M. P. A. Jackson, D. G. Roberts, and S. Snelson, eds., Salt tectonics: a global per- ... of sediments above a detachment layer of salt or shale is common on pas-.
Cobbold, P. R., P. Szatmari, L. S. Demercian, D. Coelho, and E. A. Rossello, 1995, Seismic and experimental evidence for thin-skinned horizontal shortening by convergent radial gliding on evaporites, deep-water Santos Basin, Brazil, in M. P. A. Jackson, D. G. Roberts, and S. Snelson, eds., Salt tectonics: a global perspective: AAPG Memoir 65, p. 305–321.
Chapter 14
Seismic and Experimental Evidence for ThinSkinned Horizontal Shortening by Convergent Radial Gliding on Evaporites, Deep-Water Santos Basin, Brazil Peter R. Cobbold
Dimas Coelho
Géosciences-Rennes Université de Rennes Rennes, France
Petrobras Exploration Department Rio de Janeiro, Brazil
Eduardo A. Rossello Peter Szatmari L. Santiago Demercian
Géosciences-Rennes Université de Rennes Rennes, France
Petrobras Research Center Cidade Universitária Rio de Janeiro, Brazil
Present address:
Departamento de Ciencias Geológicas Universidad de Buenos Aires Buenos Aires, Argentina
Abstract Thin-skinned gravitational gliding of sediments above a detachment layer of salt or shale is common on passive margins. Changes in surface slope result in a domain of extension upslope and a domain of contraction downslope. Contractional domains tend to occur under present-day deep water and are thus not well understood. In the deep-water Santos Basin, Brazil, a contractional domain contains a suite of salt-cored structures. Angular folds (chevron and box folds), as well as concentric folds, are common in the upper part of the Aptian evaporite sequence, which appears to comprise alternating layers. In general, angular and concentric folds form by flexural slip during shortening of mechanically layered sequences. Their occurrence in the Santos Basin is evidence in favor of horizontal contraction. The lower part of the Aptian evaporite sequence appears to be mostly rock salt. It has been squeezed out from under synclines into spaces created by growing anticlines. In places, the layered evaporite sequence has been thickened or even repeated across thrust faults and ramp anticlines. An overlying sequence of open-marine sediments has been condensed or eroded over anticlines but forms local depocenters. These depocenters are asymmetric (of foreland style) next to isolated thrusts but symmetric in synclines or between thrusts of opposite vergence. The structural styles have been reproduced in physical models, properly scaled for gravitational forces, in which salt is represented by silicone putty and sediments are represented by sand. The models were shortened horizontally by a screw jack. The experiments illustrate the importance of horizontal contraction and syntectonic sedimentation in shaping salt-cored structures. They have been used to establish criteria that may be diagnostic of contraction.
As a result of intense exploration for hydrocarbons, it has recently become apparent that thin-skinned deformation above salt layers is very common, even in stable tectonic settings, such as passive continental margins. Here, surface slopes are generated by prograding sedimentation or by basinward tilting of the entire margin, as a result of thermal subsidence or formation of new oceanic crust. If there is a layer of salt or soft mud at depth, the overlying sedimentary sequence can detach and glide
INTRODUCTION Many of the world’s hydrocarbon provinces are associated with evaporites. Impermeable layers of salt form good hydraulic seals for reservoirs, and they guide the migration of hydrocarbons. Because of their ductility and low viscosity, salt layers also provide detachment layers and form domes responsible for many structural and stratigraphic traps. 305
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downslope under its own weight. A slope of only a few degrees is sufficient. The detachment may be at a depth of up to several kilometers, and the gliding may occur over several hundred kilometers. Resulting structures may be surprisingly large and structural styles complex (see Jackson and Talbot, 1991). This has led to difficulties with seismic interpretation, especially before the arrival of modern seismic acquisition and processing techniques. Physical modeling has been of great help in investigating the mechanical origin of structures and in elucidating their styles. In a gliding thin-skinned sheet, a central plate may shift (translate) downslope with little internal deformation. However, as in plate tectonics, such a process will lead to divergence and convergence at the upper and lower margins, respectively, of the sliding plate. These margins may not be sharp everywhere. Instead, they may be domains of internal deformation (bulk strain)—extensional upslope and contractional downslope. The strain is expressed as characteristic structures, which may be salt cored. From physical modeling at the scale of an entire continental margin, Cobbold et al. (1989) have suggested that extensional and contractional domains may form where there are significant changes in surface slope, for example, at the upper and lower hinge lines of a margin or at the slope edges of prograding sedimentary wedges and deltas. Because extensional domains tend to be in proximal basin positions, such as onshore or under shallow water, they have been better studied than contractional domains. Structures characteristic of extensional domains are salt rollers in the footwalls of listric normal growth faults, salt walls (triangular in cross section) between intersecting conjugate normal faults, turtle anticlines, and salt welds (see Jackson and Talbot, 1991). Contractional domains due to gravitational gliding have been less well studied because they tend to occur in deeper water at the foot of a continental slope or a prograding delta front. Better known are contractional structures from orogenic areas, where fold and thrust belts may be underlain by salt (see Davis and Engelder, 1987; Jackson and Talbot, 1991; de Ruig, 1995; Harrison, 1995; Letouzey et al., 1995; Sans and Vergés, 1995). Nevertheless, contractional domains have been recognized along the passive margins of some continents, including the Atlantic margin of Africa (Lehner and de Ruiter, 1977), the southern margin of Australia (Wilcox et al., 1988), the southern margin of the Red Sea (Heaton et al., 1995), and the Gulf of Mexico (Weimer and Buffler, 1992). A well-studied area for salt tectonics is the Gulf of Mexico. Here, Buffler et al. (1979) described the style of folding and thrusting in the Mexican Ridges and considered the possibility that they formed as a result of gravitational gliding over Miocene shale. Worrall and Snelson (1989) reviewed structural styles throughout the Gulf, including deep-water contractional ones. Weimer and Buffler (1992) compared the styles of folds and reverse faults in three gravity-induced contractional domains of different ages: the Mississippi Fan, Perdido, and Mexican
Ridges fold belts. In the first two, the structures are cored by salt; in the Mexican Ridges, by shale. Concentrating on the Mississippi Fan foldbelt, Weimer and Buffler (1992) identified reverse faults having sediment wedges in their footwalls and asymmetric salt bodies in their hanging walls. On the Brazilian Atlantic margin, thin-skinned saltcored structures overlie a detachment layer of Aptian evaporites. Almost all authors have attributed these structures to gravitational gliding (Ojeda, 1982; Petrobras, 1983; Schaller and Dauzacker, 1986; Chang et al., 1988, 1992; Guardado et al., 1989; Dias et al., 1990; Figueiredo and Martins, 1990; Mohriak et al., 1990a,b,c; Pereira and Macedo, 1990; Carminatti and Scarton, 1991; Cobbold and Szatmari, 1991; Demercian et al., 1993; Mohriak et al., 1995). Contractional growth folds in the deep-water parts of the Campos and Santos basins were described by Cobbold and Szatmari (1991). In the Santos Basin, growth folds were identified on both dip-oriented and strike-oriented seismic lines, providing evidence for radially convergent gliding in variable directions perpendicular to the arcuate coastline. Salt-cored thrusts were identified on one regional seismic line across the Santos Basin, but this interpretation was debatable because of the poor quality of the unmigrated seismic image (Cobbold and Szatmari, 1991, their figure 9). In a more recent paper, Demercian et al. (1993) used time-migrated seismic sections and a regional structure contour map on the top of the evaporite sequence to provide further evidence for horizontal contraction in both the Campos and Santos basins. In the deep-water Campos Basin, trains of contractional growth folds have two dominant wavelengths, each proportional to total stratigraphic thickness at the time of buckling (Demercian et al., 1993, their figure 5). Salt-cored domes with sharp, straight overhanging edges were attributed to folding and synsedimentary reverse faulting (Demercian et al., 1993, their figure 6). More complex structures were attributed to compressional inversion of triangular salt walls, formed earlier and farther upslope as a result of horizontal extension (Demercian et al., 1993, their figure 3). Finally, drawing on unpublished results from physical modeling, Demercian et al. (1993) interpreted regional seismic lines with poorer definition, identifying a suite of contractional structures: growth folds, depocenters, reverse faults with sediment wedges in their footwalls, and an allochthonous tongue at the edge of the salt. The main purpose of the present paper is to provide seismic evidence for horizontal contraction in the deepwater Santos Basin, adding to the examples described by Cobbold and Szatmari (1991) and improving on them. A second purpose is to describe some experiments on horizontal contraction in layered physical models. The experiments help explain structural mechanisms and styles in areas of synsedimentary horizontal contraction. They also help to support some of the seismic interpretations of Cobbold and Szatmari (1991), Weimer and Buffler (1992), and Demercian et al. (1993).
Chapter 14—Thin-Skinned Shortening by Gliding on Evaporites, Santos Basin, Brazil
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7500000
Niteroi Ilha Grande
Santos
Cabo Frio
Rio de Janeiro 0m
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Ilha de Sao Sebastiao 00
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Slope
7250000
A
Shelf
B Paranagua
C
Abyssal plain
Sao Francisco do Sul
D 7000000
Itajai
Well 300000
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Figure 1—Location map of seismic lines A, B, C, and D (Figures 2, 3, 4, 5), offshore Santos Basin, Brazil. Contours show the approximate limits of the continental slope. Map coordinates are from the Brazilian National Grid (UTM projection).
SEISMIC EVIDENCE FOR HORIZONTAL SHORTENING IN THE DEEP-WATER SANTOS BASIN In the deep-water Santos Basin, Cobbold and Szatmari (1991) identified growth folds on both dip-oriented and strike-oriented seismic lines. They attributed the strikeparallel contraction to radial gliding of overburden in directions perpendicular to the arcuate coastline, which leads to a convergence of displacement paths along strike. A regional structure contour map on the top of the evaporite sequence provided further evidence for a pattern of radially convergent gliding in the Santos Basin. Salt-cored thrusts were identified on one seismic dip line across the basin. Here we reproduce four short seismic sections (Figures 2, 3, 4, 5) extracted from recent regional surveys. For proprietary reasons, the locations of these sections are approximate (Figure 1). Three lines run downdip, and the fourth runs along strike. They were chosen to complement the three sections previously published by Cobbold and Szatmari (1991, their figures 9, 10, 11). Thus, line B of the current paper (Figure 3) is the westward continuation of a previously published section (Cobbold and Szatmari, 1991, line C, their figure 11).
The seismic data were originally available as timemigrated versions, which we interpreted in the form of simple line drawings. For three of the lines, depth-migrated data subsequently became available and are reproduced here without interpretation (Figures 2, 3, 5). In the Santos Basin, Aptian evaporites (Alagoas Stage, Ariri Formation) may have been up to several kilometers thick when deposited (Pereira and Macedo, 1990; Demercian et al., 1993). Anhydrite dominates the evaporitic interval toward the western edge of the basin, whereas halite is more common offshore. However, the nature of the evaporite sequence is not well understood under deep water. Few wells have been drilled to date in the Santos Basin and none are in deep water. From our observations of seismic stratigraphy and structural style, we believe that the lower part of the sequence is mainly halite. In contrast, in the upper part of the sequence, seismic reflectors are evenly spaced, suggesting either regular contrasts in seismic velocity or a limited seismic bandwidth (Figures 2, 3, 5). However, since the latter possibility can be discarded, we infer that the sequence is layered. Where the sequence is folded, chevron folds, concentric folds, or box folds are the most common styles (Figures 2, 3, 4, 5). Layers tend to be uniformly thick around the folds, so we infer that folding occurred after deposition of the evaporite sequence. It is well known from physical (Text continues on p. 312)
Figure 2—Seismic section A, a deep dip line trending southeastward (to the right) down the northeastern part of the basin (see Figure 1 for location). (a) Timemigrated seismic section. (b) Drawing interpreting the time-migrated section and showing major evaporite units (patterned), bedding (thin lines), reverse faults (heavy lines with arrows), and a normal fault (heavy line). Vertical scale is in seconds (two-way traveltime). Numbers along the top refer to shot points. Folds and faults occur above a detachment layer with little internal layering interpreted as rock salt (dense L pattern). Sequence with fine bedding and angular or concentric folds is interpreted as layered evaporite (alternating dashes and L pattern). Notice small, nearly isoclinal fold (center) and thrust faults with ramp anticlines and dominant southeastward vergence. Anticline crests are eroded. Overburden thickness ranges widely from
