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Fig. 2. (A) Location map and line drawing of the main faults and folds of the basin, mapped from a level close to the top of the Mina del Carmen Formation (after Figari et al., 1999). (B) Regional seismic sections. Extensional features dominate the Eastern Sector of the Basin (B—B’ seismic line) and the Western Sector (A—A’ seismic line), where subtle evidence of positive inversion is also observed. The E—W section (C—C’ seismic section) shows the San Bernardo fold belt in the center and the increase in thickness of the Cretaceous Chubut Group to the east (after Figari et al., 1999; Hechem and Strelkov, 2002).

Figure 2 (A) Location map and line drawing of the main faults and folds of the basin, mapped from a level close to the top of the Mina del Carmen Formation (after Figari et al., 1999). (B) Regional seismic sections. Extensional features dominate the Eastern Sector of the Basin (B—B’ seismic line) and the Western Sector (A—A’ seismic line), where subtle evidence of positive inversion is also observed. The E—W section (C—C’ seismic section) shows the San Bernardo fold belt in the center and the increase in thickness of the Cretaceous Chubut Group to the east (after Figari et al., 1999; Hechem and Strelkov, 2002).

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Fig. 1. Simplified scenarios of syn-tectonic activity and expected throw characteristics in cross sections and along strike of (A) basement-involved normal faults that propagated t¢ the sedimentary cover, and (B) syn-sedimentary normal faults. For syn-sedimentary faults maximum throw values are attained in horizon C, but maximum thickness variation: correspond to unit deposited on top of C. (C—E) Throw characteristics of faults showing different periods of fault activity and post-depositional reactivation of fault planes, assumins minimum strain during fault activity. In (C) early syn-sedimentary activity is reflected by thickness variation across the fault for unit (1), throw of post-tectonic units is similar. In (D syn-sedimentary normal faults occurs during deposition of units (1) and (2). Note that maximum throw values are attained at the base of a syn-tectonic succession (horizon A) anc reduces in B. Similar throw values in overlying units suggest passive accumulation of throw after deposition of the entire succession. (E) Syn-sedimentary faults that nucleate anc propagate downward during deposition of unit (1) show similar throw values in pre-tectonic horizons (A—B—C). generally formed by propagation and coalescence of numerous minor-scale fault segments, whose interaction and displacement distribution during the extensional process strongly control the coeval topography, lateral variability of accommodation and the routes of sediment to depocenters (Alexander and Leeder, 1987; Leeder, 1993; Withjack et al., 2002). Advances in understanding the process of fault growth and the sedimentary response to exten- sional tectonic has been largely documented by using three- dimensional seismic reflections datasets (Willemse et al., 1996; Dawers and Underhill, 2000; Morley, 1999, 2002a,b; McLeod et al., 2002; Baudon and Cartwright, 2008; Lohr et al., 2008) and a limited number of field studies (Dawers and Anders, 1995; Cartwright et al., 1995; Schlische et al., 1996; Gawthorpe et al., 1997; Trudgill, 2002). Systematical throw (or displacement) measurements along fault planes constitute an useful approach to unravel the spatio- temporal evolution of fault-bounded depocenters from available 3D seismic datasets, and is mainly based in regularly spaced mea- surements of current throw of selected seismic horizons, from which both interval displacements (or throw) and isothrow con- tour maps can be obtained (Morley et al., 2004, 2007). These tools allow inference of coeval topography, changes in the location and shape of depocenters and on the number of extensional phases that occurred during deposition of the involved sedimentary sequence (Fig. 1). The structural analysis, once complemented with vertical description of lithofacies and facies associations, facies distribution maps, thickness distribution maps and several geophysical tools that help to image channels and lobes, provide key elements to predict the location of potential oil reservoirs. Consequently, characterization of reservoirs and correct emplacement of wells in syn-tectonic successions requires understanding of the relationship
Fig. 1. Simplified scenarios of syn-tectonic activity and expected throw characteristics in cross sections and along strike of (A) basement-involved normal faults that propagated t¢ the sedimentary cover, and (B) syn-sedimentary normal faults. For syn-sedimentary faults maximum throw values are attained in horizon C, but maximum thickness variation: correspond to unit deposited on top of C. (C—E) Throw characteristics of faults showing different periods of fault activity and post-depositional reactivation of fault planes, assumins minimum strain during fault activity. In (C) early syn-sedimentary activity is reflected by thickness variation across the fault for unit (1), throw of post-tectonic units is similar. In (D syn-sedimentary normal faults occurs during deposition of units (1) and (2). Note that maximum throw values are attained at the base of a syn-tectonic succession (horizon A) anc reduces in B. Similar throw values in overlying units suggest passive accumulation of throw after deposition of the entire succession. (E) Syn-sedimentary faults that nucleate anc propagate downward during deposition of unit (1) show similar throw values in pre-tectonic horizons (A—B—C). generally formed by propagation and coalescence of numerous minor-scale fault segments, whose interaction and displacement distribution during the extensional process strongly control the coeval topography, lateral variability of accommodation and the routes of sediment to depocenters (Alexander and Leeder, 1987; Leeder, 1993; Withjack et al., 2002). Advances in understanding the process of fault growth and the sedimentary response to exten- sional tectonic has been largely documented by using three- dimensional seismic reflections datasets (Willemse et al., 1996; Dawers and Underhill, 2000; Morley, 1999, 2002a,b; McLeod et al., 2002; Baudon and Cartwright, 2008; Lohr et al., 2008) and a limited number of field studies (Dawers and Anders, 1995; Cartwright et al., 1995; Schlische et al., 1996; Gawthorpe et al., 1997; Trudgill, 2002). Systematical throw (or displacement) measurements along fault planes constitute an useful approach to unravel the spatio- temporal evolution of fault-bounded depocenters from available 3D seismic datasets, and is mainly based in regularly spaced mea- surements of current throw of selected seismic horizons, from which both interval displacements (or throw) and isothrow con- tour maps can be obtained (Morley et al., 2004, 2007). These tools allow inference of coeval topography, changes in the location and shape of depocenters and on the number of extensional phases that occurred during deposition of the involved sedimentary sequence (Fig. 1). The structural analysis, once complemented with vertical description of lithofacies and facies associations, facies distribution maps, thickness distribution maps and several geophysical tools that help to image channels and lobes, provide key elements to predict the location of potential oil reservoirs. Consequently, characterization of reservoirs and correct emplacement of wells in syn-tectonic successions requires understanding of the relationship
Fig. 3. Synthetic lithostratigraphy and paleoenvironments of the Golfo San Jorge basin. Ages according to Fitzgerald et al. (1990) and Bridge et al. (2000).
Fig. 3. Synthetic lithostratigraphy and paleoenvironments of the Golfo San Jorge basin. Ages according to Fitzgerald et al. (1990) and Bridge et al. (2000).
Fig. 4. Main structural features of the Cerro Dragon field. (A) Plan view map of the structure of the study area at top of the volcanic basement, in seconds two-way travel time, ticks indicate down-throw on faults. The color map highlights the NE—SW trend of the main basement-involved normal faults, and branched relay ramps of NW—SE orientation. (B) Isochron (thickness) map (ms TWT) of the MEC-A sub-unit of the Mina del Carmen Formation, and main fault affecting the stratigraphic level; ticks indicate down-throw on faults. The reactivation of the NE—SW fault system explain the location of some depocenters of the Mina del Carmen Formation, but the change in fault orientation from NE—SW to roughly E—W characterize the “second synrift phase”, likely developed during a regional change in stress regime from purely extensional to transtensional. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Main structural features of the Cerro Dragon field. (A) Plan view map of the structure of the study area at top of the volcanic basement, in seconds two-way travel time, ticks indicate down-throw on faults. The color map highlights the NE—SW trend of the main basement-involved normal faults, and branched relay ramps of NW—SE orientation. (B) Isochron (thickness) map (ms TWT) of the MEC-A sub-unit of the Mina del Carmen Formation, and main fault affecting the stratigraphic level; ticks indicate down-throw on faults. The reactivation of the NE—SW fault system explain the location of some depocenters of the Mina del Carmen Formation, but the change in fault orientation from NE—SW to roughly E—W characterize the “second synrift phase”, likely developed during a regional change in stress regime from purely extensional to transtensional. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. Interpretation of 3D seismic reflection data of the Cerro Dragon field (See Fig. 4 for line location). (A) 3D NNW-SSE trending random line illustrating the typical half-graben geometry of basement-involved normal fault system that defines the “main synrift phase” (labeled as 1). Fault development during the “second synrift phase” was active during the deposition of the Pozo D-129-Mina del Carmen Formations (indicated as 2); evidence of a “third synrift phase” during deposition of the Cretaceous succession is interpreted by the occurrence of high-angle normal faults active during the deposition of the Upper Member of Bajo Barreal Formation (labeled as 3), the later propagated upward to the Tertiary sequences. Notation hO—h6 refers to key seismic horizons employed in the kinematical analysis of faults (hO = seismic horizon near the top of Pozo D-129 Formation; h1 = Base of the Mina del Carmen Formation; h2 = Top of lithological sub-unit MEC-A; h3 = Top of lithological sub-unit MEC-B; h4 = Top of Mina del Carmen Formation; h5 = Top of the Tobacea Section of the Lower Member of Bajo Barreal Formation; h6 = Top of the Lower Member of Bajo Barreal Formation) (B) 3D WSW-—ENE seismic section illustrating the development of a faulting event (1) affecting the Neocomian succession (N) that do not propagated significantly to the overlying Chubut Group, and extensional—transtensional reactivation of NNW-SSE fault systems (2 and 3). Note as the fault (1) died out in the Las Heras Group (N) and only produces a fault-propagation fold in the overlying succession, instead in more central segments (see A—A’ section) new syn-sedimentary faults evolves upward. Key of figures: N = Neocomian sequence (Las Heras Group), D = Pozo D-129 Formation, M = Mina del Carmen Formation (with seismic sub-units MEC-A, MEC-B and MEC-C in ascending order), B = Bajo Barreal Formation (Upper Cretaceous). BTI = Basic Tertiary Intrusive rocks (mostly of Oligocene age).
Fig. 5. Interpretation of 3D seismic reflection data of the Cerro Dragon field (See Fig. 4 for line location). (A) 3D NNW-SSE trending random line illustrating the typical half-graben geometry of basement-involved normal fault system that defines the “main synrift phase” (labeled as 1). Fault development during the “second synrift phase” was active during the deposition of the Pozo D-129-Mina del Carmen Formations (indicated as 2); evidence of a “third synrift phase” during deposition of the Cretaceous succession is interpreted by the occurrence of high-angle normal faults active during the deposition of the Upper Member of Bajo Barreal Formation (labeled as 3), the later propagated upward to the Tertiary sequences. Notation hO—h6 refers to key seismic horizons employed in the kinematical analysis of faults (hO = seismic horizon near the top of Pozo D-129 Formation; h1 = Base of the Mina del Carmen Formation; h2 = Top of lithological sub-unit MEC-A; h3 = Top of lithological sub-unit MEC-B; h4 = Top of Mina del Carmen Formation; h5 = Top of the Tobacea Section of the Lower Member of Bajo Barreal Formation; h6 = Top of the Lower Member of Bajo Barreal Formation) (B) 3D WSW-—ENE seismic section illustrating the development of a faulting event (1) affecting the Neocomian succession (N) that do not propagated significantly to the overlying Chubut Group, and extensional—transtensional reactivation of NNW-SSE fault systems (2 and 3). Note as the fault (1) died out in the Las Heras Group (N) and only produces a fault-propagation fold in the overlying succession, instead in more central segments (see A—A’ section) new syn-sedimentary faults evolves upward. Key of figures: N = Neocomian sequence (Las Heras Group), D = Pozo D-129 Formation, M = Mina del Carmen Formation (with seismic sub-units MEC-A, MEC-B and MEC-C in ascending order), B = Bajo Barreal Formation (Upper Cretaceous). BTI = Basic Tertiary Intrusive rocks (mostly of Oligocene age).
Fig. 6. 3D seismic lines illustrating typical characteristics of the different fault types identified in this study. (A) Basement-involved normal fault (Fault Type 1) vertically linked to a downward propagating fault, the later syn-sedimentary with the deposition of Mina del Carmen Formation (Fault Type 2). Notice a well-defined forced anticline over the relay structure. (B) Basement-involved normal faults showing syn-sedimentary growth strata in MEC-A sub-unit (Fault Type 2). GP = growing package. The fault is soft-linked vertically with a fault that propagates downward from the overlying Bajo Barreal Formation (Fault Type 3) (C) Typical post-Mina del Carmen fault (fault Type 3). The current throw observed in Mina del Carmen Formation is post-depositional.
Fig. 6. 3D seismic lines illustrating typical characteristics of the different fault types identified in this study. (A) Basement-involved normal fault (Fault Type 1) vertically linked to a downward propagating fault, the later syn-sedimentary with the deposition of Mina del Carmen Formation (Fault Type 2). Notice a well-defined forced anticline over the relay structure. (B) Basement-involved normal faults showing syn-sedimentary growth strata in MEC-A sub-unit (Fault Type 2). GP = growing package. The fault is soft-linked vertically with a fault that propagates downward from the overlying Bajo Barreal Formation (Fault Type 3) (C) Typical post-Mina del Carmen fault (fault Type 3). The current throw observed in Mina del Carmen Formation is post-depositional.
Fig. 7. Throw distribution of Fault Type 1 (Fault F1), see location on Fig. 4. (A) Coherence-image at levels of MEC-A sub-unit. (B) Vertical throw contours using 371 measuremen presented every 5 crosslines along the fault length, with contours spaced every 25 ms TWT. Undulated to sub-horizontal lines are hanging-wall cut-off of seismic horizons hO—h (C) Graph showing the lateral throw distribution for seven of the key horizons. (D) Interval throw graph for the selected seismic horizons. The graph is the product of subtractin throw values of the shallower horizon from the deeper horizon. If the fault displays typical growth fault characteristics then the result will be a positive number and suggests fau activity during the time of the interval, or the area lies within a part of the fault that is dying out upward. Negative values are displayed as zero on the figure, and suggest that th. particular area of the fault only experienced displacement before or after deposition of the interval (cf. Morley et al., 2007).
Fig. 7. Throw distribution of Fault Type 1 (Fault F1), see location on Fig. 4. (A) Coherence-image at levels of MEC-A sub-unit. (B) Vertical throw contours using 371 measuremen presented every 5 crosslines along the fault length, with contours spaced every 25 ms TWT. Undulated to sub-horizontal lines are hanging-wall cut-off of seismic horizons hO—h (C) Graph showing the lateral throw distribution for seven of the key horizons. (D) Interval throw graph for the selected seismic horizons. The graph is the product of subtractin throw values of the shallower horizon from the deeper horizon. If the fault displays typical growth fault characteristics then the result will be a positive number and suggests fau activity during the time of the interval, or the area lies within a part of the fault that is dying out upward. Negative values are displayed as zero on the figure, and suggest that th. particular area of the fault only experienced displacement before or after deposition of the interval (cf. Morley et al., 2007).
Fig. 8. Throw distribution of Fault F2 (Fault Type 2) (A) Coherence-image at levels of MEC-A sub-unit. (B) Vertical throw contours using 174 measurements presented every 5 crosslines along the fault length, with contours spaced every 5 ms TWT. Horizontal undulated lines are hanging-wall cut-off of seismic horizons hO—h6 (C) Graph showing the lateral throw distribution for seven key horizons. (D) Interval throw graph. The case study fault reflects syn-sedimentary activity during deposition of upper levels of Pozo D-129 Fm., MEC-A and MEC-B sub-units of the Mina del Carmen Formation. Lateral behaviors at different stratigraphic levels are better depicted by using horizon throw and interval throw graphs.
Fig. 8. Throw distribution of Fault F2 (Fault Type 2) (A) Coherence-image at levels of MEC-A sub-unit. (B) Vertical throw contours using 174 measurements presented every 5 crosslines along the fault length, with contours spaced every 5 ms TWT. Horizontal undulated lines are hanging-wall cut-off of seismic horizons hO—h6 (C) Graph showing the lateral throw distribution for seven key horizons. (D) Interval throw graph. The case study fault reflects syn-sedimentary activity during deposition of upper levels of Pozo D-129 Fm., MEC-A and MEC-B sub-units of the Mina del Carmen Formation. Lateral behaviors at different stratigraphic levels are better depicted by using horizon throw and interval throw graphs.
Fig. 9. Throw distribution of Fault F3 (Fault Type 3). (A) Coherence-image at levels of MEC-A sub-unit. (B) Vertical throw contours using 174 measurements presented every 5 crosslines along the fault length, with contours spaced every 5 ms TWT. Horizontal undulated lines are hanging-wall cut-off of seismic horizons hO—h6. (C) Graph showing the lateral throw distribution for seven of the key horizons. (D) Interval throw graph.
Fig. 9. Throw distribution of Fault F3 (Fault Type 3). (A) Coherence-image at levels of MEC-A sub-unit. (B) Vertical throw contours using 174 measurements presented every 5 crosslines along the fault length, with contours spaced every 5 ms TWT. Horizontal undulated lines are hanging-wall cut-off of seismic horizons hO—h6. (C) Graph showing the lateral throw distribution for seven of the key horizons. (D) Interval throw graph.
Fig. 10. (A) Isopach map of Mina del Carmen sub-units with indication of main paleoflow directions as obtained applying visualization attributes of fluvial channels. Topography creation during deposition of MEC-B sub-unit is inferred due to major changes of thickness across fault boundaries. After deposition of Mina del Carmen Formation most of the relie: has been smoothed. Faults active during deposition of each sub-unit are indicated as continuous, red lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 10. (A) Isopach map of Mina del Carmen sub-units with indication of main paleoflow directions as obtained applying visualization attributes of fluvial channels. Topography creation during deposition of MEC-B sub-unit is inferred due to major changes of thickness across fault boundaries. After deposition of Mina del Carmen Formation most of the relie: has been smoothed. Faults active during deposition of each sub-unit are indicated as continuous, red lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 11. Inferred Cretaceous to Paleocene stress field in the Golfo San Jorge basin. Cretaceous data (A, B) are based in sub-surface information of the Cerro Dragon field, instead Paleocene stress field (C) is based in a compilation of outcrop data of Foix et al. (2012). Map pattern faults employed to indicate fault orientations in Bajo Barreal Formation were not shown for simplicity. The scheme clearly shows the counterclockwise rotation of fault orientations, being mainly a consequence of the rotation of southern South America following the Lower Cretaceous break-up of Gondwana.
Fig. 11. Inferred Cretaceous to Paleocene stress field in the Golfo San Jorge basin. Cretaceous data (A, B) are based in sub-surface information of the Cerro Dragon field, instead Paleocene stress field (C) is based in a compilation of outcrop data of Foix et al. (2012). Map pattern faults employed to indicate fault orientations in Bajo Barreal Formation were not shown for simplicity. The scheme clearly shows the counterclockwise rotation of fault orientations, being mainly a consequence of the rotation of southern South America following the Lower Cretaceous break-up of Gondwana.
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