Evolution of the Destin Dome, offshore Florida, north-eastern Gulf of Mexico

Evolution of the Destin Dome, offshore Florida, north-eastern Gulf of Mexico Grant MacRae Department of Oceanography, Texas A & M University, College Station, TX 77843, USA

and Joel S. Watkins Department of Geophysics, Texas A & M University, College Station, TX 77843, USA

Received24 September 1990; revised26 September 1991; accepted 11 October 1991 Structural and stratigraphic relationships indicated by seismic reflection data suggest that uplift of the Destin Dome anticline resulted mainly from salt movement during Late Cretaceous to Early Cenozoic time. A thick succession of Lower Cretaceous sediment was deposited in the central part of the basin seaward of the present day Destin Dome. Sediment loading squeezed the salt both updip onto the shelf and downdip into the basin. A step in the pre-salt basement limited movement of salt and controlled the location of the dome. Vertical movement of salt was accommodated by extensional faulting. During the Early Cenozoic, sediment touchdown onto the pre-salt basement formed a salt weld between the anticline and the De Soto Canyon diapir field, cut off the supply of salt and stopped growth of the structure. The minimum accumulation thickness of the 'mother' salt is estimated to be 760 m and the original volume of salt deposited at the present day location of the Destin Dome anticline on the Florida shelf is estimated to be 1000 km 3. The volume of salt in place today is in excess of 2300 km 3. Keywords: differential sediment loading; Callovian salt; Destin Dome, Florida, USA

Introduction The area offshore Florida and Alabama (Figure 1), which incorporates the Destin Dome, Pensacola and western part of the Apalachicola OCS areas, is underlain by widespread Middle Jurassic salt (Figure 2). Over 4800 km of migrated seismic reflection profiles in an approximately orthogonal grid (Figure 3) provided the basis for describing the distribution of salt and the effect of salt tectonism on the overlying sediments in the Destin Dome region, offshore Florida and Alabama. The focus of this paper is to show that growth and development of the Destin Dome anticline, located on the Florida shelf, is associated mainly with the updip flow of Middle Jurassic salt towards the margin of the north-east Gulf Basin in response to Lower Cretaceous differential sediment loading and basin subsidence through time.

Previous w o r k

Tectonic and geological framework Geophysical studies by Buffler and Sawyer (1985) and Buffler (1989) suggest that the sedimentary succession in the Destin Dome region of the north-east Gulf of Mexico is underlain unconformably by thick transitional crust formed by crustal stretching and attenuation during the Middle Jurassic. This Middle Jurassic tectonic activity produced a north-west trending zone of alternating basement highs and lows, with a wavelength of 200-300 km, characteristic of the

eastern Gulf of Mexico basin margin (Klitgord et al., 1984; Buffler and Sawyer, 1985; Buffler, 1989). The De Soto Canyon Salt Basin represents one such basement 'low'. Buffler and Sawyer (1985) define the basement as all crustal rocks, including the Late Triassic-Early Jurassic rift sequences, lying beneath a widespread unconformity that is overlain by Middle Jurassic evaporites or equivalent rocks. During the Late Jurassic, following deposition of Middle Jurassic (Callovian age) salt and evaporites in the Gulf of Mexico basin, the onset of drifting began with the injection of oceanic crust in the deep central Gulf (Pilger, 1978; Klitgord et al., 1984; Buffler and Sawyer, 1985; Pindell, 1985; Salvador, 1987). This emplacement of oceanic crust separated the main Gulf of Mexico basin into the two major salt provinces observed today: the Sigsbee salt basin offshore Mexico, and the northern Gulf salt basin of the US Gulf Coast (Liro, 1989). By the end of the Tithonian, the basic configuration of the modern Gulf of Mexico basin had been established (Buffler and Sawyer, 1985; Pindell, 1985; Salvador, 1987). Palaeogeographic reconstructions by Salvador (1987) suggest that marine waters entered the Gulf of Mexico basin during the Late Bathonian and Callovian resulting in the accumulation of extensive Middle Jurassic Callovian age salt and evaporites or equivalent rocks. These strata were deposited unconformably over the pre-salt basement surface. Above the salt, the Upper Jurassic succession consists of a thick section of conformable shallow marine carbonates, shales and

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coarse terrigenous clastics deposited over board shelfal areas. Lower Cretaceous sediments of the Florida carbonate platform consist of a thick sequence of shallow water, cyclic, carbonate-evaporite sequences, basinward of the Tithonian shelf edge (Corso, 1987; Salvador, 1987). The top of the Lower Cretaceous is defined by a regional unconformity, the Middle Cretaceous sequence boundary (MCSB). The underlying Lower Cretaceous shallow water sediments are overlain by Upper Cretaceous deep water sediments (Mitchum, 1978; Addy and Buffler, 1984). In the absence of significant amounts of clastic sediment input and continued regional margin subsidence, the Upper Cretaceous to Middle Oligocene strata consist mainly of non-argillaceous shallow to deep I

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Antoine et al. (1967) first described structural features of the continental shelf and slope in this area from seismic reflection data. Subsequent investigations (Uchupi and Emery, 1968; Bryant et al., 1969; Martin, 1972; Garrison and Martin, 1973; Martin, 1978; Mitchum, 1978) described the shallow structure and stratigraphy of the post-Early Cretaceous sediments on the outer margin of the platform from geophysical and core data collected from the shelf, slope and Florida

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Evolution of the Destin Dome: G. MacRae and J. S. Watkins

Figure 3 Distribution of seismic data used in the study. Seismic data courtesy of Digicon Geophysical Corporation

escarpment, and from extrapolations with industry wells onshore. Addy and Buffler (1984) established a seismic stratigraphic framework for the shelf near the Destin Dome. This framework separated the Jurassic into two preliminary sequences, the lowermost of which is Jurassic salt, two stratigraphic units in the Lower Cretaceous and five post-MCSB sequences entering into the Late Tertiary. The regional distribution of Jurassic salt and associated salt structures in the northern Gulf of Mexico (Figure 2) is well documented by Martin (1978, 1980). More recently, MacRae (1990) established a detailed seismic stratigraphic framework for the Upper Jurassic and described the effects of salt tectonism on the overlying strata in the Destin Dome region.

Salt tectonism

Overview Mechanisms of salt migration, the development of salt structures and the influence of salt tectonics on sedimentation are extensively documented (Trusheim, 1960; Lehner, 1969; Bishop, 1978; Humphris, 1979; Jackson and Seni, 1983; Jackson and Talbot, 1986; Talbot and Jackson, 1987). From studies of salt migration in northern Germany, Trusheim (1960) recognized that gravity alone could initiate and amplify salt structures and defined the process of gravity-driven autonomous salt flow as 'halokinesis'. Similarly, and more recently, Jackson and Talbot (1986) in their discussion of salt dynamics conclude that gravity spreading is the most important mechanism of salt flow and that buoyancy and differential loading mainly contribute to the vertical development of salt structures. Lateral variations in thickness (including surface slope), density, strength or strain rates in a layer of salt or its cover can result in differential loading of salt (Jackson and Talbot, 1986).

Trusheim (1960) suggests that the initiation, growth and external morphology of salt structures are largely dependent, in both shape and size, on the primary thickness of the 'mother' salt formation and the weight of the overlying strata. From studies of salt structure development in the east Texas salt basin, Jackson and Seni (1983) and Seni and Jackson (1983) agree that there is a progressive increase in the structural maturity of salt towards a basin centre, which parallels an increase in the original thickness of the salt layer. Salt structures generally evolve from immature, low amplitude structures, concordant with the overlying sedimentary cover (seen near basin margins) to high amplitude, discordant, intrusive structures (salt diapirs in basin centres) which have pierced the overburden (Jackson and Talbot, 1986). Upbuilding and downbuilding are important concepts which need to be considered when addressing the growth of salt structures. Barton (1933) introduced the downbuilding hypothesis as a medium for the development of salt dome growth in the Gulf Coast area. In downbuilding, the crest of a salt structure is regarded as stationary, while the base is regarded as moving downwards. Barton (1933) termed the opposite process upthrusting. Based on centrifuge modelling, Jackson et al. (1988) refined these original concepts and called the equivalent of Barton's upthrusting and downbuilding, pure upbuilding and pure downbuilding, respectively. Figure 4 schematically shows the relationship between different processes of dome growth and those intermediate to upbuilding and downbuilding. The updip flow of salt, or the migration of salt from the central part of a basin towards the basin margin, is a rare and unusual phenomenon. Trusheim (1960) quotes field examples from the Mansfeld (Germany), Ebro (Spain) and Transylvanian (Romania) basins where this regional behaviour of salt is attributed to differential

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Evolution o f the Destin Dome." G. MacRae a n d J. S. Watkins

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sediment loading of the underlying salt mass, the configuration of the basin and the rate of basin subsidence. More recently, Jenyon (1985)invoked a similar mechanism for the development of diapiric structures in the southern Permian basin of the North Sea. He suggests that the diapiric features described are due to the movement of Zechstein salt towards the basin edge in response to maximum overburden loading over the thickest salt in the basin centre.

Salt distribution Salt in the Destin Dome region is widespread and characterized by a comprehensive suite of salt

structures (Figure 5). Thcsc include salt rollers or salt anticlines, salt pillows, salt swells, salt diapirs and ridge-like features. Thin salt, generally less than 200 ms thick (two-way travel time), characterizes the basin west of approximately 86.5°W, which includes the Mississippi-Alabama shelf and the deep water slope area of the De Soto Canyon (Figure 5). Structures include small salt swells, salt rollers, salt anticlines, a large north-west trending salt ridge and a cluster of prominent salt diapirs concentrated near the head of the De Soto Canyon. East-west trending, low relief, broad, elongate anticlines or rollers are found on the Mississippi-Alabama shelf. Basinward dipping growth faults occur on the seaward flanks of these structures.

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Evolution of the Destin Dome: G. MacRae and J. S. Watkins minimum accumulation thickness of the 'mother' salt of Thicker salt is found along the shelfal areas east of at least 760-900 m. The inferred minimum salt approximately 86.5°W, offshore Florida. The average thickness is similar to the estimate of Salvador (1987) salt thickness is 300-400 ms. The salt is more for the north-eastern Gulf of Mexico before plastic continuous and sheet-like than salt further west. Along deformation and the original salt thickness in the the margins of the basin to the north and east, the salt Mississippi-Alabama-Florida area, offshore Alabama thins to less than 100 ms and onlaps the base of salt or and western Florida, interpreted by Lopez (1989). equivalent (BSE) surface (Figure 6). A minimum accumulation thickness of 760 m yields The dome and the flanking Destin Fault are evident an original volume of salt, within the area defined by in the salt isochron map (Figure 5). The Destin Dome is the heavy dashed lines (which contains 971.5 lease a large doubly plunging structure extending 80 km blocks) in Figure 5, of approximately 12 500 km 3 and a along strike and it is up to 35 km wide downdip. At its present day volume of salt of 11 500 km 3. Similarly, the crest, the dome is over 900 ms thick. Regional seismic volume of salt within the Destin Dome anticline, line 2 (Foldout 1) shows its broad anticlinal nature and circumscribed by the 400 ms isochron, is approximately the north-west-south-east trending Destin Fault on its 2300 km 3. An initial thickness of 760 m for the shelfal flank. The Destin Fault approximates the 'mother' salt would be equivalent to a volume of regional peripheral fault trend of the northern Gulf approximately 1000 km 3 within the present day Destin Coast basin and roots at the BSE surface. Dome. At its crest, the dome attains a maximum thickness in excess of 1820 m (Figure 5 and Foldout 1). Salt thickness and volume Hence, a significant volume of salt must have migrated Seni and Jackson (1983) suggest that the original into the Destin D o m e to account for this substantial thickness of salt in the East Texas diapir province can change in salt volume and its present day size. be estimated from uniformly thick, undeformed sheet-like salt within a basin. South of the Destin Evolution of the Destin Dome Dome, the salt is sheet-like, relatively undeformed and 300-400 ms thick (Figure 5). An average interval Structural and stratigraphic relationships suggest that velocity of 4.5 km/s for salt yields an estimated initiation and uplift of the Destin Dome anticline

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(A) Deposition of sheet-like Callovian salt; (B) U p p e r Jurassic s e d i m e n t a t i o n and early (minor) salt f l o w ; (C) evacuation of salt d o w n d i p and m o v e m e n t of salt f r o m the basin l a n d w a r d o n t o the present day shelf due to Lower Cretaceous differential sedi ment loading; (D) cessation of g r o w t h of the d o m e due to removal of the salt source by sedi ment t o u c h d o w n d o w n d i p . Progressive b a s i n w a r d tilt occurs f r o m (B)-(D). Bold a r r o w s in (C) and (D) indicate the relative m a g n i t u d e of differential loading. BSE = Base of salt or equivalent; MCSB = M i d d l e Cretaceous sequence b o u n d a r y . Vertical scale, 1.5 cm = 5 kin; horizontal scale, 1 cm = 5 km

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resulted from salt movement in Middle Cretaceous to Early Cenozoic time. The uniform thickness of the Upper Jurassic strata (Smackover Formation and Cotton Valley Group sediments) over the Destin Dome and the lack of faulting indicate that no significant movement took place during the deposition of these units (Foldout 1). Thinning and onlap of post-MCSB units, on the other hand, show that uplift and faulting were taking place during the Late Cretaceous and Early Tertiary. Pervasive small-scale extensional faulting over the crest of the structure offsetting shallower units further confirms the late growth and development of the dome, as suggested by earlier studies of Ball et al. (1982), Addy and Buffler (1984) and Ball et al. (in press). In the previous section, the minimum accumulation thickness of the 'mother' salt was estimated to be 760 m. At its crest the salt-cored Destin Dome has attained a thickness in excess of 1820 m. Studies by Trusheim (1960), Jackson and Seni (1983) and Seni and Jackson (1983) show a progressive increase in the structural maturity of salt towards a basin centre (characterized by the growth of diapiric salt structures), which parallels an increase in the original thickness of the salt layer. The failure of the Destin Dome to develop into a mature salt diapir is attributed to its location on the modern continental shelf near the basin margin where the initial deposition of salt is interpreted to be thin or absent (Figures 5 and 6). Hence some tectonic or sedimentary process must have initiated the movement of salt onto the Florida shelf. Thickness variations in the Lower Cretaceous sequences of the north-east Gulf of Mexico basin (Corse, 1987) suggest such a mechanism. Figure 7 schematically illustrates the interpreted growth history of the Destin Dome. Middle Jurassic salt is initially deposited unconformably on a horizontal BSE surface underlain by Triassic-Early Jurassic rift-fill sediments and thick transitional crust (Figure

7A). Along the margin of the basin, landward of the monoclinal 'step' in the BSE surface (Figure 6 and Foldout 1), the original salt deposition is thin and onlaps the BSE surface, whereas thick sheet-like salt accumulates further basinward. Upper Jurassic carbonates and clastics are deposited over the salt sheet in a slowly subsiding shelfal environment (Figure 7B). The sediment thickness is sufficient to load the salt and initiate low relief salt swells by gravitational flow downdip associated with slow but progressive margin subsidence. This early (minor) salt flow induces no significant deformation or thinning of the overlying Upper Jurassic succession. Thinning and onlap of salt onto the BSE surface and the absence of salt withdrawal features are prevalent around the shelfal margin of the basin. There does not appear to have been enough salt updip of the dome to account for the volume of salt observed. The source of salt for the growth of the Destin Dome must therefore originate downdip of the Destin Dome. Furthermore, the deposition of Upper Jurassic strata directly onto the BSE north and east of the dome (Foldout 1) effectively removes the influence of differential sediment loading. The thicknesses of the Lower Cretaceous sequences indicate that sediments accumulated in the central part of the north-east-Gulf of Mexico basin immediately seaward of the Destin Dome (Figure 8). We infer that loading by these sediments initiated updip salt flow to the north and north-east and downdip salt flow into the basin to the south-west (Figure 7C). The relative magnitude of differential loading by the Lower Cretaceous sediments (as indicated by the bold arrows in Figure 7C) is larger basinward of the dome because of the greater thickness of overlying sediments downdip of the Destin Dome. The monoclinal 'step' in the BSE surface probably restricted the movement of salt shelfward and locally controlled the development of the dome. Evacuation and withdrawal of salt downdip, in response to Lower Cretaceous sediment loading,

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E v o l u t i o n o f t h e D e s t i n D o m e : G. M a c R a e a n d J. S, Watkins

provided a source for the continued growth of the salt diapir field seaward of the Destin Dome in the De Soto Canyon and vicinity (Foldout 1). The growth of the Destin Dome is created by shallow extensional faulting over the crest of the structure and is evidenced by the thinning and onlap of Upper Cretaceous and Early Cenozoic strata onto the MCSB (Foldout 1 and Figure 7D). Later during the Early Cenozoic, sediment touchdown (Upper Jurassic strata rest directly on the BSE in response to complete evacuation and withdrawal of salt) between the anticline and the De Soto Canyon diapir field formed a salt weld and blocked further salt movement into the Destin Dome, resulting in the cessation of growth of the structure. Backstripped sections using seismic line 2 (Foldout 1) further confirm that the site of maximum salt withdrawal is from the 'weld zone', following the usage of Jackson and Cramez (1989), and show that the structure developed by downbuilding as salt moved updip into the dome under the influence of Cretaceous sediment loading. Figure 4 illustrates these concepts as the crest, midpoint and base of a salt structure are tracked through time. In Figure 7, the base and midpoint of the dome become deeper with time and so the dominant process is one of downbuilding. Furthermore, the crest of the dome is also deeper towards the present day, which indicates a component of subsidence superposed on the downbuilding.

Conclusions The original distribution of Callovian salt in the Destin Dome region of the north-eastern Gulf of Mexico is thought to have been sheet-like with an estimated minimum accumulation thickness of 760 m. Differential sediment loading by thick Lower Cretaceous sediments, and subsequent movement of salt updip onto the Florida shelf, appears to be the principal mechanism responsible for the uplift of the Destin Dome anticline. Structural and stratigraphic relationships within the Mesozoic and Cenozoic succession and salt volume estimates support this model. Most growth occurred during Late Cretaceous to Early Cenozoic time. The updip edge of the dome appears to be controlled by a monoclinal 'step' or change in relief of the pre-salt basement surface around the periphery of the basin.

Acknowledgements The results presented herein are part of the Gulf of Mexico Stratigraphic and Structural Synthesis Project, a joint research programme conducted by Texas A&M University and the University of Texas Institute for Geophysics. We especially thank the industry sponsors of this joint research project for their technical and financial support. Special thanks to Digicon Geophysical Corporation for permission to publish key sections of their speculative seismic data. Ray Martin and Jake Hossack provided helpful comments and criticism. 508

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