Godzilla vs Bambi When Sheets and Domes Collide

Godzilla vs. Bambi: When Sheets and Domes Collide Bureau of Economic Geology Mike Hudec – AGL Mary Cubanski, Carrie Decker, David Quintanilla, Susan Lock-Williams, Shkelqim Muskaj, Greg Plate, Michael Sorgenfrie – Veritas Hampson-Russell AGL - 2006

Abstract Bureau of Economic Geology Thinning of autochthonous salt towards the downdip edge of the northern Gulf of Mexico Basin produced a belt of domes that is currently being overridden by salt sheets sourced from further updip. The roofs of these domes are deformed by the overriding sheets. Three styles of roof deformation have been observed: • Roof remains in place and relatively intact. This is favored by a strong roof and minimal sheet override. • Roof is transported, but remains relatively intact. In some This is favored by a strong roof with large-scale sheet override. • The roof is dismembered and transported. The remains of the dome may be seen as small fragments scattered along the base of the sheet downstream from the dome. The is favored by a thin, weak roof and large-scale sheet override. AGL - 2006 2

Interactions Between Sheets & Domes Bureau of Economic Geology Sea-floor Bathymetry Slope Map, NE Gulf of Mexico Domes • In the northeastern GOM, the Sigsbee Canopy has not yet overridden the edge of the underlying salt layer. • Sheets sourced from further updip are overriding domes sourced from thinner salt near the basin margin. 3 km AGL - 2006 3

Sigsbee Canopy About to Override a Dome Bureau of Economic Geology 3 D PSDM Image along Sigsbee Escarpment W Overriding sheets 1, 500 Depth in m Dome E • The dome lies in a reentrant along the Sigsbee Escarpment, so is being overridden from two directions. • The dome roof will comprise the suture once the salt bodies merge. 15, 000 Seismic data © Veritas Marine Surveys AGL - 2006 10, 000 m 4

Interactions Between Sheets & Domes Bureau of Economic Geology Approximate edge of deep salt 50 km • In the north-central GOM, the Sigsbee Canopy has advanced beyond the edge of the underlying salt layer. • Do a series of overridden domes lie beneath the Sigsbee salt Canopy? How can we distinguish them from canopy feeders? What happened to the dome roofs? AGL - 2006 5

Base-Salt Structure of a Plug-Fed Sheet Bureau of Economic Geology Time 1 Section Map Basal cutoff 1 marks sheet outline at Time 1 Base-salt flat Base-salt ramp Time 2 0 1 1 Basal cutoffs 2 Section line • Salt sheets climb upsection away from the feeder, forming a funnel-shaped base of salt. • Base-salt cutoffs define the advance history of a salt sheet. AGL - 2006 6

Mapping Sutures on Base of Salt Bureau of Economic Geology Map View Section view Suture • Sheets climb upsection to meet at a suture. The climbing cutoffs form concentric rings around the feeders. Junctions between concentric patterns mark sutures. • If base of salt has not been deformed, the suture is a high in the base of salt, and the location of youngest subsalt cutoffs. AGL - 2006 7

Base-Salt Map in Deepwater GOM Bureau of Economic Geology Dome 3 Dome 2 Dome 1 10 miles AGL - 2006 • Lows in the base of salt are feeders. • Base salt contour indicate that several of these did not extrude significant sheets. • Interpreted as domes that were overrun by sheets from further updip. 8

So Where are the Sutures? Bureau of Economic Geology NW 3 D PSDM Seismic Traverse across Two Overridden Domes SE Depth in feet 7, 500 Dome 1 Dome 2 32, 500 Seismic data © Veritas Marine Surveys 50, 000 ft • There are no continuous intrasalt reflectors near either dome interpretable as sutures. • Either the dome crests were exposed at the time of overriding, or the roofs have been removed. AGL - 2006 9

Overridden Dome Scenario 1: Roof Deformed but Not Transported Bureau of Economic Geology Overridden roof Overriding sheet Dome Suture defined by base of salt Tim Dooley • When the amount of overriding is modest and the roof of the overridden structure is strong, the roof remains connected at its updip edge. • Diagnostic criteria: – Intrasalt reflectors connect to suture defined by structural high and reversal of cutoff climb on base of salt. – Roof typically distorted into an hyperbolic shape by the weight of the overriding sheet. AGL - 2006 10

Example of Dome with Untransported Roof Bureau of Economic Geology 3 D PSDM Image W E 1, 500 Base-salt suture Depth in m Minimal roof transport ? Dome 3 10, 500 Seismic data © Veritas Marine Surveys 10, 000 m • This dome lies near the edge of the canopy, so has been only moderately overridden. • Its roof is therefore largely in place. AGL - 2006 11

Overridden Dome Scenario 2: Roof Transported but Still Intact Bureau of Economic Geology Transported roof Overriding sheet Suture defined by base of salt Dome Tim Dooley • When the amount of overriding is much larger and the roof of the overridden structure is strong, the roof may be bulldozed by the overriding sheet, but remain otherwise intact. • Diagnostic criteria: – Intrasalt reflectors define a continuous event, but this does not connect to a suture defined by structural high and reversal of cutoff climb on base of salt. – Roof typically distorted into an hyperbolic shape by the weight of the overriding sheet. AGL - 2006 12

Example of Dome with Transported Roof Bureau of Economic Geology Base-Salt Map W 3 D PSDM Image E 7, 500 R oo fb lo ck Depth in feet High in base salt Weld 20, 000 ft 10 miles 35, 000 Seismic data © Veritas Marine Surveys • Diagnostic criteria of transported roof block: – Low-amplitude seismic character, because shale-prone. – Possibly discordant to subsalt strata, because base is welded. – Cores a high in the base of salt. AGL - 2006 13

Overthrusting of Dome 2 Roof Bureau of Economic Geology Oblique View of Base of Salt, Looking South Roof blocks Dome 2 Sa di lt f re lo ct w io n • The roof has been pushed against the downslope side of the feeder. As a result, it has been welded against the SE flank of the dome. • The upslope part of the roof has been pushed down into the dome, but the downslope part has been thrust up into the overriding salt. AGL - 2006 14

Imbrication of Dome 2 Roof Bureau of Economic Geology Base-Salt Map N S 4, 500 Depth in m 10, 500 10 km Seismic data © Veritas Marine Surveys 5 km • The roof of Dome 2 broke into three large pieces as it was pushed SE against the dome’s wall. • The force of the advancing salt caused these pieces to partially override one another, creating an imbricate geometry. AGL - 2006 15

Overridden Dome Scenario 3: Roof Dismembered Bureau of Economic Geology Overriding sheet Dome Suture defined by base of salt Roof fragments Tim Dooley • When the amount of overriding is large and the roof of the overridden structure is weak, the roof may be dismembered. • Diagnostic criteria: – No continuous intrasalt reflectors in the area. – Small bumps along the base of salt, each cored by a piece of the roof. If large enough to be imaged, these bumps should be shale-cored, and floored by a weld. AGL - 2006 16

Example of Dome with Dismembered Roof Bureau of Economic Geology 3 D PSDM Seismic Traverse NW SE 7, 500 Dome 2 Transported fragments Depth in feet Dome 2 Transported roof fragment 32, 500 10 miles Seismic data © Veritas Marine Surveys 50, 000 ft • A series of fragment-cored bumps on the base of salt lie downdip from dome 2, recording the dispersal of its roof. • Other fragments to the NE are derived from other roofs. AGL - 2006 17

Why Did Domes 2 and 3 Behave Differently? Bureau of Economic Geology NW Dome 2 Roof Fragment SE 1, 250 ft Dome 2 W Dome 1 Transported Roof E 6, 000 ft VE x 2 Seismic data © Veritas Marine Surveys 20, 000 ft • The roof of Dome 1 was much thicker and stronger than the roof of Dome 2 (the seismic sections are at the same scale). AGL - 2006 18

Conclusions Bureau of Economic Geology • • AGL - 2006 Thinning of autochthonous salt towards the downdip edge of the northern GOM salt basin produced a belt of domes that are being overridden by salt sourced from further updip. Three styles of dome-roof deformation have been observed associated with overridden domes: 1. Roof is in-place and relatively intact. Favored by strong roof and minimal sheet override. 2. Roof is transported, but relatively intact. Favored by strong roof with large sheet override. 3. Roof is dismembered. Favored by large sheet override of a weak roof. 19