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by: Korbin Gutmann


Korbin Gutmann

GPA 3.91

Karl Mueller

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Karl Mueller
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This 126 page Class Notes was uploaded by Korbin Gutmann on Thursday October 29, 2015. The Class Notes belongs to GEOL 3120 at University of Colorado at Boulder taught by Karl Mueller in Fall. Since its upload, it has received 33 views. For similar materials see /class/231938/geol-3120-university-of-colorado-at-boulder in Geology at University of Colorado at Boulder.




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Date Created: 10/29/15
Outline o Stress o Lithology Strength o Deformation styles Geologic time o Deformation movements o Attitudes o Scaling Relationships o Examples Stress Force Area Compressive Tensile ii Lx Shear gt 4 Lithology Most stiff Least stiff fluid Granite Dolomite Arkose Quartz Sandstone Greywacke Coarse grained limestone Fine grained limestone Siltstone Marl mudstone Shale Anhydrite Halite of rock Strength Depth oln addition to lithology strength is a function of pressure temperature preexisting weakness chemical reactions time Geologic Time Scale TRIASSIC Tz A PER N P P N 3 2 O N O a m z lt I D SILURIAN s 0RDOVICLKN O PALEOZOIC Pz Archean Proterozoic T A CREI ACEOUS K JURASSIC J CAMBRIAN Age ofMammals Age of Regulus Agcnf Amphihims Age of Fish Age of Mali nc lnvcni bum PREC AM BRI ANP 6 Pximi the aquatic plums D c at riu 313 as about 3 100 million 339eformaiiti0 n 3998 Du Cfivlie39 Brittle 39 39 397 Deformation Movements BEFORE AFTER Rigid Body Translation Rigid Body Rotation Change in Shape Change in Volume deis Planes Fau s Bedding Contacts Lines Slickenlines and grooves Mineral growth Piercing points 00 Maine Thrust 3C 0 A E 0 v 4 C D E D 0 E Q 2 39C q 0 C 4 O C D l L Length of fault trace or rupture cm Examples Faults o Folds Joints Dikes o Shear zones Extensional veins D kgs Shearzones San Gregario fault zone California Shearzones San Gregario fault zone California Geology 3120 Sedimentary Structures Outline o Review the geologic history exercise from last time Contacts primary structures and secondary structures How to determine which way is up Cross bedding graded ledding reverse graded ledding o Determining way up using top surface features ripples mudcracks raindrops footprints o Determining way up using bottom surface features load casts Determining way up using features within rocks geopetal bioturbation stromatolites flame structures pillow lavas Block model for exercise Geologic History 12 Ma dike 15 Ma dike Normal fault 20 Ma sed 22 Ma sed Erosion Thrust fault Folding Layer B 60 Ma sed Layer G 70 Ma sed 80 Ma sed Contacts Primary Structures amp Secondary Structures Contact a boundary between rock units Primary structure structures that form during lithification Secondary structure structures that form after lithificaiton Contacts Primary Structures amp Secondary Structures Contact a boundary between rock units Primary structure structures that form during lithification Secondary structure structures that form after lithificaiton Crossbedding Checkerboard Mesa Zion National Park Flow direction Younger Concave up Graded bedding Younger Older Decrease in depositional energy with sedimentation Example flood deposits turbidity currents Graded bedding Younger Older Decrease in depositional energy with sedimentation Example flood deposits turbidity currents Reverse Inverse Graded Bedding Younger mi 9 O 791 33 a w w Increase in depositional energy with sedimentation Example debris flows a lot less common than normal graded beds Oscillation water 62 Flow direction air or water Desiccation of muddy sediments Mud cracks Mud cracks Raindrops Limited to terrestrial sediments i lt J v Current direction Side view res i nat uraquotlquot carpenter s level Shell or cavity in the rock Matrix Infill material le calcite Stromatolites gtgtgtgtgtgtgt Sharks Bay Australia Cyanobacteria grow upward toward the surface uhquot udes into material above 1 Less dense material intr Caused by rapid loading of turbidite sands Pillow lava V notch References Slide 8 httpwwwutahpicturescomCheckerboardhtml Slides 915 17 Busch R M and D Tasa Laboratory Manual in Physical Geology 3rd Ed American Geological Institute and National Association of Geology Teachers 260 p 1990 Slide 18 httgwwwdiscoverwestcomauhablinhtml Slide 20 httQvolcanoesusgsgovProductsquossarvPillowLavahtml httpvolcanoesusgsgovProductsPglossaryancientseqhtml Geology 3120 Outline Listric Normal Faults Planar Normal Faults Grabens and HalfGrabens Missing Stratigraphy o Exercise 01 bisects acute 320 35 Cross section of Moab Anticline just north of Colorado River SOUTHXEST NO WHEAST M 0 quot Moob Moab ontlcllne 0 Fm Wes arm Ca0yon Feel ooo 5000 4000 4000 3000 Y 3000 2000 o X y 2000 0 I I 39 ODO 39 l 000 J 39 Sea level v PF L Sea level 39 Ph looo MOAB VALLEY SALTCORED V Iooo 39 ANTICLINE 39 2ooo pr 2ooo 3000 3ooo 39 pt I M 39 7 4000 39L M 4ooo 39 L tLPm 5ooo I L quot 39 T 5ooo 39 Highly coniuluml below 30001quot R Thin Iu39lltlnl depum nnl shown Morphology Halfgrabens Detachment fault Conjugate fault orientations Synthetic faults Listric fault Horst Halfgrabens Detachment fault lisTr39ic quot39sPODrIShapadquot fnuFT reduces in dip as if goes deeper quotFUN overquot an ricl ine Find the Listric Normal Fault Ea 332 25 5322623 m mamaquot mango Y V 391 quot A quotquot1 1 IUR o t rmWn 4 L1 vm w J 3 x son28v J 2 3EH Sh I a r sin uM wnE 39 A WINI 1 l gltll AKIInvylhvhKr a 4 K I k If I Il b nl rrlfl 4quot lI ns lmhmm thrhoxf k If 3 Yin I 151 osoww oEmww E Ban 9502 053 1 mamaquot 53226820 Halfgrabens Detachment fault Conjugate fault orientations Halfgrabens Synthetic faults Detachment fault Black Mountains Death Valley q Vljlan39gi39ng Wall Full Graben Half grabens Detachment fault Conjugate Horst fault orientations Half grabens Synthetic faults Main fault 3 Detachment fault x 39 may quota Thingvellir Graben Iceland 39Thmgu39elllr Igw abab I Hengill A an c Ocean Heimae Ve st manna eyjar ldfell Surts 23 El Pangaea Scm Th mgrve r mbm Edam Th mgrve r Graham Edam Normal sec on Missing Normal section section Drill Hanging wall Drill hole 1 xxxzxzxxxxxzx x Izrzx 11er xxxxxx Locate the Normal Faults Problem Salt Tectonics The following notes are abbreviated versions of Mark Rowan s threeday industry short course on salt tectonics Introduction Diapirs and diapirism Diapir initiation during differential Distribution and origin of salt basins loadmg Diapir initiation during extension or Mechanics of salt deformation contracuon Active and passive diapirism Salt withdrawal structures and welds Reactivation 0f diapirs during Turtle structures extens10n or contraction Expulsion rollovers Diapir interiors and margins Welds Neardiapir deformation folding and faulting Tectonic styles of salt deformation Thickskinned extension Collisional mountain belts INTRODUCTION There has been an enormous revolution in our understanding of salt tectonics in the past decade or so see Jackson 1995 for an excellent history of salt tectonics research The beginnings of the revolution date back a little farther within some of the exploration companies but their ideas only became public starting in about 1989 Our increased understanding of the geometry and evolution of salt bodies and associated strata is due in large part to the fortuitous convergence of advances in four areas Seismic imaging There has been a steady improvement in seismic data acquisition and processing over the years But with the advent of such techniques as pre stack depth migration images of salt bodies became much clearer with improved pictures of the bases of salt sheets and overhangs and the steep anks of many diapirs e g Ratcliff 1993 Experimental and numerical modeling Attempts to model salt deformation have been made for many decades eg Nettleton 1934 but until fairly recently both salt and its overburden were modeled as viscous uids Starting in the late 1980s however B Vendeville and coworkers started modeling the overburden as a brittle material more in keeping with the known mechanical behavior The results demonstrated salt s more passive role of reacting to rather than causing deformation e g Vendeville and Jackson 1992a b and fundamentally changed the ways most people look at salt deformation Structural restoration The technique of cross section restoration was first applied to salt structures in the late 1980s eg Worrall and Snelson 1989 In the past decade numerous people have used restoration to reconstruct the history of salt movement and associated deformation of surrounding strata Field studies Armed with new ideas various researchers have reexamined exposed salt basins throughout the world leading to improved understanding of the processes of saltrelated deformation In this course we will concentrate on the new ideas about salt tectonics Many of the illustrations used here are examples of the four areas of advance listed above Much of the work has been concentrated in the northern Gulf of Mexico but the impact of the advances has spread to salt basins worldwide Thus we will also examine the geometries and structural styles of salt from such places as the North Sea the Red Sea offshore West Africa offshore Brazil the Precaspian Basin and onshore Mexico DISTRIBUTION AND ORIGIN OF SALT BASINS Salt basins are found throughout the world Fig l but a quick look will show that they occur primarily in rift basins and along passive margins as well as in their deformed counterparts such as the AlpineHimalayan system According to a review by Jackson and Vendeville 1994 many salt deposits were formed during the early postrift phase including the basins of offshore Brazil offshore West Africa the US Gulf Coast and the Red Sea Fig 2 Others were formed either during rifting or during lulls between distinct rift episodes for example many of the basins on either side of the northern Atlantic Ocean Fig 2 Finally a few salt basins appear to be older than rifting namely those in the North Sea and Persian Gulf Fig 2 Many rift basins have a similar history one that is conducive to evaporite deposition They form during extension of the continental crust and grabens are initially filled with nonmarine clastics because of the high heat ow and associated regional uplift during rifting Subsidence of the grabens either during rifting or more typically during postrift thermal and loading subsidence leads to marine incursion If the climatic conditions are appropriate it is during the transition from nonmarine to marine environments that evaporites are formed often episodically In the typical but not universal scenario continued subsidence leads to true marine conditions Rift basins have a distinct basement architecture made up of grabens and halfgrabens segmented by transverse structures such as accommodation zones or transfer faults Fig 3 An example from the Brazilian margin is shown in Figure 4 where the rift system includes a series of NW SE oriented transfer faults The rift geometry coupled with the rate of sedimentation and the relative time of evaporite formation determines the areal extent and thickness distribution of evaporites Salt may be restricted to individual halfgrabens Fig 5 it may be regionally tabular Fig 6 or it may have an intermediate geometry with a regional distribution but significant thickness variations Fig 7 Salt commonly occurs in paired basins on either side of oceanic crust such as across the Gulf of Mexico the South Atlantic and the North Atlantic Thus it has generally been thought that evaporite deposition occurs only on continental crust with subsequent oceanic spreading separating a once contiguous basin into two parts Fig 8 Many passive margins have seawarddipping re ectors or SDRs Fig 9 7 wells show that these consist of subaerial basalts that are considered the initial expression of oceanic spreading Autochthonous salt in parts of offshore Brazil occurs at a stratigraphic level above the SDRs leading to a model in which salt deposition postdates the onset of oceanic spreading Fig 10 Salt in the South Atlantic offshore West Africa or Brazil is interpreted to occur above the breakup unconformity and thus is underlain by a combination of landward continental crust and basinward oceanic crust Fig 11 There is then a transition to shallowwater carbonates and then deeperwater facies The interbedded nature of the saltcarbonate transition and the similar seismic velocities means that there is typically a low acoustic impedance contrast and thus no good top salt re ector for the autochthonous salt layer In contrast the salt is usually in contact with underlying clastic redbeds or basement so that there is a good basesalt re ector 3 The evolution for the northern Gulf of Mexico is shown in Figure 12 Initial rifting during the Early Jurassic resulted in the SSW movement of Yucatan away from N America whereas oceanic spreading resulted in southern movement and rotation about a nearby pole This means that basement structures will have different orientations in the thinned continental crust and the oceanic crust The boundary between the two is interpreted to be in the approximate position of the presentday shelf edge so that most of the deepwater province is underlain by oceanic crust Fig 13 So instead of one salt basin on continental crust that was subsequently split by spreading Fig 12 there were most likely two salt basins with the downdip edges defined by the incipient spreading center Fig 13 Although most salt basins are closely associated with rifting salt deposits can form anytime conditions are appropriate for evaporite formation Thus any restricted basin with an arid climate is a potential salt basin The modern examples are the sabkha deposits of the Arabian peninsula but these are rare in the geologic record and similarly the types of salt basins common in the past are not observed today Warren 1999 One ancient setting for evaporite deposition is a basin with an open marine connection that gets closed off This will lead to the development of various evaporite facies as the basin essentially dries up Fig 14 Examples include basins with narrow entrances that become emergent during major sealevel drops such as the Mediterranean Messinian salinity crisis and the Red Sea Another example is when plate tectonic motions close off a basin as in the case of the Precaspian Basin during the Permian Fig 15 The Precaspian Basin example nicely shows the kinds of vertical and lateral facies variations commonly found in evaporite basins Although salt dominates the basin center there is also interbedded anhydrite that becomes more dominant towards the northern margin There can also be significant components of both carbonates and siliciclastics that are concentrated along basin margins Another example is provided by the central North Sea where nonsalt facies in the proximal part of the basin disappear towards the center Fig 16 Of course the presence of other lithologies means that there can be significant even coherent re ectivity within the evaporite layer Fig 17 om mo r39 I y k u 39 39 Iaull scdimenlary pmxm emu wnrld1Si John and others 1994 Giant Zrverdmp lHeanne d Arc SrCrand Banks1H avadox hhs nxas IZ lelv Lou siana M uil n 212ipaqwa31cabmda32 Gahon47 Aqmzame55Soumem Nnnh Sea h Nnnhw 5 German 57Nonhem Nonlv Sea SBTmmsa 51 DneprDunm sz th Caspian dwTadjik EEZagros 7usuez 71 Araman 72412 Sea East 74v0man n E Sungim brScwM Urwssissippl Tbrsabmas wPetencmapas 234mm ZsroviemerUcayaH 265mm Sumo Z7Cam Mamas 30 Kwanza 39E55auuiva AO Arlny AIFuhginn Ab Canmbnan AwanianSUrSovlhAdnancSW CarpalluanSZTvamyivan an erveaIKavu77iBoha1BayWruonaparle BOCanmng m Amadm Nnnpmduclivz Hmukcm SMnncmn uqu Bunk5 Bn1 mom Canyon HamHna 1575mm Texas 17 Slgsbee 200mm 21 min 24 armivmhas 26 May 19 Nuuquen n iibena 34Sene al hrCumums 35mm Somali 974mm mm Sea Wesl 13241gerlanrAilmran 43 naieanc 4 7am nninc SJUgurian SdRhodan ian 59Nordka bo Yenise Khninnga 55ltTabriz YazdKalm 5mm Kermnn grDead sea 73 Hadmamaui 754m Range mommy 7E hanghan BQrWonmough 8340 1cer 4 Flmders Figure 1 Locations of salt basins around the world Jackson and Talbot 1991 Fxgure z zammmuy 1921 mum mare asyrnmemcn gwmemes are sapasable DANAKlL DEPRESSION MODEL Holocene Volcano Mer eurquot mom so MI Lambarene Holst 30 Mi Fxgure 6 r L continuous tabular saltbody Parodon 1981 p vanable Lhc1mess because xtpama yfdlsmthz mum architecmreAdams198 9 Depth km Aptlan salt equwalent Postbreakup 5 Fall line Sea level COB v oceanic Subaerial volcanic crust Congggntal crust Abyssalplain Figure l 1 Regional cross section across Angolan margin showing salt deposition above both continental and oceanic crust Jackson et al 2000 5 1m North America pmm m teammates Fulani m Cam mm5 4 aw ha hve Pastpnsman V Vulcamsm w ldges mafpgl uem mu 95 55 an 75 l North 3 c pnsum day cumms llmlmc x New mg and nialm Ameri a Africa 1 2D 2D Seamuspmrd lmmghhzlz mm u Gulfv bm 39 mlmvSOAM pol Mum5cm North America 7 yum my 25mm I a an 75 Early Cretaceous 130 Ma Mama 1 as an 75 7a Figure 12 Presentday geometry of the Gulf of Mexico and three reconstructions to the Early Jurassic beginning of ri ing Late Jurassic onset of sea oor spreading and Early Cretaceous end of sea oor spreading Pindell et al 2000 I n V Figure 13 Map showing distribution of oceanic and continental crust in the Gulf of Mexico 7 note that most of the deepwater province is underlain by oceanic crust Reitz 2001 SalsmlmHy mappublnzunss quotamt 2mmquot sapachsa ar Smtthelal 1993 CJ 1WDquot lenhgtas ti Pemuansa Hahn mm I u cmmm Swamiastlcs Figure 16 Facies distribution within the Zechstein ofthe central North SeaStemrt and Claxk 1999 Seconds TWT Figure 17 Undeformed and deformed examples of the Zechstein showing interbedded facies 1999 ark and Variations in moblhty SteWaIt and C1 MECHANICS OF SALT DEFORMATION Rock salt is a very different material from other more typical sedimentary rock types There are several key factors that play roles in dictating its behavior First salt is much weaker than other lithologies under both tension and compression Fig 18 Even overpressured shale almost always has more strength than salt In fact the curve for wet salt in Figure 18 falls essentially on the axis of zero strength The reason is that salt deforms as a viscous material that effectively ows with ow rates up to 15m per year in exposed diapirs in Iran Talbot et al 2000 Flow is by a combination of Poiseuille ow due to overburden loading and Couette ow due to overburden translation Fig 19 The viscous nature of salt means that it forms a constantstrength albeit very weak layer between normal sedimentary layers whose strength increases with depth Fig 20 Thus salt serves as an excellent detachment surface into which faults sole Second salt has a constant density of about 22 gcm3 irrespective of burial depth it actually get less dense with depth due to temperature effects This is in contrast to other sedimentary strata such as sands and shales that become compacted during burial and thus become more dense Fig 21 Thus salt is more dense than its surrounding strata when it is near the surface but is less dense once it is buried beneath 10001500m of sediment The relatively low density of buried salt was historically used as a rationale for models in which salt punches its way through more dense overburden until it reaches its level of neutral buoyancy This might be valid if the overburden also behaved as a weak viscous uid but in fact the overburden is a brittle material with real strength Figs 18 and 20 The underconsolidated nature of shallow sediments in places like the northern Gulf of Mexico may suggest a viscous nature but this is incorrect In fact there are plenty of fault scarps at the sea oor that attest to the brittle style of deformation of even very young sediments Vendeville and Jackson 1992a argued that the strength and brittle nature of the overburden means that the density contrast plays only a limited role in salt tectonics Instead they showed that salt should be viewed as a pressurized uid and that it is the differential uid pressure that drives salt ow Fig 22 There are three key messages in this figure 1 density is only a secondorder factor 7 the scenario is similar whether the overburden is of lesser equal or greater density than the salt 2 if there is a differential load on the uid salt and there is a place for it to go salt will ow and 3 conversely the salt cannot just push its way into the overburden because of the overburdens strength In other words for salt withdrawal and diapirism to occur 1 there must be an open path to a nearsurface salt body 2 the overburden must be thin and weak faulted enough for the differential uid pressure to overcome the overburden strength in which case it m deform the thin overburden or 3 some other process eg tectonic must create space Put another way salt does not drive salt tectonics instead salt merely facilitates and reacts passively to external forces This is a key point that underlies almost all subsequent discussion 16 We Quanzrte Deplh 1 km an umes Figure 18 Strength ofvarious rock types in both tension and compression Jackson and Vendeville 1994 osno Ln Strength 8 Viscous salt A Fxgure zu Simple Krlayermudel nuns trustwALh aweak but cunstantrstxength saltlzyerbetween twu bntde layers whuse strenth increases mm depth Vaudeville and Jacksun 1993 SALTWITHDRAWAL STRUCTURES AND WELDS Differential loading induces a differential uid pressure that drives salt withdrawal and minibasin formation The initial load may have been induced by a variety of means extension of the salt contraction emplacement of a depositional lobe etc basin initiation will be discussed later In any case salt displaced from beneath the protobasin moves laterally into anking areas Fig 23 The dynamic nature of the salt ow results in bathymetric highs adjacent to the minibasin This sets up a feedback process where the minibasin is a low that receives further sediments39 this in turn increases the differential pressure driving further salt withdrawal and ow into the anking highs Fig 23 The process continues until the suprasalt minibasin touches down on the subsalt strata forming a salt weld indicated by pairs of dots As the minibasin grows the load increases compared to the thin overburden above the anking highs Thus withdrawal and subsidence rates gradually increase through time Fig 24 Note that subsidence is not driven by the sediment added during any short time interval but by the net differential load that exists at any time Thus the existing basin drives salt ow active sedimentation just adds to the differential load and is thus a secondorder effect Eventually as the source layer thins viscous drag forces inhibit lateral ow of salt and subsidence rates slow even though there is a large differential load Minibasin subsidence ceases once the weld forms Figure 23 Evolution of a minibasin subsiding into salt a initiation due to extension shortening deposition of a sand lobe etc b subsidence and growth of basin even in the absence of sediment input because of the differential load on the salt and c cessation of subsidence once weld forms 20 Turtle structures Although the center of a minibasin will stop subsiding once the weld forms the anks which are still underlain by salt may collapse forming new anking depocenters and inverting the original depocenter into a turtle structure Figs 25 and 26 It is uncertain why some minibasins collapse symmetrically to form turtles and others don t collapse at all It may have to do with the geometry of the wedge of strata above the anking salt if it is relatively long and thin it is mechanically easy to fold and thus collapse if it is a short thick wedge it will not fold as easily Another possibility is that extension plays a role Experimental models with no lateral movement resulted in simple minibasins whereas extension helped ank collapse and the formation of turtle structures Fig A classic turtle structure from the Precaspian Basin shows the weld with some remnant salt the initial depocenter that is now inverted the anking depocenters and the adjacent salt diapirs Fig 28 Note that crestal faulting due to bending of the strata is a common secondary feature Subsidence in this example started right after salt deposition as shown by the variable thickness of the oldest suprasalt strata Moreover the turtles in this basin are linear features along and parallel to the basin margin Fig 29 suggesting that extension may have played a role In other cases there may be a prekinematic section that represented condensed sedimentation on an in ated salt high before collapse and formation of the initial basin An example from the northern GoM Fig 30 shows that initial minibasin subsidence did not start until probably the Paleogene on the order of 100 million years after the salt was deposited Two maps from a nearby area show how salt was in ated from the Late Jurassic through the Oligocene reds and yellow in Fig 31a and then collapsed to form turtle structures in the Miocene blues in Fig 3 lb The age of a turtle structure is defined here as the age of touchdown of the initial basin and the start of ank collapse ie the yellow horizon in Fig 28 This age can vary significantly across a basin even between adjacent turtles The timing is dependent on when subsidence began the sedimentation rate and the initial salt thickness Thus one cannot simply correlate between turtles Moreover the presence of a turtle tells us nothing about facies there are of course turtle structures that have reservoirquality sands eg Thunder Horse in the GoM but there are also turtles around the world where the strata forming the turtle are composed dominantly of shale carbonate or even evaporites Generally once a basin is subsiding into salt it will keep subsiding regardless of the depositional setting and the minibasin will be filled with whatever sediment is available including slumps off the adjacent highs 21 Finure 2 39 touched down and formed a weld Figum 26 Experimmtal model ofa turtle structure eounesy ofBVendevi11e Early sediment deposition Early sediment deposition 397 i A I r Depocanier subsidence Dapucaniar subsidence and passive dlaplrlsm and passive diaplrlsm 7 5 Salt dapiellon andl depucantar grounding 14quot Salt depletion and deposentar grounding Unfolding of depeneniara Into t mile structu res Burial no further deformation 177 a Figure 27 Model results of minibasin subsidence showing no turtle formation in the absence of extension a and turtle formation triggered by extension b courtesy of B Vendeville 24 Figure 28 Example ofatunle s1ructure from the Precaspian Basin with an inverted central depocenter above a r rurt Also note that subsidence began immediately after salt deposition meters Frgure 30 Examp e ofa turtle structure from the northern Gulf ofMexrco wrth the same features as the prevrous H W r r r r r r r to eventual collapse andmlnlbasln forrnatlon data courtesy of GX Technology 26 Expulsion rollovers Another type of withdrawal structure is the socalled expulsion rollover which is essentially a halfturtle In expulsion rollovers the initial basin touches down and welds out just as in turtles but ank collapse is then asymmetric so that the depocenter shifts progressively in one direction forming a growth monocline Fig 32 As the weld grows in length salt is displaced basinward where it in ates and lifts a condensed overburden An example of an expulsion rollover from the Precaspian basin is shown in Figure 33 7 the yellow horizon marks when the weld first formed and minibasin growth shifted from the central depocenter to the ank collapse as salt was driven into an allochthonous body Another purported example is provided by the famous Cabo Frio structure in offshore Brazil Fig 34 whose proposed evolution is shown in Figure 35 However regional patterns of extension and contraction in the Santos Basin suggest that the Cabo Frio structure is actually a landwarddipping normal fault where the footwall moved basinward to create the accommodation An important point is that progradation must occur early in the basin history if it is to drive salt movement When progradational geometries form above a thin prekinematic section the differential load is great enough to drive subsidence and in ation Fig 36a If progradation is late however the net load differential is not as large and the overburden is thicker and stronger so that no deformation takes place Fig 36b 27 D sta Proxwma Slawed Dasm Pragradmg wedges mp 5 39 Fquot I 4 cmday Q m Easement 03 San thmkens and ng39adatmn D epucen e We ovevbuvden Cummues Proximal Vamts C mm WWW mm emml overs n m newnan p y c 9 Rem pHka Reackve mapw Monocline advances Expmswan ruHmer RaHuvev syncime M sanwem e Rmaleddowmap Rouoveyauvances 1 WWW mmeau 7 0 cm Rmaled down aps Samoa layer Prekmema claysv m Synkmemancwedges Figure 32 Model 39 quot 39 39 in ating salt plateau Ge et al 1997 I ionrollovei tructureinthePrec piax a39 39 gmechaincieri icchangefrom eldto thickening ontothewelt on innit thetiming ofinitial his I 1 Figure 3 Expl minning onto 18 i l touchdown and welding of the Northwesl Southeast a and plats amp vanauun gure 34 Twu secnunsthruughthe Cabu Fnu fault Zane uffshure Bram Suppumng mtezpretanun as asaltrthhdrawal feature Ge 81 a1 1997 30 Soulheasl 3 End Middle Alman Salt mlckens b End Cenomanlan d End Campanlan e End Maasmchllan 39 Passive ulaplr 0 End Paleocene dlslallV NFiotatedo A Salt Weld Non Progvadallon hwesl Helm pillow Salt reduction Figure 35 Model evolution ofthe Cabo Frio structure progressive evacuation in ates distal salt which eventually evolves into a diapir that gets buried Ge et al 1997 Depth km Welds Welds may have variable geometries They can form along the original antochthonons salt layer when the snbsiding overburden comes into contact with the snbsalt strata Fig 37 7 a nice example of a socalled primary weld is shown in Figure 38 7 or they can be inclined due to evacuation above a dipping base salt Fig 39 Welds are not always obvious as shown in Figures 40 and 41 7 they are identified by discontinuous highamplitude re ectors often with angular discordance between strata on either side but this can also re ect an unconformity or onlap surface The seismic character re ects the fact there are pods of remnant salt along the weld which are there because the top and base of salt do not have perfectly matching geometries Ultimately correctly identifying and interpreting welds requires a good mental image of the threedimensional salt geometry and its evolution over time Welds can also be vertical or secondary formed by the squeezing of a salt wall in response to updip extension Fig 42 A GoM shelf example is shown in Figure 43 where a vertical weld is indicated by a teepee structure beneath the landward edge of an allochthonous socalled tertiary weld However there are many cases where vertical welds are overinterpreted Teepee structures can form along strikeslip faults or normal faults that have been reactivated during shortening and many apparent vertical welds are simply migration artifacts below the edges of overlying salt bodies and minibasins An exposed vertical weld in La Popa Basin Mexico is illustrated in Figures 44 and 45 It is 25 km in length but is a true weld only over the southeastern half To the west it consists of continuous remnant evaporite 100200 m thick and in between it consists of patchy evaporite separated by true welds However this would not be known from a seismic profile because of the difficulty in imaging steep structures Another exposed weld this time from the Flinders Ranges in South Australia is shown in Figure 46 It extends upward from a triangular diapir pedestal formed above the autochthonous salt layer and separates two minibasins with very different thicknesses and facies The weld has remnant sandstones along it that were originally deposited within the evaporite sequence and is bordered on one side by a shale sheath 32 Primary gall weld Figure 37 Formation of primary salt weld as the overburden subsides into the autochthonous salt and comes into contact with subsalt strata Jackson and Cramez 1989 33 5 a A Figure 40 Time discontinuous D seismic from the Louisiana shelf With an allochthonous weld indicated by amphtude re ectors and structural discordance migrated 3 high 36 Figure 41 Interpretation of Figure 28 showing weld geometry Salt wall Shrinking mm 7 Decolllmenl fault all tongue e exmnsinn due mmln heave quotSquot s ghorkening due a reduction of stem xgure A 15 accummudated by squeenng ume all Well Jacksun and Cma 1989 DIAPIRS AND DIAPIRISM It used to be thought that density contrast alone was responsible for the initiation and growth of diapirs 7 the idea was that salt once it became buried deeply enough to create a density inversion first bulged into a salt pillow and then punched through to the surface Instead we now know that density is a secondary factor and that diapirs are triggered by a variety of mechanisms Diapir initiation during differential loading Simple differential loading can lead to the formation of diapirs We have already seen how the formation of turtle structures and expulsion rollovers in ates adjacent salt and triggers diapirism e g Figs 25 and 27 Another similar process is shown in Figure 47 where progradational loading causes localized in ation where the salt thins over basement steps A possible example may be in southeastern Mississippi Canyon where many of the diapirs are located above apparent basement steps Figs 48 and 49 and thus may have been triggered by differential loading and local in ation 38 Distal Proxxmal a V Semv salved basln Progradahun e D Slow aggmdallcn Sal EMlclme A4 4 Basement 0 Passive wall San anhclme Staned Sall sheel W Slow aggjada mn Truncaled cresq 9 Slow aggradanon Passive wall a 2a m Step 2 Step I Source layer l Preklnnmalm layer 27 Synkmemahclayers Figure 47 Model in which the original salt thins abruptly over basement steps these serve as nucleation points for salt in ation and diapir growth Ge et al 1997 Figure 48 Strike line in southeastern Mississippi Canyon GoM showing the Louann salt stepping down presumably over basement steps Rowan et al 2000a data courtesy of WesternGeco


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