CANADUN JOURNAL OF EXPLORATION GEOPH”SICS “OLD 29. Pm 2 ,tECEMBER 19931, P 429~439
3-D SEISMIC EXPRESSION OF A CRYPTOEXPLOSION STRUCTURE’
J. HELEN ISAAC~AND ROEERTR. SSEWART~
Mclosh, 1989: Sharpton and Ward, 1990). Impact craters can AWTRACT he termed either simple or complex, the main difference being the presence of multiple ring structures and central An enigmaticcircular itr”Ct”ce is observedon three-*immsionai uplift in a complex crater. The morphological change takes (3-D, ,eimlic d&mfrom Jilmcs River. Alberta It hasan uuter diammr uf 4.8 km anda raisedcentral uplift mrruundrdby a ring ,ynform. place at an excavation cavity diameter of ahout 2 km in sedi- The Centraluplift hasB diameter“1 2.4 km and if6 crestappears to mentary rocks and 4 km in crystalline rocks (Dence. 1972). he aho”,400 m abovercgkml levels.The top of ihe StmClllr~ii ill B The principal feature of a complex impact crater is a central depth“f aho”, 4500,” and is belowtile LOW“f previousrc”n”“lic interest.Consequently. the featurehas ll”L beenpcnctrated by any peak, or group of peaks. surrounded by a llat tloor inside a wc,,s in the surveyarea. me seismic&lra inteqmation indicatr, terraced rim (Dencr, 1965). Complex crater central peaks are that thedimrbed scdimmtrare Cambrian in age.tt ii cstimilledIhat composed of def(mned and fractured rocks which arc uftcn the structure was formed during the Late CambrianIu Middle older than the country rock surrounding the structure. Partial Devonianlime petid andsuffered SeYcre W”bi,,” Mrw ,llCdrpositim “1 ,hCwcrlying Middle andUpper “ev”“ian carbonares.Rim fml,5, collapse of the central peak may he the source of some of the prohahiycaused by slumpingOf mmria, iill the dqmsion. are hreccia filling the surrounding rim nynform. observrd “” rhc ““tel slope of the synforrn. thversc faults are Without geological evidence it is difficult to establish the rrvealed by the seismicdata undernrathIhe feuture.The crnlral uplift appears1” havecoherent interm, rr,,rc,ionr andthe .illl”““l ot origin of a structure observed on seismic data alone. since uplift is *em to decreasewith increasingdepth. The whulr featurehas similarly shaped features can result from different causes. ,ile “wphologicalcharacteristics “fii ,“CIe”n,empact hlruclure. However, observed morphological and tectonic features such as central uplift. changes in structural uplift with depth or fault patterns. may provide compelling evidence for the origin of the structure. INTRODUCTlON Several enigmatic circular structures have been described Economic aspects of impact craters from observations on seismic data from the Western Canadian Mineral deposits such as metals and hydrocarbons are Sedimentary Basin (WCSB) (Sawatrky, 1976: Isaac and associated with about twenty percent of all known impact Stewart, 1993) and other parts of Canada (Scott and Hajnal, craters and in wme cases the economic implications are sig- 1988; Jensa et al., 1989). These structures are often imaged nificant (Masaytis, lY89). In some structures, interpreted to well on seismic data. Cryptoexplosion structures have a char- be impact craters, commercial hydrocarbon accumulations acteristic circular tu polygonal morphological outline and have been found (Table I: Sawatzky, 1972; Brenan et al., display evidence of violent disruption during their formation 1975: Cnrpenter and Carlson, 1992). The significance for the hut lack evidence of volcanic material to confirm a volcanic petroleum industry of impact craters as hydrocarbon-hearing origin. Cryptoexplosion structures can result from meteorite was discussed in detail by Donofrio (198 I ). impact or diatreme intrusion (Nicolaysen and Ferguson, The geophysical responses of impact structures were sum- 1990). Diatreme structures are thought to he caused by marised by Pilkington and Gricvc (1992). Several probable abrupt and violent exsolution of deep gasses and fluids hut impact craters have heen imaged with seismic data, use of arc nor well documented, geophysically. In contrast, the which allows good estimates of a structure’s size and age. 3- mechanics and morphology of impect craters have been studied D seismic data, in particular, afford three-dimensional and documented in great detail (e.g.. Roddy cf al., 1977: images unohtainahle by other methods.
1Presented at the 19th Annual National Mming. Canadian Ocophysical Union. Ranff, Alhena. May 9, 1991. Manuscript received by the Editor Augusl 10, IO%: ,CYi\Cd lnanurcnpt received November 1”. 1993. 2Depannient of Geology and Geophysics. The University of Calgary. Caigary. Alberta T2N IN4 The authon would like 10 ackn”wlrdfe Husky Oil “perations Ltd.. Calgary, for their generous donation Of the 3~D *risrnic &?,a, horehole (lilta and “SC Of computer facilities. In panicular, we would like to thank Mr. Murray Watt\. Mr. Lawrence Mewho* and Mr. David Emery of Husky Oil. Mr. Raymon*, Probat and Mr. Dean Simon. whu prcviouly interprcW* this intrresring feature on the 3~D data. shared thysir knuwlcdge with us. Vfe also thayk Ms. Susan Miller of the l?UWES Prujecr a! The University 01 Calgary fur data managemenr. The several anonymous rerlrrers are thanked for lhelr Eunst~UCtIve ideas. Tills WOrk was pantally Sup- ported hy the CREWES Project.
CEC 429 DrcEmher,991 J.H. lSAAC imt, R,R. STFNAKT
Table 1. Tabulation of somecommercial hydrocarbon accwn~lations associated with probableimpact craters.
‘iam 4ce Hydrucsrbuns * Jiewfield, SasknKhewar ,4 rriassic/ Commercinl oil field discovered in Iumssic ,968. Production of up 1” 6S &id (400 hhlid) from Mississippian carbonate hrecci:, and In-silu Mississippian in the ruised rim Pay thicknesses from 4 m to SO “1. Estimzed reserves: 3.2 x 10” mB120 MMhbl) recoverable. 0 = 14% k = 400 md. ted Wing Creek, Nortt :1 friassici Commercial oil field discwercd in Iakota limssic lY7?. X70 m of pay in Mississippian carhonxtc hrcccia rrom u 1.6 km diameter arra within ihc 6.5 km diameter centml Uplifl. Kcscrvoir rocks arc stesply dipping and intensely faulted. Estimated rescues: 6.4 x IOh- I x IO” nG(40 ~70MMhhl) ---.-rccwcrahlc. hvpone, North D;tkou , :nd Oil shows fwnd in I977 ill Camhro Zamhrian Ordovician sands draped over lhc raised rim. Some production from highly-fracrured Prrcxnhri;in gncisi- -- schist. \mes, Oklahoma Lower Oil and gas production from dolomite 3rdovician on crater rim and from hrecciated granite and dolornitc on crater floor. Estimatedpotential T~SCIYCS: ...-“-. - “-over7 x 106n+ (SOMMhhl). .“..“- iteen River. Alherr:! 2 pre Lntc Precambrian hasemcn, complex C~~IX~““~ uplifted 7h0 m above regional levels. Producing 95 m’id (fiXI bhlid) oil. - Also gas presentin the Slave Point.
Within the WCSB there xc two widely known str”ct”res impact could have caused the breakup of Gondwanaland and believed to be impact crz~lers: Steen River (Winzer, 1972) demonstrates the geological importance of this subject. and Eagle Butte (Sawat/ky, lY76: Lawton et al.. 1993). I” addition, several other str”ct~res in Alberta can be identified GEOI.OGICAI. SETTING on proprietary seismic data, including thawat James River. A 45.km’ 3-D seismic survey (Figure I) was acquired in The subject of impact cratering has significance beyond the James River area of Albert” (Twp. 34, Rge. 7. W5) in current economic implications. The well-know” extinctions 19X6 for Canterra Energy Ltd. (now Husky Oil Operations at a “umber of geological boundaries (e.g., Permian/Triassic. Ltd.). The survey was designed to image beneath the Cremceous~ertiary) may be the result of meteorite impacts disturbed Cretaceous section and to delineate an Upper (Alvarez cf al., 1980; Lampto”, 1986; Raup. 1991). The Devonian Lcduc carbonate structure observed on 2-D meteorite impact theory for mass extinction has rccrntly seismic data. The Leduc structure is at a depth of about gained momentum wilh the possible impact c~~tcr discovered -2800 m subsea (4050 m below ground level). The main at Chicxulub, Mexico (Hildebrand et al., 1991). A proposal hydrocarbon reservoir at James River is the Upper by Ohrrbeck et al. (1993) suggests that a giant asteroid Cretaceous Cardium Formation with the Leduc Formation
ClMi 430 3-D SElSMlC EXPRESSl”N OF A CKYl”lOtXPLOSrON
2 2 PASKAPOO
\ 1 mite - \ 16irn \ \ EDMONTON GP 3~0seismic ILlrw wme 0 seismicEeCIions A-A Tl”x slice 0 z Cryptoerpms~“”s!wuct”rB I \ 6 9 5 tj $ Ek Fig. 1. Outline 0, James River 3-D seismic survey with Iocations 0f BELLY RIVER seismic iines and wells. -f2 2 2 =- LEA PARK t; being an additional target. In this area, structural and strati- COLORADO GP graphic traps exist and major reserves of oil and sour gas 5 have been found nearby (e.g., Caroline. Harmattanj. A 2500 CARDIUL generalised stratigraphic chart for this part of southwestern Alberta is shown in Figure 2. Most of the wells in the area covered by the 3-D survey are shallow Cardium tests but three were drilled deeper. Mobil et al. James 8.14.34.07WS was drilled in thr project 3000 a: area in 1969 and penetrated 14 m of the Middlr Devonian MANNVILLE GP Elk Poinr Formation before T.D. at -3017 m subsea (4240 m 8 log depth). Drillstem testing of the Ireton-Elk Point interval produced gas to surface from the Leduc Formation, which MlSSlSSlPPlbN- was subsequently cored. The gas discovery was not commer- 3500 BANFF cial. Canadian Hunter et al. Ricinus 8-19.34.07W5 was 2 drilled in IYX2 10 test the Leduc Formation. It penetrated 2Y WABAMUN m of the Elk Point Formation before T.D. at -3164 m subsea (4444 m log depth). The 8-l’) well production tested gas WINTERBURN GP 4000 from the Mississippian Pekisko Formation and oil from the WTO Cardium Formation but was plugged and abandoned. Husky WOODBEND GP Caroline III-lh-34-07WS was drilled in 1990 to lest a strut- ture interpreted to be a Leduc reef. The well drilled 28 m into the Swan Hills (Beaverhill Lake) Formation before T.D. a.t -3012.5 m subsea (4274 m TVD). A Leduc reef with 3Y m of gross gas pay was encountered but the discovery was non- commercial.
SEISMIC DATA INTERPRETATION The 3-D seismic survey was acquired and processed by Geophysical Services Inc. (now Halliburton Geophysical Services Inc.). Summaries of the acquisition and processing parameters are listed in Tables 2 and 3, respectively. Fig. 2. Generalised stratigraphic colmn of southwestern Albelta.
CJEG 43, I,crc,,,brr ,Y’W I.H. 6AAC an* K.K. STEWAK,
Table 2. Acquisition parameters. ACQUISITION PARAMETERS
SOUKT Vihmscis 4 vibrators (TR4) sweepfrequencies IO 70 “7. I7 east/westlines SO0m apart shotpolnt I”ter”aI 80 m Receivers: 9 inlinr per group over 10 m 27 nonh/so”th ha 320 m apart group interval 80 m
Table 3. Processingflow.
Rotation to zero phase o~erator!en.~~~..R0..~s.._l..l m+?..msI -..--...“..l_.. 15 fold
40 x 40 m bin size
Although the high-cut trapezoidal filter frequency limits In order to identify the seismic retlectors beneath the Elk were 60/70 Hz, in the section of interest the highest signal Point, it is necessary to estimate the thicknesses of the Elk frequency was about 40 Hz. Point and the underlying Cambrian sediments. Three quarters of a circular feature about 4.8 km in diameter In this area of Alberta, the Middle Devonian Elk Point car- is observed deep in the section, below two seconds two-way bonates unconformably overlie elastic sediments of Upper time. Since the survey had been designed to delineate the Cambrian age (van Hees and North, 1964). James River is shallower Leduc structure it does not entirely cover the close to the western edge of deposition of the Elk Point deeper circular feature. This structure is below the zone of which, consequently, is very thin here. Shell et al. Caroline previous exploration interest and has not been drilled. 6.36.34-06W5, which was drilled about IO km to the north- Correlation of seismic markers with local geology indicates east of the project area, penetrated 22 m of Elk Point before that the structure is in the Cambrian section. encountering the Upper Cambrian. The sonic log from this Digitized sonic and density logs are available for the two well shows a sharp increase in velocity to about 7000 m/s at deepest wells, which penetrated the Elk Point Formation, and the top of the Cambrian, followed by a decrease in velocity are used to create synthetic seismograms. A Butterworth to about 5500 m/s (Figure 4). The well 8.1%34.07W5 had band-pass filter with frequency cutoffs of 15/20 35/40 Hz is penetrated 29 m of Elk Point, so the Elk Point Formation is applied t.o the synthetic seismogram. The synthetic seismo- assumed to be around 30 m thick here. An interval velocity gram from the well 8.19.34.07W5 and the seismic date of 6000 m/s is assumed, giving a two-way interval traveltime around that location correlate quite well (Figure 3). The peak of IO ms. At a depth of over 4000 m, a 30-m thick section is at 2145 ms is interpreted to represent the top of the Elk Point beyond the resolution of the seismic data so the Elk Point Formation (peak indicating a positive reflection coefficient). and Cambrian events cannot be identified separately. The
CJL” 432 Occcml’ir 1’9? i-0 SEKMC EXPRESSlON OF A CKYPTOEXPL”SlOII
WELL - SEISMIC TIE 8-19-34-07W5
SONIC SYNTH. SEISMIC 2000 W 2200
,160O i h MANNVILLE 3200 1700 ; NORDEGG PEKISKO 36OOa j ;
VELOCITY (M/S) $
Fig. 3. Correlationof seismic data with synthetic seismogramfor 8.19.34.07W5
6-36-34-06W5 SONIC LOG North, 1964). Lower Cambrian sediments were not deposited here; the Middle Cambrian carbonates and shales were AT (p/m) deposited directly on Precambrian igneous and metamorphic F$ z 5 basement rocks. Assuming interval velocities of 5500 mls for the Upper Cambrian elastics and 6300 m/s for the Middle Cambrian carbonates and shales gives approximate two-way interval -ELK POINT traveltimes of 55 ms and 125 ms, respectively, putting the _ 3975 top of the Precambrian as the peak at 2125 ms. E The 3-D seismic data are interpreted on a Landmark inter- F active workstation. The following events are correlated and -CAMBRIAN mapped: Mannville event, Top Cambrian (base Devonian). 4000 Intr;l-Cambrian and Precambrian. The Mannville event is picked to observe str~cf~re in the section above the feature of IIlImIllIlll1 interest, in this cast about 2650 m shallower. It is the first strong, continuous retlcctor above the zone of interest that Fig. 4. Sonic log from the well 6.36.34.06W5 showing 23 m of Elk Point above the Cambrian section, which has a 5-m thick high-velocity can be picked reliably across the survey area. The Intra- zone at WE top. Cambrian is picked to demonstrate the morphology of the structure under investigation and is not intended to represent Top Cambrian is picked, therefore, as the same peak as the a continuous reflector. In places this retlector can be seen to Elk Point top, at 2 145 ms on the seismic data. be truncated by the unconformity representing the end The Upper Cambrian sediments in the study area are pre- Cambrian-Middle Devonian hiatus. Across the central uplift, dominantly siltstones and shales while the Middle Cambrian the Irma-Cambrian event conesponds to the Top Cambrian sediments are calcareous shales and argillaceous carbonates. unconformity event. Regional geological maps show the Upper Cambrian section A northwest-southeast seismic line A-A’ (Figure 5) shows at James River to be about 150 m thick and the Middle clearly the Top Cambrian angular unconformity and the Cambrian section to be about 400 m thick (van Hees and truncation beneath it of dipping reflectors. The locations of
CJEG 433 Dcrctnbc, IYVi 0 gs 4000 Ei is- H z TOPcambrian 4500 g IntraCambrian
blue: peak WTtIO”gh 1 km
Fig. 5. NW-SEseismic line over the cryptoevplosionstwctwe with interpretedhorizons. The truncation of dipping events beneath the Top Cambrian unconformityand the eroded central uplift are *een clearly
seismic lines are shown on Figure I and on the time structure compressive forces, which could have been the result of the and iwchron maps (Figures X. 9 and IO). The dipping retlec- rebound of impacted material. There is a zone of very poor tars fkm synforms on either side of the central part of the seismic retlectivity between the dipping rrtlectors of the ring stwx”re, which appears to be uplifted. The amount of uplift depression and the Top Cambrian event. Such a seismic is seen to decrease to zero through the Top Cambrian to response might indicate a zone filled chaotically with brrccia. Precambrian interval. The Precambrian section appears to hr Thr circular nature of the structorc is not immcdiatrly relatively undisturbed underneath the central uplift, implying apparent on the seismic lines but is strikingly clear on the that Precambrian rocks have not been uplifted. The centrzill 2168 ms time slice (Figure 7). Successive time slices are uplift is interpreted to be composrd of Cambrian rocks. The used to observe the lateral movement of rrflcctors with depth thickness of this uplifted central portion is estimated to bc and to aid in the interpretation of the structure. about 400 m (120 ms traveltime). The uplifted areil appears After the horizons are correlated, severad time structure to have coherent internal reflections and can be seen to have maps are made. Some of the maps show clearly the circular suffered erosion prior to the deposition of the overlying shape of the strocturc and are illustrated here. At the Middle Devonian carbonates of the Elk Point Formation. Precambrian level there are indications of a circular structure On some lines in the survey, rim faults are evident. hut it is not known whether they are artifacts caused by Examples are shown on a north/south line B-B’ (Figure 6). anomalous interval velocities in the overlying central uplift. These rim faults appear to be present only within the struc- ture itself and do not affect the deeper section. Such faults Stncking velocities indicate slightly higher interval velocities are characteristic of complex impact craters and are prohahly for the central uplift compared to the ring depression but the caused hy slumping of sediments into the cavity. On this velocity data are not consistent. Lineations arc seen in the line, reverse faults affecting the Middle Camhrinn and southeast but they do not appear to reflect the major ~“nes of Precambrian sections can also he seen. These faults map in a deep Precambrian faulting that are often related to diatreme concentric pattern underneath the structure hut are confined StrUCtUreS. to the northwestern part. They arc thought to he contrmpora- The Intre-Cambrian time structure map (Figure 8) and neous with the formation of the structure and indicate Intra~Camhrian-Precambrian isochron map (Figure Y) are the 1 km blue:peak r*d:tm”gh
Fig. 6. N-S seismic line showing rim faults. deeper reversefaults and the possible breccia zone. as indicated by the poor seismic reflectivity, most significant. The circular shape and annular rim synfwm impact craters are obeyed. The diameter of the central uplift are clearly visible. The diameter of the entire feature is 4.8 of B complex impact crater, L)\,<, is related to the overall km and of the central uplift, 2.4 km. The circular shape evi- diameter, D, by Dy,, - 0.220, for impact structures on 811the dent on the Top Cambrian time structure map (Figure IO) terrestrial planets (Pike, 19X5). Using these relationships and might he a result of the selective resistances to erosion of the the extrapolated dips of the central uplift gives a preerosion circular diPping strata beneath the unconformity surface or of overall diameter of 6 km, with a central uplift 1300 m in differential compaction. At the Mannville level. later tectonic diameter and 600 m thick. events with a strongly linear northwest/southeast orientation are superimposed on the preexisting structure. OKIGIN OF THE STRUCTURE Three-dimensional seismic data can be usrd to visualise a Let us review geological processes which can produce cir- structure in different ways. The directions of dip of the Intra- cular StrlKt”reS: Cambrian went are calculated with Landmark software and a) Impact mapped (Figure 11) to give another three-dimensional image The morphology of the structure - its circular shape, ring of the structure. The grey-scale image plots the dip directions depression, fault patterns and central uplift - is very similar of the surface from north (white) tu south (black). This pic- to that observed at impact craters. The geometry corresponds ture shows clearly the geometry of the structure, particularly reasonably well with the scaling equations for impact craters the raised central uplift and the rim synform. although, since there has been considerable erosion of the It is clear that the structure was eroded prior to the drposi- structure, its original dimensions can only he estimated. [ion of Devonian strata so an attempt is made to estimate its Regional gravity maps only show regional trends and the original dimensions. The maximum original size of the fee magnetic maps available to us do not cover this area. The ture is estimated by extrapolation of the observed truncated observed decrease with depth of the structural uplift in the dipping events of the central uplift to a maximum preerosion core of the feature and the cohrrent reflections in the uplift height, assuming no change in dip. This gives a maximum tend to suggest an cnplosivr SOUI’C~from above rather than original uplift of 700 m and a maximum original overall below. diameter of 7 km. These values compare fairly well with The fall of a single meteorite is rare, since bodies entering estimates calculated such that scaling equations for complex the Earth’s atmosphere usually break up due to aerodynamic
(I;ti 435 Dw‘mhir 1YY W E IkIll Fig. 7. Horizontaltime slice at 2168 ms showing the circular plan of the beds stress, unless they burn up first (Melosh, 1989). Meteorite c) Volcano showers fall “ver an area known as the scattering ellipse, The conical shape of the central uplift is similar t” that of with the largest masses falling in the forepart of the ellipse a volcano but the internal reflections observed in the central and the smallest in the rear (Krinov, 1962). Observations on uplift would not be expected from the core of a volcano. additional seismic data ill the vicinity of the study area of There is no record in the literature of volcanic activity in more possible impact structures covering such an elliptical Alberta during the Cambrian, Ordovician, Silurian or Lower area would provide additional evidence for an impact origin. Devonian Periods. If this structure were a volcano, one b) Diatreme would expect the section t” be disturbed below the structure Craters associated with diatremes are caused by the explo- to a considerable depth, something which is not seen here. A sive release of highly compressed gasses and fluids by central distinct, deep rim synform, as observed here on the seismic venting. The proposed impact origin of such structures as data, is not generally associated with a volcano. Sudbury. Ontario and Mans”“, Iowa is disputed by some d) Salt dissolution authors (e.g., Nicolaysen and Ferguson. 1990; Officer and During Upper and Middle Cambrian times, the environ- Carter. 1991). Criteria for internally driven cryptoexplosion ment of deposition varied between shallow marine, with sttuctures include the alignment of a few such stmctwes on a elastic influx, and a submergent carbonate shelf. The sedi- lineament, since they are thought t” be associated with the ments are predominantly siltstones, shales, calcareous shales reactivatitln “f preexisting deep linear zone\ of weakness and argillaceous carbonates with no evidence of salt (van (Nicolaysen and Ferguson, 1990). There are no obvious deep Hers and North, 1964). Since there is no evidence for the linear faults in the Precambrian on the James River 3-D survey. existence of salt, salt dissolution is unlikely to be the cause Also, there does not appear to be a large Precambrian base- of the structure. Furthermore, salt dissolution generally leads ment uplift here, as seen on s”me structures interpreted as to funnel-shaped structures without a central uplift diatremes (Officer and Carter, I99 I ). (Anderson and Brown, 1992).
CEG 436 ,,eri,,+cr I’,‘)? I 0 St:l’;h,lt’ EXPIIISSION 01, A t’I~Yi’l~o~\,‘l~osIoN
Lines - I.. c - >- - ,- ,- - +- L” N 10 N 2x3 ru N -a3uwcvIoI-Jmru Ob-cuW*cnms.t~cDDh-NW*mco oo~,~o”ooo~~ooo”o”ooDDOOOOO
TWO-Way I30 I30 Time (ms) 2110
: 00 90 2 c
5 (1 fill
JO I 0
B - IU w &. m cr, 4 al UJ I- - C. *- +.- I- c I- - I- N a7 N r\, Z” N IV 0 0 0 c3 0 c3 0 c, ” 0 tl NWP~m-J~~~D.-NWP~ncn N “o”“ooDc>oooo”ooc3o t
Fig. 8. intra~cambrian ,ime~Str”Ct”re map. showing clearly me CirCUlar shape 0, me StwCt”fe.
Lirles c I- - - ,- - I- t--+-NNNNNroN cz+-NWP~Jv~~mcmc>,-NWPcn~ c2D~>0D000”-NW*~~-=mwOD”OOU”0000DDOOOO
130
90
50
ItI Nl B t - N w * “, cm -.I m u3 +- - *. I- - ,- - I- ,.A I- N n7 N a> n, rk2 LX> “0000000”~c,NWP~~~~m~~,-~“~~~~~ OD”“OO”OOa”“D0000
.- Ikm
Fig. 9. Intra~-Cambiian-Precambr,an i*OChlOn map
<‘I,,
Lines c I.. - e I- z.-. ,.. I- c - C” N ns 02 n2 m N _ ~ o Ic- ~ (pI ~ pI FD c, ,-. CL> w .P cn 53 -.I CD co c, +- N w I+ Two-way Time (ms) I 31:) IJO 2 2 G 1 !‘ N t Fig. 10. Tap Cambrian time-structure maps Ths unconformity surface reflects the selective resistances to erosion of the dipping strata beneath, Lines w I- b.. +- r c >- *- c I-. CL> N N 1v N N N ~,+.N~.~~cT,~~~Ds.-cuWPcn.m CNW*~m~mw00”“U0000”0~,““~~” c2c3c7oDo”“o 1x0 9 II 5U IO -NW~m~~Cn~,~-,--t-‘~--,--I--, cu 0, N co N N m N o”o~oooc3oot- 2,-J w P 01 a --I m co 0 e. ro w P cr. 53 t Dc,Oc?O”OC300C>caD”0”0 1 km Fig. 11. Ink-Cambrian dip-azimuth map showing the directions of dip of the intra-Cambrian event I I, ,, 438 ,~P,I,,IIIII ,‘/‘I/ 3-u SENWC EXPRESStON OF A CRYPT”EXPLOS,ON e) Shale plug Hrcna”. K.I... Pc,cr,“n. H.I.. an* Srni,h. H.J.. ,075. The origin of Red Wing Creek StI”Ct”le: McKenzie County. North Dakota: Wyoming Geol. Asin. Shale plugs are upwellings of shale and can look similar to Earth ski. ““Il. 8. 1.41. salt plugs, tending to be free of internal coherent reflections. crpcntcr. R.N. an* Carlson. R., IWZ, The A”lCl impact cra,cr: Okla. GC 0,. The observed central uplift appears to have internal reflec- Notes 52. 6. X8-223. “encr, M.R.. ,965, The ex,~a,~rres!,ia, origin 01 Canadian Cralerh: N.Y. tions. One would also expect to see B velocity pull-down AUd. sci. A”“. 123. ‘)‘a-900. beneath a shale plug, due to its lower velocity compared to IW2. The nature and si~nifiuance of wrewial impact \IIIIC~L,~C~: the surrounding calcareous material, and that is not observed Pn,c‘ZJlh lnlrmiil. cku,. Gong.. sect. 15. 77.89. D<,nofrio. K.K.. ,9x,. impact cwters: implicatims f,,l hsement hydrocar~ here. ho” production: J. PCIT. GWl. 3. 279-302. t] Limestone dissolution Hildrbrand. A.K., Fmlirld. G.T., Kring. LA.. Piikinpwn. M.. Camargc Limestone is present in the sedimentary section here but, L.A.. ,acobsrn, S.B. and Boynton, w,v.. I’)‘),. Chirruluh crater: 2 pos*i~ hle Crc,aocous~Terriq boundary im,,acr Cramer on ,hc Yucatan ~cninrula. as with salt dissolution, it is hard to find a model for dissolution Mrxic,,: Gcillngg I‘9 Xh7~X71. in an annular shape with the observed faulting and coherent Isaac. J,H. and stewin. R.R.. 1w1. 3-u scisnlic ch;lr;,ftelil;irio,, rof poisit,lc lnC,e”ri,c mpact Criltccs: Presented I’),h A”“. Nat. beg.. (‘an. ~;eophys. reflections in the central uplift. Most collapse structures Un.. Banff. Alhelm associated with limestone dissolution are either sinkholes or Jama. L.F.. Pe-Piper. c.. Rohrrrwn. F.B. and Friedsnseich. 0.. IW4. linear features (Jenyon and Fitch, 19X5). Montagnais: ri whmarine impact *~IUC~UIC on the Scotian Sheli. eastern Cmada: B”,,. GC”,. sue. A”,. 101.45”~463. Jenyon. M.K. and Fitch. A.A.. 1985, Scismii rrflectiw~ intrrprrtation: Ccwnpl.. Mono I, IIO. 8. Krinov. EL.. 1X12. Giant rnetwritcs: Pcrgmwn Press. Inc. The enigmatic anomaly observrd on the 3-D seismic data L;impkx. C.. 19X6. Mais e~tin~tion5: Franklin Wms. at James River has the morphological characteristics of a Lawon. LX:.. Stewan. ILK. and Ci~uk R.. 1W3, The gcophybical bigniiurr ,,I ,hC mlgie Butte impact CrawI: Plesantrd IYd, A”“. NX hog.. Can. meteorite impact structure. It has a circular shape, rnised Grophy\. Un.. Eh”fi. Alhem circular central uplift, annular rim synform and rim faults. h4asaytis. V.L.. 19x4, ‘The cc,m,mic geology Of impxl cm,cr\: t”lCrnhl. The amount of central uplift is seen to decrease with depth CLVO,. RC”. 31, 922~Y11. Mchh. IL.. iW9 Impact cr;llering: 3 geologic process: Oxford Lniv. and continuous retlections are seen within the uplift. The Press. 1°C. structure is confined to the interval between the Top Nicnlnym~ LO. and Fcrguion. J.. IWO Ccyptocnpiosion strucIure5. rhwk drformation and sidrmphiir concemiilim rclalcd 50 explnslve venring ot Cambrian unconformity and the top of the Precambrian, fhids aswciated with alkaline ultlamafic milgmar: Teclnnuphys~ 171. leading to the conclusion that the central uplift is composed 103~115. of Cambrian rocks. The structure is 4.X km in diameter and “herhck. V~R.. Marshal,, J.R. and Aggarwal. H.. lW3. tmpx,*. lil,ilCS. and the breakup ,>I Gondwanaland: J. ckut. I”L. ILI’~. has a central uplift 2.4 km in diameter which is raised about tmccr. (‘B an* cartel. N.L.. ,441, A lrYirW of the StrilClw~, prinhgy. 400 m above regional levels. It is estimated that the stmcture ;md dynamic delurmnlion chnracterislics 01 some cnigmaiic trccrstrial was formed during Late Cambrian to Middle Devonian time rnucturei: Fxrh Sci. Rev. 30. 1~49. Pik, K.J.. ,985. somr molpholofic ry\lcr”mic\ 0,~ compiex impart StmC- and suffered severe erosion before the deposition of the over- ,urcs: MC,e”ritiCS 2”. 4wlX. lying Middle and Upper Devonian carbonates. The disturbed, Pikington. M. and Grieve. R.A.F., 1992, The geophysical signature of eroded rocks in the central uplift and the breccia-filled rim miestriili impact craters: Rcr. Gcophys. 30. lhlK X I, Raup. D.M.. 1441. Extinction: h*d gcnei or had Id’?: Norlon and Co. synform could be exploration targets for hydrocarbons. R,>ddy. 0.L. Pcpin, R.0 and Merrill. K.“,. ,977. Edi., tmpac, antI cxpb There is structural closure and there could be fracture porosity rim crwring: Pcrgar”“” Prsrs, 1°C. Sawatrky. H.B.. 1972. Vicwulicld a pmducinp fossil ciatec: J. C;w kc. and permeability introduced into the central uplift due to Expl. tirophys. 8, 22-40. deformation and erosion. The structure is estimated to extend ~, 19x. TWO pcohable Iate Crrtilcrous as,r”hlemcs in wes,em canada from depths of 4500 m to SO00 m below ground level (3250 F..ilg,c R”,,C. Alhem and ““rm, Saskatchewan: CeophyricI 41. 126,~ ,271. m to 3750 m subsea). Scott. 13. and H;~jnd Z.. 1988. Scirmic signature of the Hau@ton stmct~r~: Mrtroritic5 23, 239~247. Sharpton. V.L. and Ward. P.D.. 1990, t+Js.. Global catastmphrs in Earth history: an intcrdisciplinar~ conference on impacts, vdciinim, ilnd miihr mumlily: Gcol. Sm. Am., Special Faper 247. van Hees. H. and North. F.K.. 1964. Cambrian. in McCiossan. R.G. and Glaisrcr, R.P.. Et.. Geological hiswry of svcscern Canada Ah Sot. P.Sr. tie,,,.. 2,).33. Winrsr. S.R.. 1972. The Steen River ar~mblemc. Alberta. Canada: Pmt. 24th ,“,cml. Geol. clmg.. sect. 15. 14X456. CIBG 439 mcenlhcr IYYl