-TR 1–2 (lower Aptian) and LKEGR-TR 1–3 (upper Aptian) subcycles. The LKEGR-TR 2–1 (upper Aptian–lower Albian) and the LKEGR-TR 2–2 (lower–middle Albian) subcycles constitute the LKEGR-TR 2 Cycle. The LKEGR-TR 3 and the LKEGR-TR 4 cycles consist of a single T-R cycle. Recognition of these T-R cycles is based upon stratal geometries, nature of cycle boundaries, facies stacking patterns within cycles, and large-scale shifts in major facies belts. The T-R subcycles are characterized by shifts in major facies belts that are not of the magnitude of a T-R cycle. The cycle boundary may be marked by a subaerial unconformity, ravinement surface, transgressive surface or surface of maximum regression. A single T-R cycle consists of an upward-deepening event (transgressive aggrading and backstepping phases) and an upward-shallowing event (regressive infilling phase). These events are separated by a surface of maximum transgression. The aggrading phase marks the change from base-level fall and erosion to base-level rise and sediment accumulation; this phase signals the initiation of the creation of shelf-accommodation space. The marine transgressive and flooding events of the backstepping phase are widespread and provide regional correlation datums. Therefore, these T-R cycles and subcycles can be identified, mapped, and correlated in the northeastern Gulf of Mexico area. The progradational events associated with the regressive infilling phase represent a major influx of siliciclastic sediments into the basin, the development of major reef build-ups at the shelf margin, and a significant loss of shelf-accommodation space. These T-R cycles are interpreted to be the result of the amount of and change in shelf-accommodation due to a combination of post-rift tectonics, loading subsidence, variations in siliciclastic sediment supply and dispersal systems, carbonate productivity and eustasy associated with a passive continental margin. The T-R cycles, where integrated with biostratigraphic data, can be correlated throughout the northern Gulf of Mexico region and have the potential for global correlation of Lower Cretaceous strata.  2002 Elsevier Science Ltd. All rights reserved. K W: Lower Cretaceous; Gulf of Mexico; transgressive-regressive cycles.

1. Introduction The Cretaceous and Paleogene strata of the Mississippi Interior Salt Basin (MISB) area, onshore northeastern Gulf of Mexico, consist of evaporite, carbonate, and nonmarine, marginal marine, and marine terrigenous siliciclastic sediments. Stratigraphic analysis of the Upper Cretaceous and Paleogene deposits of the eastern Gulf Coastal Plain were published by Mancini et al. (1996), and by Baum & Vail (1988) and Mancini & Tew (1991), respec0195–6671/02/$35.00/0

tively. From these previous studies, it is clear that the establishment of a stratigraphic framework that includes a biostratigraphic component is fundamental to the correlation of these strata for the interpretation of the geologic history of this region. Therefore, the purpose of this paper is to provide a stratigraphic framework that is integrated with biostratigraphy for the Lower Cretaceous strata of the MISB area, northeastern Gulf of Mexico. The establishment of such a stratigraphic framework facilitates the correlation of key sedimentologic trends evident in these strata in  2002 Elsevier Science Ltd. All rights reserved.

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the Gulf of Mexico and is useful for the global correlation of these events in the Lower Cretaceous section. The sequence stratigraphic relationships of the Upper Cretaceous and Paleogene deposits in the onshore area of the northeastern Gulf of Mexico are useful for establishing a stratigraphic framework for this geologic time period. In this case, thirdorder (1–10 myr in duration), unconformity-bounded depositional sequences, as defined by Mitchum et al. (1977), Van Wagoner et al. (1988), and Posamentier et al. (1988) can be recognized in surface exposures based on physical surfaces and stratal geometries, and associated systems tracts can be traced regionally in outcrop. However, in studying the Lower Cretaceous shelf deposits in the subsurface of this region that are characterized by mixed carbonate and siliciclastic deposition and in which stratal patterns have been driven by low-frequency tectonic and eustatic events associated with post-rift, passive margin conditions, a stratigraphic analysis based on the cyclicity of sedimentation patterns within cycles recorded in these strata (transgressive-regressive cycles) rather than only an analysis based upon the characterization of sequence boundaries has utility in establishing a stratigraphic framework for correlation of diagnostic events in the Lower Cretaceous strata. Transgressive-regressive (T-R) cycles are of particular interest because the documentation of these cycles in the rock record provides an approach for intrabasin correlation and interbasin regional and potential global correlation. T-R cycles or sequences have been described by Johnson et al. (1985) for Devonian strata, Steel (1993) for Triassic and Jurassic strata, Embry (1993) for Jurassic strata, and Jacquin et al. (1998) for Cretaceous strata. T-R cycles consist of a transgressive phase that includes an upward deepening event, and a regressive phase that includes an upward shallowing event (Johnson et al., 1985). The transgressive portion of T-R cycles consists of a backstepping phase, but may also include an aggrading phase; the regressive portion of these cycles consists of an infilling phase, but may also include a forestepping phase (Jacquin & de Graciansky, 1998). The events of the aggrading phase mark the change from base level fall and erosion to base level rise and sediment accumulation. This aggrading phase signals the initiation of the creation of shelf-accommodation space. T-R cycles and the associated phases have been driven by the amount of and change in shelfaccommodation space resulting from base-level changes (eustasy and tectonic and loading subsidence) and by sediment accumulation (amount and rate of sediment supply to and/or on the shelf).

Recognition of T-R cycles is not independent of cycle boundary identification. Embry (2002) employed a subaerial unconformity or shoreface ravinement unconformable surface to identify the unconformable portion of the boundary of a T-R sequence and a maximum regressive surface to recognize the conformable portion of a sequence boundary. He used a maximum flooding surface to divide a T-R sequence into transgressive and regressive systems tracts. In this paper, T-R cycles are defined by a combination of factors including stratal geometries, nature of cycle boundaries, facies stacking patterns within cycles, and large-scale shifts in major facies belts. The cycles are separated by discontinuities in the sedimentary record as recognized by abrupt changes or interruptions as observed in seismic reflection profiles, in well log signature patterns, and/or in cored sections obtained from the drilling of wells. A discontinuity may consist of an unconformable surface, such as a subaerial unconformity, ravinement surface, or transgressive surface at the cycle boundary. In the marine system, the boundary of a T-R cycle may be recognized by a conformable surface of maximum regression, which separates an upward shallowing interval from an upward deepening interval. The aggrading section of the transgressive phase rests on a subaerial unconformity, and the top of this section is delimited by an erosional ravinement surface or transgressive surface. The top of the overlying backstepping section of the transgressive phase is marked by a surface of maximum transgression. The surface of maximum transgression separates the upward deepening interval of the T-R cycle (transgressive phase) from the upward shallowing interval of the cycle (regressive phase). The regressive phase in shelf strata consists of an infilling section. The forestepping section of the regressive phase occurs in continental slope deposits. The top of the regressive phase is delimited by a surface of maximum regression. A T-R cycle is composed of one or more cycles, each of which consists of an upward-deepening event and an upward-shallowing event. If the T-R cycle consists of more than one cycle, the couplet of upward-deepening and upward-shallowing is referred to as a subcycle. A surface of marine flooding separates the transgressive and regressive sections of a subcycle. Generally, these subcycles occur in the transgressive backstepping phase of a T-R cycle. The marine transgressive and flooding events of the backstepping phase of a T-R cycle are widespread and provide regional correlation datums. However, during this transgressive event, a minor decrease in shelfaccommodation may produce an upward-shallowing event that results in a regressive-infilling section.

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Figure 1. Basins and uplifts in the onshore northern Gulf of Mexico area as modified from Pilger (1981), and structural features of the Mississippi Interior Salt Basin area as modified from Mink et al. (1990).

Although this event defines a shift in facies belts, it is not of the magnitude of the progradational event of the regressive-infilling phase of a T-R cycle that represents a major influx of siliciclastic sediments into the basin. The subcycle boundary generally consists of an erosional transgressive surface landward and a conformable surface of maximum regression seaward.

2. Geologic setting The MISB (Figure 1) is one of a series of salt basins formed by the initial rift configuration of alternating crustal highs and lows resulting from translateral movement along the original Pangean rift margin that was initiated during the Late Triassic and Early Jurassic (Winkler & Buffler, 1988; MacRae, 1994). The northern and eastern margins of the basin are defined by the limit of Jurassic salt and the Mobile Graben (Figure 1). The distribution of salt structures indicates that original salt thickness was moderate along the northern and northeastern regions of the basin, whereas salt thickness was greater in the central and southwestern portions of the basin. The Jackson Dome, which is a buried Late Cretaceous volcano and associated atoll structure (Dockery, 1998), pro-

foundly modified the configuration of the MISB in west-central Mississippi and also altered the distribution of strata in this area. The southern margin of the basin is defined by the northern margin of the Wiggins Arch (Figure 1). The Wiggins Arch is a mass of relict continental crust left behind during the rifting between North America and South America. It is an elongate structure oriented in a general west–east direction that bifurcates to the west, with the southwestern flank forming the Hancock Ridge (Grazier, 1988). The southern margin of the arch is steeply sloping and the northern margin is gently sloping. The arch exerted strong influence on the sedimentation patterns in the basin during the Mesozoic. The southwestern and western margins of the basin are not as clearly defined as the other margins, but are recognized on the basis of the distribution of salt-related structures. The depositional history of the strata of the MISB is directly linked to the tectonic history of the basin, which is closely related to the origin of the Gulf of Mexico (Wood & Walper, 1974). The Gulf of Mexico is a divergent margin basin characterized by extensional rift tectonics and wrench faulting (Pilger, 1981; Miller, 1982; Salvador, 1987; Winkler & Buffler,

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Figure 2. Absolute ages, chronostratigraphic units, and lithostratigraphic units of Lower Cretaceous strata in Texas, Louisiana, Arkansas and the Mississippi Interior Salt Basin area.

1988). Its history includes a phase of crustal extension and thinning, a phase of rifting and sea-floor spreading and a phase of thermal subsidence (Nunn, 1984). Based on the distribution of crust type, Sawyer et al. (1991) proposed the following as a model for the evolution of the Gulf of Mexico region and related MISB. The Late Triassic–Early Jurassic early rifting phase was characterized by large and small halfgrabens bounded by listric normal faults and filled with nonmarine siliciclastic sediments (red-beds) and volcanics. The Middle Jurassic phase of rifting, crustal attenuation and formation of transitional crust was characterized by the evolution of a pattern of alternating basement highs and lows and the accumulation of thick salt deposits. The Late Jurassic phase of seafloor spreading and oceanic crust formation in the deep central Gulf of Mexico was characterized by a

regional marine transgression as a result of crustal cooling and subsidence. Subsidence continued into the Early Cretaceous, and a carbonate shelf margin developed along the tectonic hinge zone of differential subsidence between thick and thin transitional crust. During the Early to mid-Cretaceous, regional erosional events are recognized to have occurred in the Valanginian and Cenomanian (Figure 2) reflecting times of sea-level fall in the northern Gulf of Mexico. The Early Cretaceous depositional system in the northeastern Gulf of Mexico was dominated by fluvial-deltaic to coastal siliciclastic sedimentation updip and the development of a broad carbonate shelf with a low relief shelf margin downdip that approximates the boundary between thick transitional crust and thin transitional crust (Winkler & Buffler, 1988; Sawyer et al., 1991). The development of a carbonate

Transgressive-regressive cycles in Lower Cretaceous strata

shelf margin during the Early Cretaceous is believed to have resulted from a combination of a change in the slope of the basement that is marked by a crustal hinge zone and Jurassic sediment depositional patterns (Dobson, 1990; Sawyer et al., 1991). Although Jurassic depositional patterns were greatly affected by basement topography, sediments deposited at the close of the Jurassic reflect an infilling of the basement low areas and a general progradation (Dobson, 1990; Dobson & Buffler, 1997). This progradation was interpreted by Dobson (1990) to have resulted in the change from a carbonate ramp to a rimmed carbonate platform margin. This platform margin was a characteristic feature of the northern Gulf of Mexico region throughout the Early Cretaceous. The Lower Cretaceous shelf margin was exposed during the early Late Cretaceous (mid-Cenomanian) by a major lowering of sea level in the Gulf of Mexico. This sea-level fall has been attributed to a combination of regional igneous activity (Jackson Dome) and global sea-level fall during the mid-Cenomanian (Salvador, 1991). A Late Cretaceous marine transgression followed this regional erosional event, and this, in combination with an increase in siliciclastic sediment influx resulting from the Laramide orogeny, affected deposition in the Late Cretaceous and into the Cenozoic (Salvador, 1991). 3. Stratigraphy and paleontology Detailed biostratigraphic studies of Lower Cretaceous strata of the MISB are not available. There are, however, regional lithostratigraphic correlations of Lower Cretaceous strata of the surface and subsurface of Texas, Arkansas, Louisiana, and Mississippi (Imlay, 1940; Forgotson, 1957; Stricklin et al., 1971; Anderson, 1979; Pittman, 1984, 1985, 1989; Yurewicz et al., 1993) that enable correlations with the subsurface units in the MISB (Figure 2). The detailed biostratigraphic information available from surface exposures of these strata and from cores from wells (Adkins, 1932; Hazzard, 1939; Imlay, 1945; Douglass, 1960; Young, 1967, 1986) can then be used to estimate the geologic ages of the Lower Cretaceous units in the MISB. 3.1. Hosston Formation In the subsurface of the MISB, the Hosston Formation (Figure 2) consists of sandstone, shale, occasional limestone nodular beds, and thin lignite beds (Nunnally & Fowler, 1954; Dinkins, 1969, 1971; Devery, 1982). The boundary between the Cotton Valley Group and Hosston Formation is

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unconformable (McFarlan, 1977). Throughout much of the basin area, the formation is a fluvial-deltaic, conglomeratic, medium- to coarse-grained sandstone (Dinkins, 1971). However, in the mid- to downdip areas of the basin the formation consists of coastal and shallow marine interbedded sandstone and calcareous shale (Applin & Applin, 1953), and in areas south of the basin proper (south of the Wiggins Arch) it includes interbedded limestone and shale (Warner, 1993). The Hosston Formation is generally devoid of agediagnostic fossils; thus determining the age of the unit is problematical. Petty et al. (1995) published microfossil occurrences for three wells in the offshore continental shelf region of the northeastern Gulf of Mexico. A sample from near the base of the Hosston Formation in a well in Viosca Knoll Block 117 yielded the dinoflagellate species Muderongia simplex, which Lentin & Williams (1989) reported as ranging from the Valanginian to lower Barremian stages. A sample approximately one-quarter of the way up from the base of the Hosston Formation in a well in Mobile Block 991 contained the dinoflagellate species Druggindium ‘A’ of unpublished taxonomic affinity, and Phoberocysta neocomica s. l. (several subspecies were described as belonging to this species) of Hauterivian age. Calcareous nannofossil specimens of the species Nannoconus steinmanni, which is a typical Tithonian to latest Barremian microfossil, were observed approximately one-third of the way up from the base of the Hosston Formation in Chevron MS 87 well in Mississippi Sound Block 57 (well 4 on Figure 3). McFarlan (1977) correlated the upper part of the Cotton Valley Group with the top of the Berriasian Stage and the lower part of the Hosston Formation to the uppermost part of the Valanginian Stage; thus, almost all of the Valanginian Stage is missing. These observations indicate a late Valanginian–early Aptian age for the Hosston Formation of the MISB area. 3.2. Sligo Formation The Sligo Formation is a subsurface unit throughout the Gulf of Mexico, and only occurs along the southern margin of the MISB and south of the basin proper. It consists predominantly of marine shelf, calcareous, fossiliferous shale associated with thin shallow marine, fine-grained sandstone beds in the area of the MISB (Devery, 1982). South of the MISB and in Louisiana and Texas, the formation is primarily marine shelf and reef limestone (Anderson, 1979; Warner, 1993). The Hosston Formation-Sligo Formation boundary is characterized by a change in lithology and in well log signature.

Figure 3. Cross section of Lower Cretaceous strata from the updip area of the Mississippi Interior Salt Basin to the Lower Cretaceous shelf margin in the offshore area of the northeastern Gulf of Mexico. T-R, transgressive-regressive; TA, transgressive aggrading phase; TB, transgressive backstepping phase; RI, regressive infilling phase.

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Transgressive-regressive cycles in Lower Cretaceous strata

Biostratigraphic data (foraminifera) and stratigraphic observations indicate an early Aptian age for the Sligo Formation in the northeastern Gulf of Mexico area (Petty et al., 1995). Specifically, the benthic foraminifera Choffatella decipiens occurs at the top of the Sligo Formation (Petty et al., 1995), and the highest occurrence of this microfossil is in lower Aptian strata (van Hinte, 1976). 3.3. Pine Island Shale The Pine Island Shale, James Limestone, and Bexar Formation of the MISB area are stratigraphically equivalent to the Pearsall Formation of Texas, Arkansas, and Louisiana (Forgotson, 1957). The Pine Island Shale occurs in much of the onshore area of the northern Gulf of Mexico area (Imlay, 1945; Forgotson, 1957; Anderson, 1979). In the MISB, the Pine Island Shale consists primarily of marine shelf, dark gray to black, calcareous, fossiliferous shale (Dinkins, 1969). These shale beds become micaceous and silty updip and are interbedded with fine- to medium-grained sandstone, whereas downdip the shale beds are interbedded with fossiliferous limestone (Dinkins, 1969). The Sligo Formation-Pine Island Shale boundary is characterized as a discontinuity in lithology and well log signature. Biostratigraphic and stratigraphic data indicate that the Pine Island Shale is equivalent to the Hammett Shale Member of Texas. Young (1986) assigned the Hammett Shale Member to the Dufrenoyia rebeccae (ammonite) Zone of late Aptian age, and Hazzard (1939), Imlay (1940), and Forgotson (1957) made a similar age assignment for the Pine Island Shale in the subsurface of Arkansas and Louisiana. Hazzard (1939) reported the occurrence of the ammonite genera, Dufrenoyia, Hypacanthoplites, and Pseudosaynella in the Pine Island Shale of Arkansas. These observations indicate a late Aptian age for the Pine Island Shale of the MISB area. 3.4. James Limestone The James Limestone occurs in the southern portion of the MISB and south of the basin proper. Lithofacies include low-energy marine shelf limestone and shallow water high-energy grainstone and reefal boundstone (Loucks et al., 1996). The Pine Island Shale-James Limestone boundary is gradational as observed from well log signatures and lithologic descriptions. Regional lithostratigraphic correlation indicates that the James Limestone is equivalent to the Cow Creek Limestone Member of Texas (Imlay, 1945; Forgotson, 1957). Biostratigraphic data (ammonites,

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foraminifera) indicate a late Aptian age for the Cow Creek Limestone Member (Adkins, 1932; Imlay, 1945; Forgotson, 1957); specifically, the member occurs in the Aptian Dufrenoyia justinae (ammonite) Zone (Young, 1986). Also, species of Orbitolina have not been recovered from the member or the James Limestone (Imlay, 1945). These paleontologic data and stratigraphic observations indicate a late Aptian age for the James Limestone of the MISB area. 3.5. ‘Donovan’ Sandstone The term ‘Donovan’ sandstone rather than Rodessa Formation is used informally in this study to designate the predominantly sandstone interval in the MISB that occurs between the top of the Pine Island Shale and the base of the shaly interval below the Ferry Lake Anhydrite throughout most of the basin. The shaly interval is assigned to the Bexar Formation–Rodessa Formation interval. The term ‘Donovan’ has been used for many years to refer to the hydrocarbonproductive, fluvial, sandy interval between the top of the Hosston Formation and the base of the Ferry Lake Anhydrite in the Citronelle Field of Mobile County, Alabama (Eaves, 1976). The ‘Donovan’ interval has traditionally been referred to as the Rodessa Formation in Mississippi and Alabama. In this study, it was considered useful to differentiate the typical Rodessa lithology (predominantly carbonate) that occurs south of the MISB proper from the atypical siliciclastic interval that occurs within the MISB. The ‘Donovan’ sandstone is also interpreted to occur within a cycle separate and distinct from the Rodessa carbonate beds. Well log correlations indicate that the ‘Donovan’ interval grades from predominantly sandstone in the updip portion of the MISB to a siltstone and shale interval in the downdip portion of the basin. The interval is interpreted to be equivalent to the James Limestone based on lithostratigraphic correlation, and thus is assigned to the upper part of the Aptian Stage. The ‘Donovan’ sandstone beds represent the first significant siliciclastic progradational unit above the Hosston Formation. 3.6. Bexar Formation The Bexar Formation is defined as the interval between the base of the Rodessa Formation or the Glen Rose Limestone undifferentiated, and the top of the Cow Creek Limestone Member or James Limestone (Forgotson, 1957). The type well of the Bexar Formation is in Bexar County, Texas, where the unit consists of marine shelf, black, calcareous

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shale and thin, dense, finely crystalline limestone beds (Forgotson, 1957). The Bexar Formation is well defined along the southern margin of the MISB and south of the basin proper, where it consists of a thin interval of marine shelf lime mudstone and shale between the top of the James Limestone and the carbonate rocks of the Rodessa Formation (Loucks et al., 1996). It is more extensive in Louisiana (Anderson, 1979) and Texas (Forgotson, 1957) than in the MISB area. The James Limestone-Bexar Formation boundary is disconformable as observed from core studies (Loucks et al., 1996). The Bexar Formation is of late Aptian age (Forgotson, 1957) and was assigned to the late Aptian Kasanskyella spathi (ammonite) Zone by Young (1986) in Texas. 3.7. Rodessa Formation The Rodessa Formation traditionally has been defined as the stratigraphic interval between the top of the Pearsall Formation and the base of the Ferry Lake Anhydrite (Forgotson, 1957). In this paper, we assign most of this interval to the ‘Donovan’ sandstone. The Rodessa Formation as recognized herein is the marine shelf and reef carbonate interval between the top of the Bexar Formation and the base of the Ferry Lake Anhydrite. The shale interval between the top of the ‘Donovan’ sandstone and the base of the Ferry Lake Anhydrite is assigned to the Bexar Formation– Rodessa Formation interval (Figure 3). The Rodessa carbonate beds occur along the southern margin of the MISB and south of the basin proper, and the Bexar shale–Rodessa shale beds occur in the central and northern parts of the MISB. The transition from siliciclastic to carbonate sediments occurs in the southern portion of the MISB (Figure 3). In the southern portion of the MISB, the Bexar Formation separates the carbonate beds of the James Limestone from the carbonate beds of the lower part of the Rodessa Formation. Several anhydrite beds in the upper part of the Rodessa Formation are regionally extensive, occurring from east Texas across Louisiana to Mississippi, and in offshore areas correlate stratigraphically with the lower portion of the Glen Rose reef limestone (Pittman, 1985). Growth of the Glen Rose reef kept pace with sea-level rise and resulted in restricted circulation conditions in broad lagoonal areas behind the reef. These dynamics are widely acknowledged to have formed the subaqueous anhydrite beds of the Rodessa Formation–Ferry Lake Anhydrite–Mooringsport Formation interval (Sarg, 2001). The contact of the Rodessa Formation with the Bexar Formation is gradational as

observed from well log signatures and lithologic descriptions. Regional stratigraphic correlation indicates that the Rodessa Formation is equivalent to the ‘lower’ Glen Rose Limestone of Texas (Imlay, 1940; Forgotson, 1957). The ‘lower’ Glen Rose Limestone (post-Bexar Formation to Corbula bed, sensu Stricklin et al., 1971) is richly fossiliferous in outcrop. Douglass (1960) assigned the lower portion of the Glen Rose Limestone to the benthic foraminiferal Orbitolina texana Zone of earliest Albian age, and Young (1986) placed the lower portion of the Glen Rose Limestone in the Hypacanthoplites cragini and the Douvilleiceras mammillatum ammonite zones of earliest Albian age. Imlay (1940) reported an early Albian age for the Rodessa Formation of Arkansas and Louisiana. These paleontologic data and stratigraphic observations indicate an earliest Albian age for the Rodessa carbonate beds in the MISB area. 3.8. Ferry Lake Anhydrite The Ferry Lake Anhydrite is one of the most distinctive lithostratigraphic units in the Gulf Coastal Plain and is used widely for regional correlation. Although it is a subsurface term, equivalent strata crop out in Arkansas in the lower part of the De Queen Formation (Pittman, 1984) and in the middle of the ‘lower’ Glen Rose Limestone of Texas (Pittman, 1989). The Ferry Lake Anhydrite was deposited as subaqueous lagoonal gypsum that was subsequently neomorphosed to anhydrite (Lock et al., 1983; Loucks & Longman, 1985). The build-up of the Lower Cretaceous (Glen Rose) shelf margin reef restricted circulation of marine waters to its greatest extent during deposition of the Ferry Lake Anhydrite. In the MISB, the Ferry Lake Anhydrite consists primarily of massive anhydrite, shale, and limestone (Nunnally & Fowler, 1954). The Rodessa Formation-Ferry Lake Anhydrite boundary is sharp and is recognized at the base of the first massive anhydrite as observed from well log signatures and lithologic descriptions. Scott (1939) assigned the Glen Rose Limestone (with anhydrite in the basal part) to the Albian Stage, based on the occurrence of the ammonite Knemiceras roemeri. Imlay (1940) assigned the Ferry Lake Anhydrite (Glen Rose anhydrite of Hazzard, 1939) to the middle of the lower part of the Albian Stage, based on stratigraphic relations with the Glen Rose Limestone. Pittman (1989), studying the regional stratigraphy of the Glen Rose beds, determined that Ferry Lake Anhydrite equivalent strata occurred in the ‘lower’ Glen Rose Limestone, stratigraphically below the regional marker Corbula bed, and Douglas (1960)

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and Pittman (1989) reported that these strata are in the Orbitolina texana Zone (below the lowest occurrence of O. minuta), indicating an early, but not earliest, Albian age. Thus, an early Albian age is concluded for the Ferry Lake Anhydrite of the MISB area based on paleontologic data and stratigraphic observations. 3.9. Mooringsport Formation The Mooringsport Formation in the MISB is defined as the interval between the top of the massive anhydrite beds of the Ferry Lake Anhydrite and the coarse siliciclastic sediments of the Paluxy Formation. This interval is equivalent to the middle portion of the Glen Rose Limestone of Texas (Imlay, 1940). Strata equivalent to the Mooringsport Formation crop out in Arkansas within the upper portion of the De Queen Formation. In this area, the De Queen Formation consists of interbedded siliciclastic sediments, limestone, and evaporite (gypsum and halite) (Lock et al., 1983; Pittman, 1984). Dinosaur footprints also occur in the De Queen Formation (Pittman, 1984). Significantly, the marine invertebrate fauna associated with this unit attains its highest diversity in the upper portion of the De Queen Formation (Mooringsport Formation). In Louisiana, the shallow marine carbonate interval between the top of the Ferry Lake Anhydrite and the base of the upper part of the Glen Rose beds is the Mooringsport Formation, which is a carbonate unit in downdip areas and a siliciclastic unit in updip areas in the MISB (Yurewicz et al., 1993). South of the MISB proper, the Mooringsport Formation includes carbonate and calcareous shale beds (Davis & Lambert, 1963). In much of the MISB area, the Mooringsport Formation consists of marine shelf, dark gray shale and shallow marine, fine-grained sandstone. The shallow marine sandstone beds become more prominent updip, and marine shelf and reef limestone beds occur downdip (Dinkins, 1966; Baria, 1981). Generally, the calcareous interval is higher stratigraphically in the Mooringsport section in areas south of the MISB than in the basin proper. The Ferry Lake AnhydriteMooringsport Formation boundary is sharp but conformable (Dinkins, 1969). Updip of the limit of deposition of the Ferry Lake Anhydrite, the Mooringsport Formation is recognized as a predominantly shaly interval between the sandstone beds of the ‘Donovan’ interval and the sandstone beds of the Paluxy Formation. This recognition of the unit is significant because it results in the assignment of all shale beds between the ‘Donovan’ interval and Paluxy sandstone beds to the Mooringsport Formation,

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including shale beds equivalent to those that occur below the Ferry Lake Anhydrite, where the Ferry Lake Anhydrite is present. These shale beds are assigned to the Bexar Formation–Rodessa Formation interval in the central and northern parts of the MISB. Regional stratigraphic correlation indicates that the Mooringsport Formation is equivalent to the middle portion of the Glen Rose Limestone of central Texas (Imlay, 1940). In Texas, the Corbula bed in the middle of the Glen Rose Limestone, and Glen Rose strata stratigraphically above this bed, contain ammonites of the Douvilleiceras mammillatum Zone (Young, 1986) of middle early Albian age. Also, these strata have been assigned to the lower part of the benthic foraminiferal Orbitolina minuta Zone, indicating an early Albian age (Douglass, 1960; Pittman, 1989). Imlay (1940) reported an early Albian age for the Mooringsport Formation of Arkansas and Louisiana. These biostratigraphic data and stratigraphic observations indicate that the Mooringsport Formation of the MISB area is of early Albian age. 3.10. Paluxy Formation The Paluxy Formation in the MISB is the stratigraphic interval between the top of the Mooringsport Formation and either the base of the carbonate rocks of the Andrew Formation (in the southern portion of the basin) or the thickly bedded sandstone and shale interval of the Dantzler Formation (in updip areas). In Arkansas, the Paluxy Formation is the siliciclastic unit that overlies the Mooringsport Formation (Imlay, 1940). In Texas, the Paluxy Formation, which is overlain by the Walnut Formation, is considered to be the basal unit of the Fredericksburg Group (Hayward & Brown, 1967; Young, 1967). In Louisiana, the Paluxy Formation is the unit that overlies the Glen Rose beds and is overlain by the Fredericksburg Group (Yurewicz et al., 1993). In the MISB, the Paluxy Formation consists chiefly of fluvial, coastal, and shallow marine fine- to coarse-grained micaceous sandstone beds interbedded with carbonaceous shale (Nunnally & Fowler, 1954). The lower and upper portions of the formation often include thicker shale units than the middle portion of the unit, which is predominantly sandstone. The Mooringsport Formation-Paluxy Formation boundary is gradational as observed from well log signatures and lithologic descriptions. Sandstone beds assigned to the Paluxy Formation in updip areas grade into shale beds of the Mooringsport Formation downdip. Regional stratigraphic correlation indicates that the lower portion of the Paluxy Formation is equivalent to the upper portion of the ‘upper’ Glen Rose Limestone

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of southwestern Texas (Imlay, 1940). The upper portion of the ‘upper’ Glen Rose Limestone of Texas occurs within the Hypacanthoplites comalensis (ammonite) Zone of latest early Albian age and the Metengonoceras sp. (ammonite) Zone of early middle Albian age (Young, 1986). This interval also corresponds to the middle and upper parts of the Orbitolina minuta (foraminiferal) Zone (Pittman, 1989), the upper portion of which is within the middle part of the Albian Stage (Douglass, 1960). The upper portion of the Paluxy Formation in north Texas was assigned to the middle Albian Oxytropidoceras salasi (ammonite) Zone by Young (1967, 1986). The upper portion of the Paluxy Formation in the MISB has been correlated with the lower portion of the Walnut Formation of Texas (Forgotson, 1957). The lower portion of the Walnut Formation lies within the Metengonoceras hilli (ammonite) Zone of middle Albian age (Young, 1986). Imlay (1940) reported a late early Albian age for the Paluxy Formation of Arkansas and Louisiana. These biostratigraphic data and stratigraphic observations indicate that the Paluxy Formation in the MISB area ranges in age from late early to middle Albian. 3.11. Andrew Formation The Andrew Formation is the stratigraphic interval between the sandstone beds of the Paluxy Formation and the thickly bedded sandstone and shale beds of the overlying Dantzler Formation (Eargle, 1964). The stratigraphic relationships of the Andrew Formation with the various formations of the Fredericksburg Group of Texas are not well understood. It is possible that it is equivalent to at least the upper portion of the Walnut Formation, and may be equivalent to the entire formation. This interpretation is based on similar stratigraphic relationships between the nonmarine sandstone beds underlying both the Walnut and Andrew formations and the transgressive nature of these two formations. It seems clear, however, that the Andrew Formation is equivalent, in part, to the Fredericksburg Group in Louisiana. The Andrew Formation is usually assigned to the Washita-Fredericksburg Groups undifferentiated in the MISB. This undifferentiated group assignment also includes the overlying Dantzler Formation. The Andrew Formation consists of marine shelf, fossiliferous limestone and shale (Eargle, 1964). The limestone beds of the Andrew Formation are prominent downdip and along the southern margin of the MISB. The Paluxy Formation-Andrew Formation boundary is characterized by a discontinuity in lithology and in well log signature.

The Fredericksburg Group in Texas (which includes the Walnut Formation and the Edwards Limestone) was assigned by Young (1967, 1986) to the Metengonoceras hilli, Oxytropidoceras salasi, Manuaniceras carbonarium, and Manuaniceras powelli ammonite zones of middle Albian–early late Albian age. Regional lithostratigraphic correlation indicates that the Andrew Formation is equivalent, in part, to the Fredericksburg Group of Texas. These biostratigraphic data and stratigraphic observations indicate that it is of middle–early late Albian in age. 3.12. Dantzler Formation The Dantzler Formation is the stratigraphic interval between either the top of the carbonate rocks of the Andrew Formation or the sandstone beds of the Paluxy Formation and the base of the Upper Cretaceous Tuscaloosa Group. In the MISB, it is usually referred to as the Washita-Fredericksburg Groups undifferentiated. It includes fluvial, thickbedded, lignitic, fine- to medium-grained sandstone and carbonaceous shale (Nunnally & Fowler, 1954) that were interpreted by Chasteen (1983) to represent a stacked series of braided stream deposits. Coastal and marine shelf, calcareous sandstone that contains limestone nodules is present in the lower Dantzler Formation (Nunnally & Fowler, 1954). The Andrew Formation-Dantzler Formation boundary is transitional as observed from well log signatures and lithologic descriptions. In the southern portion of the MISB, these transitional beds have been assigned to the Fredericksburg Group. The boundary between the Dantzler Formation and the Tuscaloosa Group is unconformable (Dinkins, 1971). The Andrew Formation–Dantzler Formation interval in the MISB differs from the corresponding interval in Louisiana. For example, although the Aptian and early Albian units (Sligo Formation, Pine Island Shale, James Limestone, Bexar Formation, Rodessa Formation, and Mooringsport Formation) are of comparable thicknesses in southern Mississippi and in central Louisiana, as observed in the Southern Minerals No. 1 well (well 3 on Figure 3) in the Sandy Hook field of Pearl River County, Mississippi, and in the Placid Louisiana Central 227 well in La Salle Parish, Louisiana, the interval from the top of the Mooringsport Formation to the top of the Dantzler Formation in Mississippi is approximately 396 m (1300 ft) thicker than the thickness of this interval in Louisiana. In some areas of the MISB, the Dantzler Formation is as much as 518 m (1700 ft) thicker than this interval in central Louisiana. The location of both of the wells discussed above was on the ancient

Transgressive-regressive cycles in Lower Cretaceous strata

continental shelf area during the Early Cretaceous. Similarly, the Fredericksburg and Washita groups in north Texas are less than 152 m (500 ft) thick (Kessinger, 1982) compared to a total thickness of approximately 762 m (2500 ft) for these two groups in southern Mississippi. Clearly, the MISB area experienced much greater accommodation and siliciclastic sediment supply during the late Albian and early Cenomanian than either Texas or Louisiana. The biostratigraphy for the Washita formations in Texas is well known, although various authors have defined the group differently. Some have included the Kiamichi Formation in the Fredericksburg Group (Kessinger, 1982), whereas others have placed the Kiamichi Formation in the Washita Group (Young, 1967). Young (1967, 1986) recognized numerous ammonite zones in the Washita Group (including the Kiamichi Formation), which range from early (but not earliest) late Albian to early Cenomanian in age. Regional lithostratigraphic correlation indicates that the Dantzler Formation is equivalent, in part, to the Washita Group of Texas and, in part, to the Fredericksburg Group of Louisiana and the offshore northeastern Gulf of Mexico. These biostratigraphic data and stratigraphic observations indicate that the Dantzler Formation in the MISB is of late Albian age. 4. Transgressive-regressive cycles Lower Cretaceous transgressive-regressive (T-R) cycles and subcycles are recognized based on an integrated study of well log signatures, core analyses, lithologic well log descriptions, and regional seismic reflection profiles. Well log signatures provide the primary means for the recognition and correlation of the T-R cycles in the MISB area. Facies stacking patterns within cycles, as recognized from well log signatures, are used to characterize the T-R cycles and subcycles. Abrupt changes or discontinuities in well log records are utilized to delimit cycle boundaries and phase changes within a cycle. Where available, core and lithologic well log descriptions are used to identify the lithologies and facies of the transgressive and regressive phases. Interruptions in sedimentation or discontinuities are recognized from core and lithologic well log data, and unconformities are observed from the core data or inferred from changes in well log patterns. Reflection configuration and termination are recognized using seismic reflection profiles. The characteristics of the seismic sections are used to support the interpretations regarding the T-R cycles based on well log signatures, core analyses, and lithologic well log descriptions. Biostratigraphic criteria are

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the principal means for assigning the cycles to a Cretaceous stage. The geochronology of Cretaceous stages of Gradstein et al. (1995) is used to age-date the cycles. The T-R cycles are recognized on the basis of large-scale landward or seaward shifts of major lithofacies belts. The tops of these T-R cycles are identified by the presence of major progradational sandstone beds (‘Donovan’, Paluxy and Dantzler units), and the bases of these cycles are recognized by the occurrence of marine shale and argillaceous limestone (Sligo/Pine Island, Bexar/Mooringsport, and Andrew units). The tops of the T-R subcycles are recognized by the presence of the development of reefs. Figure 3 presents a cross section from the updip area of the MISB to the Lower Cretaceous shelf margin in the offshore area of the northeastern Gulf of Mexico showing lithostratigraphic units, lithofacies changes and T-R cycles. In general, the following well log responses were used to recognize the T-R cycles, subcycles and associated transgressive/regressive phases. A change from higher to lower gamma ray and/or from more to less positive SP log responses identifies the discontinuity in the log records used to recognize the surface of maximum transgression, which separates the transgressive from the regressive phase of a T-R cycle. Higher gamma ray and/or positive spontaneous potential (SP) log signatures are interpreted as representing shale, clay, and argillaceous limestone beds. Lower gamma ray and/or negative SP responses are interpreted as sandstone beds. High resistivity and/or high density log responses are interpreted as carbonate rocks. Very high resistivity and/or very high density log signatures are interpreted as anhydrite beds. With respect to seismic reflection data, the following generalizations were used to recognize the T-R cycles. Thin (one or two seismic cycles), concordant, parallel seismic reflection configurations are interpreted as marine strata of the transgressive backstepping phase. These reflectors are characterized by onlap reflection termination. Thick (several seismic cycles), oblique, progradational seismic reflection configurations are interpreted as prograding clinoforms of the regressive infilling phase. These reflectors are characterized by offlap (downlap) reflection terminations. Four transgressive-regressive (T-R) cycles have been recognized in Lower Cretaceous strata of the MISB area (Figure 4). These are the LKEGR-TR 1 (Lower Cretaceous, Eastern Gulf), the LKEGR-TR 2, the LKEGR-TR 3, and the LKEGR-TR 4 cycles. Three T-R subcycles are recognized in the LKEGR-TR 1 Cycle: the LKEGR-TR 1–1, the

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Mid.

Ferry Lake Anhydrite

‘

‘

et al

Figure 4. Absolute ages, chronostratigraphic units, lithostratigraphic units, and transgressive-regressive cycles and subcycles for the northeastern Gulf of Mexico, and transgressive and regressive facies cycles for Western Europe.

LKEGR-TR 1–2, and the LKEGR-TR 1–3 subcycles. The LKEGR-TR 1–1 Subcycle represents a transgressive aggrading phase associated with the LKEGR-TR 1 Cycle. Two T-R subcycles are recognized in the LKEGR-TR 2 Cycle: the LKEGR-TR 2–1 and the LKEGR-TR 2–2 subcycles. The LKEGR-TR 3 and the LKEGR-TR 4 cycles are each composed of a single T-R cycle. The LKEGR-TR 4 Cycle is not well developed in the MISB; however, this cycle occurs in the offshore area of the northeastern Gulf of Mexico and in the onshore area of the northwestern Gulf of Mexico (Texas). The duration of the LKEGR-TR 4 Cycle in the northeastern Gulf of Mexico and in the MISB area is in question because the cycle is truncated by the mid-Cretaceous (mid-Cenomanian) unconformity. 4.1. LKEGR-TR 1 Cycle The LKEGR-TR 1 Cycle (upper Valanginian–upper Aptian) includes the Hosston Formation, Sligo

Formation, Pine Island Shale, James Limestone, and ‘Donovan’ sandstone. The Hosston Formation unconformably overlies the Cotton Valley Group in the northern Gulf of Mexico area (McFarlan, 1977), and a significant hiatus of 4 myr is associated with this subaerial unconformity. Based on core analysis and well log lithologic descriptions, the Hosston Formation includes interbedded sandstone and calcareous shale in downdip areas of the MISB, and consists of conglomeratic, coarse-grained and mediumgrained sandstone in the remainder of the MISB area (Figure 5A). The LKEGR-TR 1–1 Subcycle (upper Valanginian–lowermost Aptian) represents the early stage or aggrading portion of the transgressive phase of the LKEGR-TR 1 Cycle. Jacquin & de Graciansky (1998) reported that aggrading sequences can develop in the early stage of the transgressive phase of a T-R cycle. A siliciclastic aggrading sequence generally consists of widespread and thick deposits that are a result

Transgressive-regressive cycles in Lower Cretaceous strata

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Figure 5. Core photographs of Lower Cretaceous facies. A, Hosston medium-grained sandstone of the transgressive aggrading phase of the LKEGR-TR 1 Cycle, Reese No. 1–A well, Jefferson Davis County, Mississippi, 4836 m (15,862 ft). B, Sligo fossiliferous wackestone of the transgressive backstepping phase of the LKEGR-TR 1–2 Subcycle, Chandeleur Sound Block 61, St. Bernard Parish, Louisiana, 6092 m (19,981 ft). C, Sligo rudist boundstone of the regressive infilling phase of the LKEGR-TR 1–2 Subcycle, 1654–6 well, Main Pass 253, 4759 m (15,610 ft). D, Pine Island calcareous shale of the transgressive backstepping phase of the LKEGR-TR 1–3 Subcycle, Waveland Gas Unit No. 1 well, Hancock County, Mississippi, 4780 m (15,679 ft). Diameter of coin is 18 mm.

of a high terrigenous sediment supply that keeps pace with the creation of shelf-accommodation (Jacquin & de Graciansky, 1998). Based on core analysis, well log lithologic descriptions and well log signatures, Hosston calcareous shale beds probably represent the transgressive section (shallow marine deposits) of this T-R subcycle, and the Hosston coarse- to mediumgrained sandstone is interpreted as the regressive section (fluvial-deltaic deposits) of this subcycle. Palynomorph, dinoflagellate, and calcareous nannofossil data support this interpretation. The late stage or backstepping portion of the transgressive phase of the LKEGR-TR 1 Cycle is

composed of the Sligo Formation and the lower beds of the Pine Island Shale, and the regressive infilling phase of this T-R cycle includes the upper silty beds of the Pine Island Shale, the James Limestone, and the ‘Donovan’ sandstone. The contact of the Sligo Formation (backstepping phase) with the underlying Hosston Formation (aggrading phase) is recognized by a change in well log pattern and lithology. Throughout much of the Gulf of Mexico, the Sligo Formation consists of fossiliferous limestone beds as discerned from core studies and well log lithologic descriptions. The lithologic contrast or discontinuity between the Sligo limestone beds and the underlying Hosston

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Figure 6. Well log pattern from the Simmons ‘A’ 1 well, Yazoo County, Mississippi, showing the well log signature characteristics of the LKEGR-TR 1 Cycle and associated transgressive-regressive subcycles from the updip area of the Mississippi Interior Salt Basin; see Figure 3 for location of the well. SP, spontaneous potential; ILM, medium induction (resistivity); TA, transgressive aggrading phase; TB, transgressive backstepping phase; RI, regressive infilling phase; RS, inferred ravinement surface; TS, inferred transgressive surface; SMT, inferred surface of maximum transgression; SMF, inferred surface of marine flooding; K1h, Hosston Formation; K1s, Sligo Formation; K1pi, Pine Island Shale; K1‘d’, ‘Donovan’ sandstone.

sandstone beds is recorded in higher resistivity and more positive SP log responses for the Sligo carbonate beds as compared to the Hosston sandstone beds (Figure 6). This discontinuity is interpreted to be an

erosional transgressive surface or ravinement surface that marks the base of the LKEGR-TR 1–2 Subcycle. The lower and middle beds of the Sligo Formation constitute the LKEGR-TR 1–2 Subcycle (lower

Transgressive-regressive cycles in Lower Cretaceous strata

Aptian). Based on core studies and well log lithologic descriptions, the lower Sligo beds in the northeastern Gulf of Mexico include lime mudstone and fossiliferous wackestone (Figure 5B), and the middle Sligo beds are fossiliferous reef rudstone and boundstone (Figure 5C). Based on core analysis, well log lithologic descriptions, well log signatures and paleontologic (ammonite, bivalve, dinoflagellate, calcareous nannofossil and foraminiferal) data, the Sligo fossiliferous lime mudstone and wackestone beds are interpreted as the transgressive section (marine shelf deposits) and the Sligo fossiliferous rudstone and boundstone beds represent the regressive section (reef deposits) of this subcycle. The lower Sligo beds are characterized by a more positive SP log reading compared to the middle Sligo beds in the MISB area (Figure 6). This change in log response is used to recognize an inferred surface of marine flooding, which separates the transgressive and regressive sections in this subcycle. The upper Sligo beds, Pine Island Shale, James Limestone, and ‘Donovan’ sandstone comprise the LKEGR-TR 1–3 Subcycle (upper Aptian). The Sligo intraformational boundary between the middle and upper Sligo beds is recognized by a discontinuity (inferred transgressive surface) in well log pattern consisting of a more positive SP log response for the upper Sligo beds compared to the middle Sligo beds (Figure 6). Seaward, this contact is inferred to be conformable with the top of the Sligo reef beds representing a surface of maximum regression, which separates an upward-shallowing Sligo section from an upward-deepening section. The lithologic contrast between the Sligo limestone beds in the MISB area and the overlying Pine Island calcareous shale beds (Figure 5D) is recorded in higher gamma ray and lower resistivity log responses for the Pine Island shale beds compared to the Sligo limestone beds (Figure 7). The ‘Donovan’ sandstone overlies the Pine Island Shale in much of the MISB. Based on core analysis and well log lithologic descriptions, the Pine Island Shale is a black, calcareous, fossiliferous shale and silty shale, and the ‘Donovan’ sandstone is a conglomeratic, fine- to medium-grained sandstone (Figure 8A) interbedded with red shale. The lithologic contrast between the Pine Island shale beds and the ‘Donovan’ sandstone beds is recorded in a more positive SP log record for the Pine Island Shale compared to the ‘Donovan’ sandstone (Figure 6). The upper beds of the Sligo Formation and the lower beds of the Pine Island Shale are interpreted as the transgressive section (marine shelf deposits) of the LKEGR-TR 1–3 Subcycle, and the upper silty beds (shallow marine deposits) of the Pine Island Shale and the ‘Donovan’

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sandstone (fluvial deposits) represent the regressive section of this subcycle. In the southern portion of the MISB and in much of the onshore area of the northern Gulf of Mexico, the James Limestone overlies the Pine Island Shale. The James Limestone and ‘Donovan’ sandstone are interpreted as coeval. Based on core analysis, well log lithologic descriptions, and paleontologic (ammonite and bivalve) data, the James Limestone is a marine shelf, bioclastic grainstone (Figure 8B) and reef rudstone (Figure 8C). The lithologic contrast between the Pine Island Shale and the James Limestone is recorded in a higher gamma ray log response for the Pine Island shale beds compared to the James packstone, grainstone, rudstone and boundstone beds (Figure 7). In the southern portion of the MISB and in the offshore area of the northeastern Gulf of Mexico, the James Limestone represents the regressive section (marine shelf and reef deposits) of the LKEGR-TR 1–3 Subcycle. The Sligo Formation (LKEGR-TR 1–2 Subcycle) and the lower beds of the Pine Island Shale (transgressive section of the LKEGR-TR 1–3 Subcycle) are interpreted as the transgressive backstepping phase of the LKEGR-TR 1 Cycle, and the upper silty beds of the Pine Island Shale and the ‘Donovan’ sandstone/ James Limestone interval (regressive section of the LKEGR-TR 1–3 Subcycle) represents the regressive infilling phase of this cycle. The facies transition in the Pine Island Shale marks the change from an upwarddeepening section to an upward-shallowing section and represents a surface of maximum transgression. This surface is recognized by a change from higher to lower gamma ray and from more to less positive SP log responses. The seismic data for the upper Sligo Formation/lower Pine Island Shale interval is characterized as concordant, parallel reflection configurations (Yurewicz et al., 1993). The seismic data for the ‘Donovan’ sandstone/James Limestone interval are characterized as oblique, progradational reflection configurations with offlap (downlap) reflection terminations (Figure 9). The ‘Donovan’ sandstone, which caps this T-R cycle, is the first of three major progradational siliciclastic units in the MISB, and the Sligo Formation and lower beds of the Pine Island Shale mark the base of this cycle.

4.2. LKEGR-TR 2 Cycle The Bexar Formation, Rodessa Formation, Ferry Lake Anhydrite, Mooringsport Formation, and Paluxy Formation constitute the LKEGR-TR 2 Cycle (uppermost Aptian–middle Albian). The Bexar Formation boundary with the underlying James

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Figure 7. Well log patterns from the Southern Minerals No. 1 well, Pearl River County, Mississippi, showing the well log signature characteristics of the LKEGR-TR 1, LKEGR-TR 2, and LKEGR-TR 3 cycles and associated transgressiveregressive subcycles from the southern portion of the Mississippi Interior Salt Basin; see Figure 3 for location of the well. GR, gamma ray; SP, spontaneous potential; ILD, deep induction (resistivity); TB, transgressive backstepping phase; RI, regressive infilling phase; SA, inferred subaerial unconformity; TS, inferred transgressive surface; SMR, inferred surface of maximum regression; SMT, inferred surface of maximum transgression; SMF, inferred surface of marine flooding; MCU, mid-Cretaceous (mid-Cenomanian) unconformity; K1s, Sligo Formation; K1pi, Pine Island Shale; K1j, James Limestone; K1b, Bexar Formation; K1r, Rodessa Formation; K1f1, Ferry Lake Anhydrite; K1m, Mooringsport Formation; K1p, Paluxy Formation; K1a, Andrew Formation; K1d, Dantzler Formation.

Transgressive-regressive cycles in Lower Cretaceous strata

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Figure 8. Core photographs of Lower Cretaceous strata. A, ‘Donovan’ conglomeratic, fine- to medium-grained sandstone of the regressive infilling phase of the LKEGR-TR 1–3 Subcycle, Citronelle Unit 28 No. 1 well, Mobile County, Alabama, 3383 m (11,097 ft). B, James bioclastic grainstone of the regressive infilling phase of the LKEGR-TR 1–3 Subcycle, Denmiss 24–8 well, Lawrence County, Mississippi, 4852 m (15,915 ft). C, James rudist rudstone of the regressive infilling phase of the LKEGR-TR 1–3 Subcycle, Denmiss 24–8 well, Lawrence County, Mississippi, 4855 m (15,925 ft). D, Bexar argillaceous lime mudstone of the transgressive backstepping phase of the LKEGR-TR 2–1 Subcycle, Chandeleur Sound Block 61, St. Bernard Parish, Louisiana, 5590 m (18,334 ft). Diameter of coin is 18 mm.

Limestone is disconformable (transgressive surface documenting erosion in the marine system) as observed from core study (Loucks et al., 1996). This contact is also recognized by a discontinuity in well log pattern and change in lithology. Throughout much of the northern Gulf of Mexico, the Bexar Formation

consists of calcareous shale and argillaceous lime mudstone (Figure 8D) as discerned from core studies and well log lithologic descriptions. The lithologic contrast between the Bexar shale beds and the underlying James Limestone beds is recorded in more positive SP and lower resistivity log responses for the

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Bexar shale beds as compared to the James Limestone beds (Figure 7). The Bexar Formation, Rodessa Formation, and Ferry Lake Anhydrite comprise the LKEGR-TR 2–1 Subcycle (uppermost Aptian–lower Albian) of the LKEGR-TR 2 Cycle. Based on core analysis, well log lithologic descriptions, and paleontologic (foraminiferal, ammonite and bivalve) data, the Bexar Formation is a marine calcareous shale and argillaceous lime mudstone, the Rodessa Formation is a fossiliferous, reef rudstone and boundstone (Figure 10A) and the Ferry Lake Anhydrite is a subaqueous anhydrite. The lithologic contrast between the Bexar Formation and the Rodessa Formation is recorded in higher gamma ray and lower resistivity log responses for the Bexar shale beds compared to the Rodessa limestone beds (Figure 7). The lithologic contrast between the Rodessa limestone beds and the anhydrite beds of the Ferry Lake Anhydrite is recorded in a higher resistivity log response for the Ferry Lake Anhydrite compared to the limestone beds of the Rodessa Formation. The Bexar shale beds are interpreted as the transgressive section (marine shelf deposits) of the LKEGR-TR 2–1 Subcycle, and the Bexar argillaceous limestone beds (marine shelf deposits), Rodessa rudstone and boundstone beds (marine shelf and reef deposits) and Ferry Lake Anhydrite beds (lagoonal deposits) represent the regressive section of this subcycle. The lithologic transition in the Bexar Formation, as recognized by a change in the gamma ray log response, is marked by an inferred surface of marine flooding, which separates the transgressive and regressive sections in this subcycle. The Mooringsport Formation and Paluxy Formation constitute the LKEGR-TR 2–2 Subcycle (lower–middle Albian) of the LKEGR-TR 2 Cycle. Based on core analysis and well log lithologic descriptions, the Mooringsport Formation is a calcareous shale (Figure 10B) in the MISB, and the Paluxy Formation is a fine- to medium-grained sandstone (Figure 10C) interbedded with carbonaceous shale (Figure 10D). The Ferry Lake Anhydrite and Mooringsport boundary is inferred to be conformable with the top of the Ferry Lake Anhydrite, representing a surface of maximum regression, which separates an upward shallowing section from an upward deepening section. This discontinuity between the Ferry Lake Anhydrite and the Mooringsport Formation is recognized by a higher resistivity log response for the Ferry Lake Anhydrite compared to the Mooringsport beds (Figure 7). This boundary marks the base of the LKEGE-TR 2–2 Subcycle. In parts of the MISB, the LKEGR-TR 2–1 Subcycle is not recognizable. In

these areas, the base of the LKEGR-TR 2–2 Subcycle and the LKEGR-TR 2 Cycle is recognized by a discontinuity (inferred transgressive surface) in well log pattern and change in lithology between the Mooringsport Formation and the underlying ‘Donovan’ sandstone. In these cases, the lithologic contrast between the ‘Donovan’ sandstone and the overlying shale beds is recorded in lower resistivity and more negative SP log responses for the ‘Donovan’ sandstone beds compared to the Mooringsport shale beds (Figure 6). In much of the MISB, the calcareous shale beds of the Mooringsport Formation are interpreted as the transgressive section (marine shelf shale deposits) of the LKEGR-TR 2–2 Subcycle, and the sandstone beds of the Paluxy Formation represent the regressive section (fluvial and coastal siliciclastic deposits) of this subcycle. In the southern portion of the MISB and in the offshore area of the northeastern Gulf of Mexico, the Mooringsport Formation includes lime mudstone, packstone, grainstone (Figure 11A), rudstone and boundstone (Figure 11B). Based on core analysis, well log descriptions, and paleontologic (foraminifera, ammonite and bivalve) data, the packstone, grainstone, rudstone and boundstone beds are part of the regressive section (marine shelf and reef deposits) of the LKEGR-TR 2–2 Subcycle. These carbonate beds are characterized by higher resistivity and lower gamma ray log signatures than the calcareous shale and lime mudstone beds of the Mooringsport Formation (Figure 7). The lithologic contrast between the Mooringsport beds and the sandstone beds of the Paluxy Formation is recorded in higher gamma ray and higher resistivity log responses for the Mooringsport Formation compared to the sandstone beds of the Paluxy Formation. The Bexar Formation, Rodessa Formation and Ferry Lake Anhydrite (LKEGR-TR 2–1 Subcycle) and lower beds of the Mooringsport Formation (transgressive section of the LKEGR-TR 2–2 Subcycle) are interpreted as the transgressive backstepping phase of the LKEGR-TR 2 Cycle. The upper beds of the Mooringsport Formation and Paluxy Formation comprise the regressive infilling phase of this cycle. The facies transition in the Mooringsport Formation marks the change from an upwarddeepening section to an upward-shallowing section and is interpreted as a surface of maximum transgression. This surface is recognized by a change in gamma ray log response from higher to lower values. The seismic data for the Bexar Formation interval are characterized as concordant, parallelreflection configurations (Yurewicz et al., 1993). The seismic data for the upper Mooringsport Formation/ Paluxy Formation interval are characterized as

Figure 9. Representative seismic reflection profile from the offshore shelf area of the northeastern Gulf of Mexico showing the seismic reflection configuration and termination characteristics of the LKEGR-TR 1 (TR-1), LKEGR-TR 2 (TR-2), LKEGR-TR 3 (TR-3), and LKEGR-TR 4 (TR-4) cycles and the Valanginian (VU) and mid-Cretaceous (MCU) unconformities. TA, transgressive aggrading phase; TB, transgressive backstepping phase; RI, regressive infilling phase. Seismic data are minimum phase and reverse polarity wavelet.

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Figure 10. Core photographs of Lower Cretaceous strata. A, Rodessa rudist boundstone of the regressive infilling phase of the LKEGR-TR 2–1 Subcycle, Waveland Gas Unit No. 1 well, Hancock County, Mississippi, 4280 m (14,039 ft). B, Mooringsport calcareous shale of the transgressive backstepping phase of the LKEGR-TR 2–2 Subcycle, Gex No. 1 well, Hancock County, Mississippi, 4105 m (13,466 ft). C, Paluxy fine- to medium-grained sandstone of the regressive infilling phase of the LKEGR-TR 2–2 Subcycle, Pilgrim No. 1 well, Walthall County, Mississippi, 4022 m (13,193 ft). D, Paluxy carbonaceous shale of the regressive infilling phase of the LKEGR-TR 2–2 Subcycle, Pilgrim No. 1 well, Walthall County, Mississippi, 4029 m (13,214 ft). Diameter of coin is 18 mm.

oblique, progradational, reflection configurations with offlap (downlap) reflection terminations (Figure 9). The Paluxy Formation, which caps this T-R cycle, is the second and most areally extensive of three major progradation siliciclastic units in the MISB, and the Bexar Formation and lower Mooringsport Formation mark the base of this cycle. 4.3. LKEGR-TR 3 Cycle The LKEGR-TR 3 Cycle (middle Albian–upper Albian) is composed of the Andrew Formation and Dantzler Formation. The boundary between the Andrew Formation and the underlying Paluxy Formation is recognized by a discontinuity (inferred transgressive surface) in well log pattern and change in lithology. The lithologic contrast between the Andrew limestone beds and the underlying Paluxy sandstone beds is recorded in higher resistivity and lower gamma ray log responses for the Andrew limestone beds as compared to the Paluxy sandstone beds (Figure 7). Based on core analysis and well log lithologic descriptions, the Andrew Formation consists of lime mudstone (Figure 11C), grainstone, rudstone and boundstone (Figure 11D), and the Dantzler Formation includes fine- to medium-grained sandstone (Figure 12A) interbedded with carbonaceous shale.

The Andrew limestone beds have higher resistivity and more positive SP log responses compared to the Dantzler sandstone beds (Figure 7). South of the MISB proper, the Andrew limestone beds are overlain by an interval of mixed siliciclastic and carbonate beds referred to as the Fredericksburg Group (Figure 13). In the MISB, beds of the Tuscaloosa Group (Cenomanian age, Mancini et al., 1980) unconformably (subaerial unconformity) overlie sandstone beds of the Dantzler Formation. The Dantzler sandstone beds have higher gamma ray and higher resistivity log readings and a more positive SP log signature as compared to the Tuscaloosa sandstone beds. The Andrew argillaceous lime mudstone beds are interpreted as the transgressive backstepping phase (marine shelf deposits) of the LKEGR-TR 3 Cycle, and the upper beds (grainstone, rudstone and boundstone) of the Andrew Formation (carbonate shoal and reef deposits), Fredericksburg Group (marine shelf deposits) and the Dantzler sandstone beds (fluvial deposits) represent the regressive infilling phase of this cycle. The facies transition in the Andrew Formation marks the change from an upward-deepening section to an upward-shallowing section and represents an inferred surface of maximum transgression. This surface is recognized by a change from higher to lower

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Figure 11. Core photographs of Lower Cretaceous strata. A, Mooringsport fossiliferous grainstone of the regressive infilling phase of the LKEGR-TR 2–2 Subcycle, Chandeleur Sound Block 61, St. Bernard Parish, Louisiana, 5072 m (16,637 ft). B, Mooringsport rudist boundstone of the regressive infilling phase of the LKEGR-TR 2–2 Subcycle, Chandeleur Sound Block 61, St. Bernard Parish, Louisiana, 5074 m (16,642 ft). C, Andrew lime mudstone of the transgressive backstepping phase of the LKEGR-TR 3 Cycle, Chandeleur Sound Block 61, St. Bernard Parish, Louisiana, 4494 m (14,739 ft). D, Andrew rudist boundstone of the regressive infilling phase of the LKEGR-TR 3 Cycle, Chandeleur Sound Block 61, St. Bernard Parish, Louisiana, 4487 m (14,716 ft). Diameter of coin is 18 mm.

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Figure 12. Core photographs of Lower Cretaceous strata. A, Dantzler fine- to medium-grained sandstone of the regressive infilling phase of the LKEGR-TR 3 Cycle, Boteler 10–7 No. 1 well, Forrest County, Mississippi, 2970 m (9740 ft). B, Washita bioclastic grainstone of the regressive infilling phase of the LKEGR-TR 4 Cycle, 1654–6 well, 2672 m (8763 ft). C, Washita rudist boundstone of the regressive infilling phase of the LKEGR-TR 4 Cycle, 1654–6 well, 2681 m (8,792.7 ft). D, mid-Cretaceous (mid-Cenomanian) unconformity, 1654–6 well, 2655 m (8708 ft). Diameter of coin is 18 mm.

gamma ray log values. The seismic data for the lower Andrew Formation interval are characterized as concordant, parallel-reflection configurations with onlap reflection terminations, and the seismic data for the upper Andrew Formation and Fredericksburg Group/ Dantzler Formation interval are characterized as

oblique, progradational reflection configurations with offlap (downlap) reflection terminations (Figure 9). The Dantzler Formation, which caps this T-R cycle, is the third of three major progradation siliciclastic units in the MISB, and the Andrew Formation marks the base of this cycle.

Transgressive-regressive cycles in Lower Cretaceous strata

4.4. LKEGR-TR 4 Cycle The LKEGR-TR 4 Cycle (upper Albian–lower Cenomanian), which is not well developed in the MISB, includes the Washita Group. This T-R cycle is recognized throughout much of the northern Gulf of Mexico. The boundary between the Washita Group with the underlying Fredericksburg Group is recognized by a discontinuity (inferred transgressive surface) in well log pattern and change in lithology. The lithologic contrast between the Washita limestone beds and the underlying Fredericksburg mixed siliciclastic and carbonate beds or siliciclastic beds is recorded in higher resistivity, higher gamma ray, and more positive SP log responses for the Washita limestone beds as compared to the Fredericksburg beds (Figure 13). Based on core analysis, well log lithologic descriptions, and paleontologic (foraminifera, ammonite and bivalve) data, the lower beds of the Washita Group include marine argillaceous lime mudstone, and the upper beds consist of packstone, grainstone (Figure 12B), rudstone, and boundstone (Figure 12C). The lower argillaceous lime mudstone beds are characterized by higher resistivity and more positive SP log readings than the upper Washita beds (Figure 13). The lower Washita argillaceous lime mudstone beds are interpreted as the transgressive backstepping phase (marine shelf deposits) of this T-R cycle, and the upper Washita packstone, grainstone, rudstone and boundstone beds as the regressive infilling phase (carbonate shoal and reef deposits) of this cycle. The facies transition in the Washita Group marks the change from an upward-deepening section to an upward-shallowing section and represents an inferred surface of maximum transgression. This surface is recognized by a change from higher to lower gamma ray log values. In the offshore area of the northeastern Gulf of Mexico, as observed from core studies and well log lithologic descriptions, the Washita Group is unconformably overlain by Maastrichtian chalks (Figure 12D) and a significant hiatus of 20 myr or more is associated with this subaerial unconformity. This determination is based on the occurrence of age diagnostic Maastrichtian planktonic foraminifera, including Globotruncanita conica (White), in the chalks overlying the mid-Cretaceous (mid-Cenomanian) unconformity (Paleo-Data unpublished report, 2000). The seismic data for the lower Washita Group are characterized as concordant, parallel-reflection configurations with onlap reflection terminations and the seismic data for the upper Washita Group are characterized as oblique, progradational reflection configu-

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rations with offlap (downlap) reflection terminations (Figure 9). 5. Stratigraphic analysis and comparison Cyclic changes in global sea level and associated relative changes in coastal onlap during the Early Cretaceous were recognized by Haq et al. (1988). These authors described 18 third-order, unconformity-bounded depositional sequences for the late Valanginian–middle Cenomanian. For the onshore Gulf Coast (East Texas, Louisiana and Mississippi), McFarlan (1977) described four sequences (Hosston–Sligo, James–Rodessa–Ferry Lake, Mooringsport–Glen Rose–Paluxy, and Fredericksburg–Washita) (Figure 14). Corso (1987) recognized three Lower Cretaceous sequences (Depositional Unit II, Hosston/Rodessa/Ferry Lake/ Mooringsport; Depositional Unit III; Paluxy/ Fredericksburg, and Depositional Unit IV, Washita) from seismic data for the northeastern Gulf of Mexico region. Goldhammer & Lehmann (1991) described two-second order (10–100 myr in duration), unconformity-bounded depositional sequences for the Valanginian–Albian for the Texas Gulf Coast and Mexico areas. These sequences included the Hosston–lower Sligo and the upper Sligo–Pearsall– lower Glen Rose–upper Glen Rose–Fredericksburg depositional sequences. Yurewicz et al. (1993) described eight third order sequences from the onshore area of Louisiana and Mississippi. These sequences included the Sligo, James, Rodessa– Ferry Lake, Mooringsport, lower Glen Rose, upper Glen Rose, Paluxy–Fredericksburg, and Washita sequences. Scott (1993) recognized five sequences from outcrop and subsurface studies from the US Gulf Coast. These sequences included the Sligo– Cupido, James, Rodessa–Sunniland and upper Glen Rose, Edwards–Stuart City, and Buda–El Abra– Devils River sequences. Scott et al. (2002) described three second-order depositional cycles (Twin Mountains–Glen Rose–lower part of the Paluxy, upper part of the Paluxy–Walnut–Goodland, and Washita, including the Kiamichi–Duck Creek– Fort Worth–Denton–Weno–Pawpaw–Main Street– Grayson–Buda) from outcrop studies in north-central Texas. The Lower Cretaceous T-R cycles recognized in the MISB area in this study correlate with the secondorder depositional cycles identified by Scott (2002) for the Lower Cretaceous section of north Texas (Figure 14). The LKEGR TR-2 Cycle corresponds to the upper Aptian–lower Albian second-order depositional sequence, and the LKEGR-TR 3 Cycle

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Figure 13. Well log pattern from the 1654–6 well from Block 253 of the Main Pass offshore area of the northeastern Gulf of Mexico, showing the well log signature characteristics of the LKEGR-TR 4 Cycle near the Lower Cretaceous shelf margin; see Figure 3 for location of the well. GR, gamma ray; SP, spontaneous potential; ILD, deep induction (resistivity); TB, transgressive backstepping phase; RI, regressive infilling phase; SA, inferred subaerial unconformity; TS, inferred transgressive surface; SMT, inferred surface of maximum transgression; MCU, mid-Cretaceous (midCenomanian) unconformity; K1a, Andrew Formation; K1f, Fredericksburg Group; K1w, Washita Group.

correlates with the lower Albian–upper Albian secondorder depositional sequence of Scott et al. (2002). The LKEGR-TR 4 Cycle corresponds to the upper Albian–lower Cenomanian second-order depositional sequence of Scott et al. (2002). Such correspondence of Lower Cretaceous cycles in the eastern Gulf Coastal Plain with those of the western Gulf Coastal Plain demonstrates the utility of T-R cycles for regional correlation across the northern Gulf of Mexico area.

The Lower Cretaceous T-R cycles recognized in this area correlate with the Valanginian–Cenomanian facies cycles reported by Jacquin et al. (1998) for the Boreal areas of Western Europe (Figure 4). The upper Valanginian–lower Aptian LKEGR-TR 1–1 Subcycle, which represents the transgressive aggrading phase of the LKEGR-TR 1 Cycle of the MISB area, broadly relates to the upper Valanginian–lower Aptian Cycle 12 of Jacquin et al. (1998). In Western Europe, the basal unconformity of Cycle 12a is a major erosional

Transgressive-regressive cycles in Lower Cretaceous strata

unconformity, and Cycle 12d is a highly aggradational cycle (Jacquin et al., 1998). In the MISB area, the lower boundary of the LKEGR-TR 1 Cycle is defined by the Cotton Valley Group–Hosston Formation boundary. This cycle boundary is a major subaerial unconformity, and aggradational processes dominated during Hosston deposition. The lower–upper Aptian LKEGR- TR 1–2 and LKEGR-TR 1–3 subcycles, which represent the transgressive backstepping and regressive infilling phases of the LKEGR-TR 1 Cycle of the MISB area, show correspondence to the lower–upper Aptian Cycle 13 of Jacquin et al. (1998). This cycle represents one of the most widespread and correlatable events in the Mesozoic record (Jacquin et al., 1998). It has excellent utility for correlation in the onshore area of the northern Gulf of Mexico. The upper Aptian–middle Albian LKEGR-TR 2 Cycle of the MISB area corresponds broadly to the upper Aptian–lower Albian Cycle 14a of the Boreal areas of Jacquin et al. (1998). Cycle 14a is a widespread and correlatable event in Western Europe, and in Tethyan areas it merges with Cycle 14b producing Cycle 14, which is one of the major marine flooding events in the Mesozoic record (Jacquin et al., 1998). This T-R cycle is the best correlation marker in the Lower Cretaceous section in the northern Gulf of Mexico. This major transgression in the region includes the development of the Glen Rose shelfmargin reef (Rodessa, Ferry Lake and Mooringsport formations). The maximum landward shift of Lower Cretaceous carbonate rocks occurs within this upper Aptian–middle Albian cycle in the MISB. The middle–upper Albian LKEGR-TR 3 Cycle of the MISB area correlates broadly with the lower– upper Albian Cycle 14b of the Boreal areas of Jacquin et al. (1998). This T-R cycle is useful for correlation in the MISB area and the northern Gulf of Mexico. The upper Albian–lower Cenomanian LKEGR-TR 4 Cycle of the northern Gulf of Mexico corresponds broadly to upper Albian–lower Cenomanian Cycle 15 of Jacquin et al. (1998). Although this T-R cycle is not well developed in the MISB, it is easily recognized in the onshore area of the northwestern Gulf of Mexico and the offshore area of the northern Gulf of Mexico. This T-R cycle is highly useful in these areas for correlation. In Western Europe, Cycle 15 may continue into the middle Cenomanian, which is a major relative sea-level downward shift in this region (Jacquin et al., 1998). The mid-Cenomanian was also a time of major relative sea-level fall and erosion in the northern Gulf of Mexico, producing the midCretaceous (mid-Cenomanian) unconformity of Salvador (1991). The correspondence of these four

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T-R cycles of the northern Gulf of Mexico to T-R facies cycles 13, 14a, 14b and 15 of Western Europe demonstrates the usefulness of the T-R cycles for global correlation of Lower Cretaceous strata. 6. Depositional history During the earliest Cretaceous (Berriasian), fluvial and deltaic sediments of the Cotton Valley Group prograded across the MISB area. As base level continued to fall a regional subaerial unconformity developed at the top of the Cotton Valley Group. The hiatus between the Cotton Valley Group and the Hosston Formation is represented by most of the Valanginian Stage. This unconformity and hiatus are recognized throughout the northern Gulf of Mexico (McFarlan, 1977; McFarlan & Menes, 1991) and in Western Europe (Jacquin et al., 1998). Coastal sands of the Hosston Formation were deposited during an initial rise in base level and increase in shelf accommodation in the southern portion of the MISB during the late Valanginian that post-dated a fall in base level and regional unconformity (Figure 15). Hosston fluvial–deltaic sediments later aggraded and prograded across much of the MISB area. In areas south of the MISB, where siliciclastic sediment supply was diminished, coastal fine-grained sands and shallow marine muds were deposited concurrently with Hosston fluvial–deltaic coarse-grained sands in updip areas during the late Valanginian–earliest Aptian. This interval is a highly aggradational to progradational section in the northern Gulf of Mexico (McFarlan, 1977; Salvador, 1991) and in Western Europe (Jacquin et al., 1998), indicating the creation of shelf accommodation during a time characterized by high rates of sediment supply. During the early late Aptian, marine mud, silt, and limestone beds of the Sligo Formation and Pine Island Shale were deposited, representing a major rise in sea level and significant marine flooding event. Downdip in the MISB and south of the basin proper, marine shelf and shelf margin reef limestone beds of the Sligo Formation and James Limestone were deposited. This Aptian interval represents a widespread transgression in the northern Gulf of Mexico (McFarlan, 1977; Salvador, 1991) and in Western Europe (Jacquin et al., 1998). The Pine Island Formation, in particular, represents a significant regional transgression observed even in updip areas of the MISB and across Louisiana and Texas (Forgotson, 1957; Anderson, 1979; Yurewicz et al., 1993). With a reduction in shelf accommodation and an increase in siliciclastic sediment supply, fluvial deposits of the ‘Donovan’

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Lower Cretaceous Cycles and Depositional Sequences Stratigraphy

‘

‘

Stages

Figure 14. Stages, stratigraphy, and transgressive-regressive cycles and subcycles of the Lower Cretaceous strata in the northeastern Gulf of Mexico and comparison of sequences and cycles recognized by other authors in the northern Gulf of Mexico.

sandstone prograded across the MISB and capped this Aptian interval. A second base-level (sea-level) rise, which occurred during the latest Aptian–early middle Albian in the MISB area, began with the transgressive deposits of the Bexar Formation, continued with the deposition of the marine mud beds of the Mooringsport Formation, and resulted in the development of the Glen Rose (Rodessa, Mooringsport) shelf margin reef. With decreased shelf accommodation, cessation of siliciclastic deposition, and periodic restriction of basin circulation, the evaporite deposits of the Ferry Lake Anhydrite accumulated during the early Albian. In downdip areas of the MISB and south of the basin area, Mooringsport sedimentation included marine shelf-limestone deposition, which was part of the development of the Glen Rose shelf-margin reef complex. A decrease in shelf accommodation and a major influx of siliciclastic sediments occurred during the early middle Albian and resulted in fluvial and coastal siliciclastic deposition of the Paluxy Formation, which caps this cycle. This cycle represents a major transgression and significant marine flooding event in the

northern Gulf of Mexico (McFarlan, 1977; Salvador, 1991) and in Western Europe (Jacquin et al., 1998). During the late middle Albian, a third rise in base-level (sea-level) and increase in shelf accommodation occurred, resulting in deposition of the transgressive fossiliferous limestone beds of the Andrew Formation in downdip areas of the MISB. Fluvial sandstone and shale deposits of the Dantzler Formation cap this middle–upper Albian cycle, and signal a major base-level (sea-level) fall represented by the mid-Cretaceous (mid-Cenomanian) unconformity. The regressive phase of this cycle represents a significant aggrading and prograding interval in the MISB. A fourth base-level (sea-level) rise and increase in shelf accommodation, which occurred during the latest Albian–early Cenomanian, began with the transgressive deposits of the Washita Group and culminated with exposure of the Lower Cretaceous shelf during the mid-Cenomanian. This marine transgression is recorded at the top of the Dantzler Formation in the MISB. However, because the MISB experienced considerably greater accommodation

'

0

600 ft

Paluxy

Scale

32 000 ft

0

200 m

Mooringsport

Scale

10 000 m

Sligo

Pin e Island

James

Bexar

Rodessa

Ferry Lake

Dantzler

Andrew

Fred e g

Was h it rick sbur

a

S

Figure 15. Schematic cross section showing the distribution of Lower Cretaceous lithofacies from the updip area of the Mississippi Interior Salt Basin to the Lower Cretaceous shelf margin and slope in the offshore area of the northeastern Gulf of Mexico from the late Valanginian to the middle Cenomanian, as modified from Yurewicz et al. (1993).

Anhydrite

Reef talus

Reef limestone

Shelf limestone

Calcareous shale

ovan

Hosston

'Don

Fine-grained siliciclastic sediments

Sandstone and shale

Sandstone

Explanation

N

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primarily as a result of subsidence and because siliciclastic sediment supply was substantially higher in the MISB during the late Albian than in adjacent areas in the northern Gulf of Mexico, the effects of this marine transgression are limited in the MISB area. In both Western Europe (Jacquin et al., 1998) and the northern Gulf of Mexico (Salvador, 1991), the post midCenomanian was a time of major base-level (sea-level) fall and erosion.

7. Conclusions Four transgressive-regressive (T-R) cycles, the LKEGR-TR 1 (upper Valanginian–upper Aptian), LKEGR-TR 2 (upper Aptian–middle Albian), LKEGR-TR 3 (middle–upper Albian), and LKEGRTR 4 (upper Albian–lower Cenomanian), are recognized in Lower Cretaceous strata of the northeastern Gulf of Mexico. Recognition of these T-R cycles is based upon stratal geometries, nature of cycle boundaries, facies stacking patterns within cycles, and large-scale shifts in major facies belts. Five transgressive-regressive (T-R) subcycles are recognized in these strata in this region. These T-R subcycles include the LKEGR-TR 1–1 (upper Valanginian–lower Aptian), LKEGR-TR 1–2 (lower Aptian), and LKEGR-TR 1–3 (upper Aptian) of the LKEGR-TR 1 Cycle and the LKEGR-TR 2–1 (upper Aptian–lower Albian) and LKEGR-TR 2–2 (lower– middle Albian) of the LKEGR-TR 2 Cycle. The T-R subcycles are characterized by shifts in major facies belts that are not of the magnitude of a T-R cycle. The LKEGR-TR 3 (middle–upper Albian) and LKEGR-TR 4 (upper Albian–lower Cenomanian) cycles consist of a single T-R cycle. The marine transgressive and flooding events of the transgressive phase of these cycles are widespread and provide regional correlation datums. Therefore, these T-R cycles have excellent potential for regional and global correlation, and the T-R subcycles are useful for regional correlation.

Acknowledgments This research was funded by the US Department of Energy (DOE), Office of Fossil Energy through the National Petroleum Technology Office of the National Energy Technology Laboratory, and the US Department of Interior through the Minerals Management Service (MMS). However, any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the DOE or MMS.

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