Structure and petroleum potential of the Dagestan thrust belt ...

BULLETIN OF CANADIAN PETROLEUM GEOLOGY VOL. 42, NO. 3 (SEPTEMBER, 1994), P. 352-364

Structure and petroleum potential of the Dagestan thrust belt, northeastern Caucasus, Russia KONSTANTINO. SOBORNOV All-Russian Research Geological Oil Institute (VNIGN1) Shosse Entuziastov, 36, Moscow, 105819, Russia

ABSTRACT Exploration boreholes and seismic reflection data in the foothills zone of the northeastern Caucasus obtained during the last decade reveal considerable differences between the surface and subsurface structures of the area. The new data suggest that this zone may be viewed as a buried thrust belt. The allochthonous assemblage of the belt is formed mainly by stacked north-verging thrust sheets made up mostly of Mesozoic carbonates and sandstones bounded at the top and bottom by conjugate detachment surfaces. The thrust sheets are interpreted to be inserted into the clastic section of the Terek-Caspian foredeep along the base of Oligocene - Early Miocene mudstones. The blind subsurface thrusts have been active since the Late Miocene. Strata above and below the allochthonous unit are characterized by independent styles of deformation. The mildly deformed foredeep clastics create a hinterland-facing monocline that is passively uplifted by underthrusting. These rocks mask the subsurface structures. Tectonic wedging in the Dagestan thrust belt was facilitated by the mechanical weakness of the overpressured mudstones of the Maykop Formation (Oligocene Lower Miocene) which prevented transmission of the compressional stress across it. The interpreted geometry of the thrust belt front implies a shortening ranging from 20 to 50 km. This interpretation of the regional structure suggests a petroleum exploration play consisting of structural traps within the buried antiformal stacks. Regionally, oil- and gas-bearing Upper Cretaceous and Upper Jurassic carbonate rocks involved in thrust sheets and sealed by Maykop mudstones are considered primary prospecting targets.

R.g~SUMF..

Des forages d'exploration et des donn6es de rfflection sismique provenant de la zone des contreforts des monts Caucase du nord-est obtenus durant la dernifre dfcennie, r6v6lent des difffrences considfrables entre les structures de surface et de subsurface de la rfgion. Ces nouvelles donnfes suggbrent que cette zone peut &re envisagfe comme une ceinture de chevauchement ensevelie. L'assemblage allochtone de la ceinture est constitu6 surtout de nappes de charriage empilfes, situfes ~ la limite nord et composfes surtout de roches carbonatfes et de gr~s mfsozo'fques limitfs au haut et au bas par des surfaces de dfcollement de m~me direction. Les nappes de charriage sont interprft6es comme 6tant ins6rfes dans la coupe clastique de l'avant-fosse Terek-caspienne le long de la base des pflites de l'Oligocfne Miocene inffrieur. Les chevauchements de la subsurface sans affieurements ont 6t6 actifs depuis le Miocene supfrieur. Les strates au-dessus et en-dessous de l'unit6 allochtone sont caract6risfes par des styles de dfformation indfpendants. Les sfdiments clastiques 16gfrement dfformfs de l'avant-fosse crfent un monoclinal faisant face ~ l'arri~re-pays et passivement soulev6 par sous-charriage. Ces roches cachent les structures de la subsurface. L'insertion tectonique dans la ceinture de chevauchement Dagestan fut facilit6e par la faiblesse mfcanique des pflites en surpression de la formation Maykop (Oligoc~ne - Mioc6ne inffrieur) qui emp~cha la propagation de l'effort de compression h travers cette dernibre. L'interprftation de la gfomftrie du front de la ceinture de chevauchement laisse supposer un raccourcissement allant de 20 h 50 Km. Cette interprftation de la structure rfgionale sugg~re une rfgion d'exploration pftrolifre qui consiste en pifges structuraux au sein des empilements de structures anticlinales ensevelies. A l'6chelle rfgionale, les roches carbonatfes du Crftac6 supfrieur et du Jurassique supfrieur renfermant du pftrole et du gaz, impliqufes dans les nappes de charriage et scellfes par les pflites de la formation Maykop, sont considfrfes comme des cibles d'exploration primaires. Traduit par Marc Charest.

INTRODUCTION

manuscripts written in the 9th century. Commercial petroleum

The eastern Caucasus is one of the oldest oil and gas producing areas in the world - oil and gas seepages were found many centuries ago; they were mentioned in Arabian

prospecting by the A n g l o - R u s s i a n Oil C o m p a n y at the Kayakent field began in 1898 (Kuprin, 1959). For many years hydrocarbon exploration in the foothills was confined to 352

STRUCTURE AND PETROLEUM POTENTIAL OF THE DAGESTAN THRUST BELT, NORTHEASTERN CAUCASUS, RUSSIA

exposed anticlinal structures. Prolific Miocene sandstones and Upper Cretaceous carbonates at depths of 0.5-3.0 km were the most important targets. Although the study area was considered a mature exploration region, the largest discovery to date was made only recently, in the early 1980's, and new data indicate the presence of very attractive untested targets. Recently acquired information implies that notwithstanding its long history of petroleum exploration, this area has new, attractive structural plays. Re-interpretation of the structural geology of the area, constrained by new seismic and drilling data, shows prominent structural disharmony at different levels of the sedimentary section. This disharmony results from the extensive occurrence of buried thrust sheets of Mesozoic rocks which are regionally oil and gas bearing. REGIONAL FRAMEWORK

The Caucasus is a northwest trending, predominantly south-verging orogen located in the central part of the Alpine fold system (Fig. 1). Structurally it represents a deformed margin of the Scythian epi-Variscan plate which extends along the southern edge of the Russian platform (Zonenshain et al., 1990). Folding and uplifting of the Caucasus resulted from closing of the Tethys Ocean that originally separated Eurasia from Gondwana and subsequent collision of the continental blocks that began in the Late Cretaceous. In the Late Cenozoic, the continental collision and compression led to the subduction of the Transcaucasus back-arc basin. This, in turn, caused the uplift of the Scythian plate margin and formation of

~$7-~" 40dE

L

---------'7-7

i~~:.:.:.:~--c~casus

VL,~2,,.i::.::_

'~

I

~

~

I

~

~

_

~

400kin ]

..

NN~"~

AraiioaonE~te ~

".

-

the Greater Caucasus in the hanging wall of the subduction zone. The Caucasus is situated to the north of the northernmost tip of the Arabian re-entrant (Fig. 1). The northward incursion of the Arabian plate during the Late Tertiary - Quaternary controlled the structure of the northern margin of the Tethys (Philip et al., 1989, Zonenshain et al., 1990). The tectonic stress generated by the continent collision strongly affected the Caucasus, which lies just ahead of the leading edge of the Arabian plate. In this area, shelf sediments of the Scythian plate were subjected to intensive detachment faulting which resulted in the formation of the Dagestan thrust belt flanked to the north by the Terek-Caspian foreland basin (Figs. 2, 3). On the southeast, the thrust belt is bounded by the Samur dextral strike-slip fault, and on the west by the Argun sinistral fault (Sobornov, 1988). The latter separates the Dagestan thrust belt from the Terek-Sunzha fold zone. Seismic refraction data indicate thickening of the crust beneath the Eastern Caucasus, where a mountain root up to 57 km thick is inferred (Krasnopevtseva, 1984). The thickened crust of the Dagestan Highlands is marked by a pronounced negative gravity anomaly. This type of crust is considered to be a manifestation of the still active north-dipping subduction of the Transcaucasus plate below the Caucasus (Fig. 4), a suggestion which is supported by data on the Late Tertiary volcanic activity and seismicity of the region. The subduction zone is marked by a seismically active Benioff plane, which is traced to a depth of about 60 km (Zonenshain et al., 1990). According to the refraction data along the VolgogradNakhichevan refraction profile across the Eastern Caucasus and earthquake observations, the internal structure of the Caucasus crust is complicated by a south-dipping, seismicallyactive zone (Krasnopevtseva, 1984; Smirnova et al., 1985).

~ /

. :."•

Iranian

353

,,,..,g°,~ ,.,****o.o;'.... ,.~,:,~*.,.::. ~rtntf-2k.:Z,2.~,~.L~ff.~.~..,

~

]

~

Legend: 1 - continental crust, 2 - oceanic and intermediate crust, 3 major sedimentary basins, 4 - recent folding at the border of the Arabian plate, 5 - relevant motion of continental blocks with respect to Eurasia, 6 - recent volcanoes, 7 - major thrusts 8 - major strike-slip

50kin

~

~

Jurassic -Eocene

~ Oligocene-Miocene ,'~ Pliocene-Quaternary

faults.

Fig. 1. S c h e m a t i c p r e s e n t - d a y plate tectonic setting of the Caucasus and adjacent areas (modified from Philip, et aL, 1989). Line A-A' corresponds to the cross-section on Figure 4.

Fig. 2. Index map of the northeastern Caucasus showing the main tectonostratigraphic units. Numbered lines show the location of the photographs on Figures 6 and 7 , and the geological and seismic profiles on Figures 8 to 12.

K.O. SOBORNOV

354

Outcropping faults are of minor structural importance in this

47OE i

430N -

~ - . ,_

.~"-..~....~"""~.,.

~~;:

PM['I

............

oH an~ gas ~ie~~ syncline

Skamkhai.Bulak~-.'g~'~'.':.),~.

% BuynaksK.'~

,PE

Jurassic-Eocene Oligocene- M i o c e n e anticline

area.

".:° ~~Dimitrovsk" ~,,,,.~,-=, L°:;';

~

~ ~

.

'

-

~

'~ '

b'~J'z(al~']'; °~'~0~.¢:~

,."%.~.'! .,,~'~..,.""~..,.""~,...~ ~

Fig. 3. Location map of the fields and geographical names mentioned in the paper.

The waveguide of this zone is marked by a drop in P-wave velocity from 6.7 to 5.9 km/s, which could be accounted for by fractured rocks with fluids in them. The zone is interpreted to be a fault which merges with the subduction zone beneath the southern slope of the Caucasus, delineating a wedge-shaped, north-tapering upper crustal slab. It seems likely that shortening which has resulted from thrusting of the upper crustal slab was accommodated by corresponding ductile thickening of the lower crust. This process, along with subduction of the Transcaucasus plate, is thought to be responsible for the presence of the orogenic root below the Caucasus. LITHOSTRATIGRAPHIC U N I T S

Surface exposures in the eastern Caucasus include rocks ranging from Early Jurassic to Quaternary. A few deep wells along the northeastern slope of the Caucasus penetrated preJurassic clastics and carbonates (Gushchin et al., 1986). Paleontological studies of these rocks suggest a Late Paleozoic Triassic age; due to the scarcity of available information, the stratigraphy of the pre-Jurassic section is still poorly understood. Geophysical data show that the cumulative thickness of the sediments along the northern edge of the disturbed belt is approximately 12 km (Averbukh et al., 1974). Tertiary strata occur mostly along the northern flank of the belt, with progressively older beds exposed to the south (Fig. 2). These rocks form a system of foothill ridges composed of southverging thrust sheets, stacked in imbricate fashion. The appearance of Cretaceous and Jurassic carbonates and sandstones at the surface defines the Dagestan Highlands (Fig. 3). Geomorphologically, this area represents a plateau which stands from 1 to 2.5 km. The structural style of the Dagestan Highlands is dominated by box and gentle, open folds. -

Figure 5 outlines the stratigraphy of the study area along with the stratigraphic distribution of the main oil and gas reservoirs. The sedimentary section of the basin consists of three lithostratigraphic units: Lower-Middle Jurassic, Upper Jurassic-Eocene, and Oligocene-Quaternary. The Lower-Middle Jurassic sequence is represented by a southward propagating deltaic complex up to 15 km thick (Sholpo, 1978). This stratigraphic unit is an unconformitybounded sequence consisting of interbedded sandstones, siltstones, and mudstones with coal measures. The sequence contains good reservoirs and source rocks (Sokolov et al., 1990), but no commercial reserve of hydrocarbons has yet been found in this part of the section. A possible explanation for this could be a scarcity of seals and the past exploration strategy based on inadequate structural concepts which relied on concentric folding and coincidence of anticlinal crests at different stratigraphic levels. The Upper Jurassic-Eocene lithostratigraphic unit is formed by carbonate-dominated rocks, deposited on the passive margin shelf of the Scythian plate. Thickness of these rocks varies from 1 to 5 km. Fractured and vuggy carbonates of this complex, sealed by up to 2 km thick argilaceous rocks of the Maykop Formation (Oligicene-Lower Miocene) are the main oil and gas reservoirs for all the major hydrocarbon accumulations of the northern Caucasus. Aptian-Albian marine shales and Maykop mudstones are the source rocks (Sokolov, et al., 1990). The Oligocene-Quaternary section forms a clastic wedge up to 6 km thick in the foredeep basin. Seismic profiles across adjacent areas of the Terek-Caspian Basin reflect the clinoform textures of this complex (Kunin et al., 1989). It is noteworthy that in the lower part of the section f o r m e d by Oligocene - Middle Miocene strata, the clinoforms propagate to the south, while in the upper Middle Miocene - Quaternary section they propagate to the north. The change of the propagation direction of the clinoforms in the Oligocene-Quaternary section corresponds to the change in provenance of the sedimentary material. This implies that during the late Middle Miocene - Quaternary a large influx of clastic material was derived from the Caucasus and deposited in the foredeep basin. The lower part of the Oligocene -Quatemary foredeep fill is represented by the Maykop Formation, Oligocene - Early Miocene in age, up to 2.2 km thick in Buynaksk syncline. This stratigraphic unit, especially its lower part, consists mainly of mudstones and siltstones. Along the deformation front, these rocks are characterized by pronounced thickness changes and abnormally high fluid pressures. Field observations and borehole data display both thrusting and intensive folding of this section (Figs. 6, 7). The lower part of the Maykop Formation incorporates several olistostrome horizons that involve numerous Cretaceous and Paleocene - Eocene limestone and marls olistoliths (Fig. 5). Some of the allochthonous fragments are immense, i.e., up to a million cubic metres in volume (V.


355

South

North Kura basin

Greater C a u c a s u s

T e r e k . C a s p i a n basin

A'

o

A

-7. * . .- *.' ..:.: :.. ~. ..

•

.."

"*..\

*:.~ i,~. . . . . . . ~'

-" " " ." -....;.. ," -......_~-.'.~.x..~"..-..,".... ,L -~ ....~::.. ..... .%,,,.~..'.%:.....,,'..,7,..

*.,

"

"~

0

!iiiiDiiiiiiiiiiiiiiiiiiiil;iiiiiiiiiiiiiii!ii:ii::::

f ( i ~ ' : : + J ~ - ' ~ ~ i i 50 km

i i ! ! i i i i i i i i i i i i ii i i~i;t;~i;~ii!i~i~i ii i i iii i i ii i i i i i ! ? .

/// ~..~:.~;

.~.

oceanic crust sediments waveguide

.

.

.

.

.

.

.

.

.

.

50 km

.

50 km

Fig. 4. Simplified cross-section of the eastern Caucasus along line A-A' on Figure 1, based on geological and seismic refraction and reflection data.

Sharafutdinov, pers. comm.). Their origin is a subject of a debate that has lasted for several decades. Some researchers suggest that the olistoliths are the product of gravity slumping of young weak beds from intrashelf ramps (Brod et al., 1960; Markus and Sharafutdinov, 1989), while others provide convincing evidence for a more recent tectonic origin of these chaotic units (Shatskiy, 1929; Dotduyev, 1990). Investigations of the structure of the Maykop Formation, based on field observations and study of subsurface data, led to the conclusion that both gravity slumping and postsedimentary thrusting contributed to the deformation of these rocks (Sobornov, 1991a). It appears that some of the chaotic horizons were formed by erosion of the paleo-shelf during the prominent Oligocene sea level lowstand, while other horizons are due to later structural delamination along a weak zone near the carbonate-mudstone interface. The Oligocene - Lower Miocene section includes several horizons enriched by organic matter (Sokolov et al., 1990). These rocks are regarded as important source beds. The Middle Miocene part of the section includes highly porous sandstones interbedded with mudstones, and contains several dozen oil pools. The uppermost Upper Miocene - Quaternary sedimentary sequence lies uncomformably on the underlying Jurassic Miocene sediments. It consists primarily of poorly-sorted sands, conglomerates, and siltstones. Thickness of these rocks reaches 2.5 km in the axis of the Terek-Caspian foredeep (Nikitin, 1987). STRUCTUREOF THE DAGESTANTHRUSTBELT - EVOLUTIONOF CONCEPTS Unlike many known thrust belts, there are no significant exposed foreland-verging thrusts in the transition zone between the northeast Caucasus and the Terek-Caspian foredeep basin. The surficial structure of this area is characterized by the extensive occurrence of hinterland-verging or antiCaucasus thrusts. These thrusts cut the Miocen section and

form foothill ridges and hills. Typically, as mapped, faults of this kind are characterized by horizontal displacements of 200300 m, but the largest of them, such as the thrust mapped to the northwest of the Shamkhal-Bulak anticline (Fig. 3), have horizontal displacement of up to 2 km (Shatskiy, 1929; Brod, 1960; Mirzoev and Sharafutdinov, 1986). The origin of this thrusting from the undeformed foreland basin toward the disturbed belt has been a matter of discussion for decades. From the 1920s to the 1960s the majority of geologists treated these faults as so-called post-erosional thrusts (Kuprin, 1959; Brod, 1960). According to this concept, the thrusts were formed by the lateral movement of material from the foredeep to the deeply eroded areas of the Caucasus as readjustment of isostatic balance. It was also generally accepted that this deformation occurred only within the Tertiary clastic section, while the underlying competent Mesozoic beds were not involved in thrusting (Fig. 8a). This structural concept was based mainly on field observations and 1-2 km deep boreholes. For several decades exploration efforts in the Dagestan thrust belt were focused on the testing of anticlinal structures which are exposed at the erosional surface (Kuprin, 1959; Brod, 1960). The dominant geological concept at that time postulated that all exposed folds were concentric. It was generally assumed that all folds had deep roots and that the structural style of the Mesozoic section was basically the same as that of the Tertiary cover. As a result, by the end of the 1960s about 40 small- and medium-sized oil and gas pools, situated in 13 fields, had been found. Most of them are located at depths from 1 to 3 km (Mirzoev and Sharafutdinov, 1986). In the late 1960s - early 1970s a shortage of untested, exposed anticlines compelled the local, state-owned petroleum association (Dagneft) to drill deeper wells within the existing fields. This made exploration more costly, but at the same time it provided a wealth of new and important information. The new data showed that for its entire width of 20-50 km the Tertiary section is underlain by folded and thrusted Mesozoic rocks (Mussaev, 1968). It soon became apparent that the

K.O. SOBORNOV

356

~'~:;i~o~,-,~i~:".~'A~f

..~.'.~'~

Quaternary 1 km

Neogene

o c~ tm

Paleoger

t~

0

Cretaceou

Jurassic

,..-.-................;.;....................~:

:.:-:.:.:.:.:-:.:.:.:.;.:.:.:.:.:.:.:.:.:.:.:.:

:::::::::::::::::::::::::::::::::::::::::::::: . ...............T;T.%..,...,,

...........

:':'"~."."."?.'" : :i:::: : :i:!:::: ::::::::::::::::::::::::::::::::::::::::::: ,......-..°.....,.......................... ,-...°.......,....°.........° ............,.. ,..°...°......,.......°...........,....... °., ,,.....-..,.....°.....°.....................

':'1":':'1":':'5:.:.:.:.:.:.:.:.:.;.:.:.;.:.:.

1 12 P- 13 l Jl,

5

Legend: 1 - carbonates, 2 - sandstones, 3 - mudstones and shales, 4 - conglomerates, 5 - coal. Fi 9. 5. Composite stratigraphic column and stratigraphic distribution of oil and gas pools of the Dagestan thrust belt.

subsurface structure of the area was much more complicated than previously thought. Deep drilling at the Duzlak and Shamkhal-Bulak anticlines (Figs. 9, 10) and several other localities, provided evidence for the extensive occurrence of thrust structures in the Mesozoic section which lacked direct surficial manifestations in the overlying Tertiary section (Sharafutdinov, 1975; Sobornov, 1988). The newly obtained data indicated that the subsurface structure of the area is dominated by north-verging thrusts. The co-existence of the opposite verging thrusts in the Tertiary section and the newly established deep-seated thrusts in Mesozoic beds was puzzling. The conventional structural

concept failed to explain this phenomenon. Moreover, the scarcity of reliable data led to the co-existence of several conflicting and inconsistent hypotheses. For example, in Figure 8 (b and c), two geological cross-sections along the same line are shown. Authors of these cross-sections used the same data but their interpretations are strikingly different. The widely accepted structural concept used to explain the relationship between the folded and thrusted Mesozoic beds and the less deformed Tertiary cover was put forward by E G. Sharafutdinov (1975) (Fig. 8b). He suggested that the development of the Mesozoic-involved buried thrust belt took place during the Oligocene, well before the formation of the antiCaucasus thrusts in the overlying Tertiary section, which represents a younger generation of thrusts. The latter set of thrusts was considered to be independent of the deep-seated thrusting in Mesozoic rocks. The revision of the structural framework of the Mesozoic section, which contains the most important oil and gas reservoirs, opened new opportunities for exploration beyond established anticlines. Due to the lack of reliable geophysical data on the structure of the oil- and gas-beating Mesozoic carbonates, exploration efforts which focussed on deep targets resulted in the drilling of a significant number of dry holes. Since the adopted structural model suggested only a limited amount of shortening, without duplication of Mesozoic section (Fig. 8b), most of the boreholes were targeted to test the regionally oil- and gasbearing Upper Cretaceous - Eocene fractured carbonates just below Maykop Formation. Drilling showed that at many localities, the crests of the exposed anticlines in the Tertiary section did not directly overlie structural culminations in Mesozoic strata. Many unsuccessful wells bottomed in the overthickened Maykop section, which is characterized by abnormally high fluid pressures, before the oil- and g a s - b e a r i n g U p p e r Cretaceous carbonates were reached. The reasons for the dramatic changes in thickness of the Maykop mudstones beneath the exposed anticlines and the migration of the anticline crests with depth were not clear. Nevertheless, several hydrocarbon accumulations were found due to stepout drilling and the application of seismic reflection profiling. The most important result of this exploration stage was the discovery of the Dimitrovsk and Shamkhal-Bulak gas and condensate fields which are currently the biggest in the Dagestan thrust belt (Fig. 10). In these fields, gas and condensate occur in structural traps formed in the Upper Jurassic and Upper Cretaceous - Eocene carbonates. Total net pay thicknesses in these fields are up to 500 m. The producing zones lie at depths of 2.0 - 4.5 km and are comprised of fractured carbonates which have highly variable reservoir properties. For example, in the 6Dimitrovsk well, the Upper Cretaceous rocks tested gas at 1.5x106 m3/d with significant condensate, while in some adjacent wells only marginally commercial gas had been obtained (Mirzoev and Sharafutdiov, 1986; Sokolov et al., 1990). Since the subsurface structure of this area does not coincide with the surface geology, these fields could hardly be detected by the traditional methods of surface observations and shallow drilling. The discovery of these fields beyond the Tertiary


357

Fig. 6. Contorted lower Maykop beds, Buynaksk syncline. R~eferto Figure 2 for location.

anticlines was mainly due to new seismic data, which provided insights into the structure of deeply buried Mesozoic rocks. SEISMICEXPRESSIONOF THE THRUSTBELT Structural concepts applied to the Dagestan foothills have evolved with progress in seismic acquisition and processing techniques. Seismic CDP data collected and processed by the Grozneftegeofizika Geophysical Trust during the last decade contributed greatly to a better understanding of the structure of the Dagestan thrust belt. Figure 11 shows a south-north oriented seismic profile through the western part of the thrust belt which trends perpendicularly to the regional structural strike. Interpretation shown in this profile is constrained by surface mapping, borehole data, and nearby seismic lines. Four deformed seiSmic-structural packages, which sequentially overlie gently south-dipping Mesozoic autochthonous rocks of the Terek-Caspian foreland basin are clearly defined. Correlation of the reflectors with relevant boreholes, and geological mapping show these packages to contain JurassicEocene, Oligocene-lower Miocene, Miocene, and PlioceneQuaternary sequences, respectively.

The lowermost Jurassic-Eocene package is marked by high-amplitude, discontinuous, curvilinear reflectors between 1.6 - 3.5 two-way time (TWT) in the southern half of the section. Convergence and offsets of reflector sets within this carbonate package suggest that it incorporates individual thrust sheets which form an antiformal stack centered on the southern edge of the section. Seismic lines constrained by drilling data show that this seismic-structural package includes the lowermost horizons of the Maykop series and older Mesozoic rocks. The overlying Oligocene - Lower Miocene package contains generally short, weak, and usually chaotically oriented reflectors. This package is also characterized by prominent thickness variations. The most remarkable thickening of this package - up to 2.0 s TWT (normally 0.6-1.0 s TWT) - occurs along the northern edge of the underlying antiform (central part of the profile). This thickening is interpreted to be due to folding and thrust repetition of the Maykop mudstones. The thick, clayey-siliciclastic Miocene sequence is outlined by a band of continuous, strong, sub-parallel reflectors that dip northward between the surface and 2.4 s TWT. The reflection

358

K.O. SOBORNOV

IOM Fig, 7. A thrust fault cross-cutting lower Maykop strata, 1 km to the northeast of the Chirkey reservoir. Looking east. Refer to Figure 2 for location.

sequence of this package suggests that the Miocene monocline is cut by south-verging thrust faults, which sole into the Maykop mudstones. Extrapolation and projection of these faults updip to the surface coincide with the foothill ridges carried by thrusts. The uppermost Pliocene-Quaternary package occurs only in the northern half of the cross-section and extends from the surface down to approximately 0.5 s TWT. It is characterized by planar reflectors of varying amplitude that dip gently to the north. This package truncates the underlying monoclinal sequence of Miocene rocks indicating the presence of an angular unconformity between Miocene and Pliocene-Quaternary sequences. BURIED THRUST BELT

The spatial relations of the interpreted seismic-structural packages, and their internal characteristics, exhibit features typical of a triangle zone. These are: 1. the wedging of the deformed, deep-seated package between considerably less deformed over- and underlying ones; 2. the approximate correspondence of the monoclinal uplift of the upper structural unit to the thick, underthrust wedge; and 3. the presence of thrust faults of opposite vergence delimiting the allochthonous body and merging at its leading edge (Gordy and Frey, 1977; Jones, 1982, 1987; Muller et al., 1988; Sobornov, 1988, 1992; Sanderson and Spratt, 1992). The most conspicuous difference between well-known examples in the Canadian Cordillera, Alps, Urals and the northeastern Caucasus foothills is the almost complete absence of the foreland-verging thrusts at the surface in the study area. However, as was shown by Jones (1987), a few hundred meters of erosion will expose the foreland-verging thrusts. The integrated interpretation of recently acquired geological and geophysical information suggests a new structural interpretation for the D a g e s t a n foothills ( S o k o l o v and Sobornov, 1986; Sobornov, 1988, 1991a; Dotduev, 1990). According to the new data, the area can be viewed as a buried thrust belt that was formed by allochthonous thrust sheets

composed predominantly of competent Mesozoic rocks (Fig. 12). The thrust sheets split the foredeep terrigenous section and rotated the overlying Miocene beds. The allochthonous assemblage is bounded by upper and lower detachments which merge beneath the axis of the Terek-Caspian foredeep at depths of 3 - 6 kin. It is composed of a system of thrust sheets stacked on top of each other, forming a wedge-shaped, duplex structure (Fig. 11). Through correlation and extrapolation of surface and subsurface data it is possible to trace the deformation front for some 300 km along the Dagestan thrust belt. A portion of the M a y k o p section was sheared off its Mesozoic carbonate substratum during the formation of the allochthonous wedge and subsequently accreted to the wedge which led to the tectonic thickening of the Maykop Formation in front thrust belt (Fig. 11). Drilling data and field observations reveal a number of folds and small- to medium-sized faults in the Maykop section that distribute the large displacement accommodated by the underlying massive thrust sheets. In many outcrops, these rocks are highly contorted (Fig. 6) and cut by thrust faults (Fig. 7) in response to the relative movement in opposite directions of the overlying and underlying beds (foreland-vergent thrusting of Mesozoic rocks, and hinterland-vergent thrusting of Miocene strata). The suggested interpretation implies that the structural discordance of the competent Mesozoic and Miocene sections is accommodated by tectonic thickening of the Maykop mudstones. Seismic and drilling data indicate that the Mesozoicinvolved forelandward (north-facing) thrusts tend to flatten out at depth, and merge into a south-dipping bedding plane probably located somewhere in the Jurassic shales. Deep boreholes in the Varanda anticline in the western part of the Dagestan thrust belt (25 km to the west of the Chirkey reservoir) show that in this area, upper Paleozoic (?) carbonates and terrigenous deposits as well as Mesozoic rocks are involved in basinward thrusting (Gushchin et al., 1986). Figure 13 shows a proposed model for the structural evolution of the Dagestan thrust belt. This model is constrained by subsurface data in the thrust belt front and surface data in the


South

A

~

B

Paleozolc(?)

North

ShamkhaI-Bulak

359

5-7 km. These depths correspond to the positions of the interpreted buried wedge-shape thrust sheets (Levkovich and Asmanov, 1985; Zakharova et al., 1990). This seismicity may be evidence for contemporary active northward propagation of the wedge along the detachment planes. The distribution of earthquake foci also corresponds with the southern continuation of the interpreted detachment zone beneath the Dagestan Highlands. In this area, earthquake foci are concentrated along a south-dipping surface to depths of 30 km (Smirnova, et al., 1985). This implies that the peripheral foreland thrust belt may represent the leading edge of the upper crustal slab delineated by seismic refraction data (Fig. 4). This provides a possible method of displacement transfer from the suture zone in the southern flank of the Caucasus via the crustal slab bounded by an intercrustal shear zone for about 200 - 300 km along the northern edge of the Caucasus.

"

EVOLUTION AND TIMING OF DEFORMATION

C

2km

2krn

The structural position of the wedge-shaped thrusts is thought to have been controlled by prominent differences in strength and rheology among the stratigraphic units involved in the deformation. The heterogeneity of the layered sedimentary section may have prevented growing thrust faults within the Jurassic-Eocene sequence from cutting through the overlying strata to reach the surface. Instead, the thrust sheets were displaced along a weakened horizon within the overpressured Maykop section, causing complex folding and multiple thrusting. The formation of the triangle zone may have been facilitated by the good lubricating properties of the Maykop mudstones which enveloped the allochthonous wedge, minimizing friction along the thrust planes. In the overlying, relatively competent, sandstone-dominated Neogene beds, the shortening of substratum was accommodated in the most appropriate way - by passive uplift and by backthrusting up to the stressfree surface, along with bedding-plane detachment faulting. The simultaneous motion along the opposite-vergent thrust surfaces enabled the allochthonous assemblage to insert itself into the foredeep fill. The considerable heterogeneity in physical properties of rocks involved in thrusting can be illustrated by comparing their densities. The present density of Mesozoic units ranges from 2.42 to 2.84 g/cm 3 while that of Maykop sediments, as measured in the laboratory and not allowing for high pore pressure, is 2.38 to 2.44 g/cm 3 (Amirkhanov et al., 1972). This difference apparently was even greater some 5 - 4 Ma ago prior to intensive dewatering and density increase in the Maykop rocks. It appears that this factor, together with the obvious ductility contrast, prevented the relatively dense and rigid lower thrust sheets to reach the surface, and led to the emergent backthrusting along the mechanically weak Maykop strata. The above mentioned factors appear to explain why the lower horizons of the Maykop Formation have been mostly encountered beneath overthrust Mesozoic-involved sheets. Up to now, these data were regarded as the most important arguments in favor of Oligocene time of thrusting (Sharafutdinov,

1

Fig. 8. Cross-sections through the Sulak salient of the Dagestan thrust belt along the same line. Refer to Figure 2 for location. A - after Brod, 1960; B - after Sharafutdinov, 1975; C - after Zangiev, 1979.

hinterland. The initial predeformed thickness and composition of the presently eroded hinterland part of the section are based on regional reconstructions and on a study of vitrinite reflectivity of the Middle Jurassic coal bearing rocks (Sokolov, et al., 1990). The latter data indicate that the Middle Jurassic section experienced subsidence to a depth of 5 - 6 km prior to the late Tertiary - Quaternary thrusting and uplift. Geometric restoration of the cross-section shown in Figure 11 indicates that up to 50 km of shortening occurred in this part of the northeast Caucasus. The existence of up to 7 km of structural relief in the western part of the transition zone between the Caucasus and the Terek-Caspian foredeep basin resulting from low-angle thrusting is also indicative of the high amount of shortening in this area. To the southeast, compressional deformation becomes milder implying decrease of shortening. In the southernmost part of the study area, the interpreted geometry of the thrust belt suggests shortening of 20 km or less. The structural interpretation implying the tectonic wedging of the assemblage of thrust sheets into the foreland depression along the Maykop mudstone is supported by data on the seismicity of the northeastern Caucasus. A number of historical earthquakes and their aftershocks (some of them of large magnitude) occurred along the Dagestan thrust belt at depths of

360

~ 0 . SOBORNOV

DUZLAK

SOUTH-WEST

NORTH-EAST 0

2 3 4 5 6 KM

KM ~

L.-M.Jurassic

,~,. U. JurassicEocene

r-~! Maykop Fm. N

U.Miocene

~

M.Miocene PlioceneQuaternary

Fig. 9. Geological cross-section through the Duzlak anticline, southern Dagestan. Note opposite vergence and difference in displacements of the surface and subsurface thrusts. Refer to Figure 2 for location (modified from Sobornov, 1991b). 1975). However this assumption apparently conflicts with different pieces of evidence which imply a younger age of the m a j o r c o m p r e s s i o n a l d e f o r m a t i o n in the n o r t h e a s t e r n Caucasus. Seismic and sedimentological data indicate that in the Oligocene - Middle Miocene sediments were derived from the north, but not from the Caucasus to the south. These include the presence of south propogating clinoforms in Oligocene - Middle Miocene section in the Terek-Caspian foredeep (Kunin, et al., 1989), and the northern derivation of clay minerals of Maykop rocks (Sarkisian and Kotel'nikov, 1980). Therefore, the major episode of thrusting and subsequent uplift in the northeastern Caucasus occurred later, after deposition of these rocks. The late Miocene tectonic event is clearly manifested on seismic sections as a prominent sequence boundary. The boundary is marked by the onlap of the uppermost seismic package onto strata which correspond to the tilted monocline of the leading edge of the buried thrust assemblage. Seismic profiles indicate that the underthrusting was active after the late Miocene as well, since reflectors in the late Pliocene - Quaternary section are tilted in front of the thrust belt (Fig. 11). In seismic profiles across the Terek-Caspian basin, this sequence is marked by the north-propagating

clinoforms (Kunin et al., 1989). Thus, the major deformation took place mainly during the Pliocene-Quaternary. The post-Oligocene thrusting is also supported by drilling at the Duzlak anticline (Fig. 9). There, the complete prePliocene section is involved in the anticlinal structure, implying the Pliocene or later age for the major folding. The late Tertiary - Quaternary timing of thrusting is also supported by stratigraphic studies which show that Lower Miocene deposits occur in an underthrust position in the Duzlak anticline (Sobornov, 1991b and references herein). It is noteworthy that in this example, which is borne out by deep drilling, there is a considerable difference between magnitudes of displacement along thrusts delimiting the wedgeshaped thrust sheet. The thrusts at the base of the sheet have a cumulative displacement of more than 3 km, that is up to 10 times that of the upper thrust verging in the opposite direction. It appears that the discrepancies between displacements along the upper and lower thrusts were at least partly caused by the accommodation of forelandward movement by layer-parallel shortening within the foredeep clastics, which resulted in their overconsolidation (Sobornov, 1991 b). Large, fluid discharge in this zone is manifested by numerous mud volcanoes and clastic


dikes cross-cutting the Miocene section. An analogy for this may be provided by deformation at the toe of the accretionary wedge at the Cascadia margin off Vancouver Island, Canada, where considerable overconsolidation of sediments is recognized from seismic reflection data (Yuan et al., 1993). Taking into account that magnitudes of displacement along backthrusts in the less explored central and western parts of the Dagestan thrust belt are much greater than in the southern one, where the Duzlak anticline is situated, and the structures

SkamkhaI-Bulak South

North

0

i~!i;/~|iQ:'~i~.:~i~!'~'::,.:.:.~:~":';~°*~°~.~o~:~g - PI ocene!;]!:~%~~ ' - " ~ - - . ~ _ Q uat e r n a ry

....,,,~!.,iiiilji:~ii.ii:!~ U.

• ~

'_

Miocene

M. M i o c e n e

~ ."

' ~-

~

;~

,

KM

I ,', ', ~ ',, ', 'i U. Jurassic-

there fit into the same general pattern, it is likely that the amount of subsurface shortening for these regions is correspondingly greater (Sobornov, 1988). A geomorphological study of the foothills shows that the wedge-shaped structures along the leading edge of the thrust belt are still active (Nikitin, 1987). The Tertiary monoclines carried by backthrusts, attributed to the growth of the subsurface allochthonous wedges, create impressive features of the present day landscape. This evidence, coupled with the high seismicity of the area, leave no doubt regarding the late Tertiary-Quaternary age of the wedge-shaped thrusts of the Dagestan foothills. The geomorphological pattern of the study area together with the structural geometry seen on seismic and geological cross-sections implies that the thrust belt involved northward propagating step faulting. This is suggested by the greater degree of rotation and uplift of the northward-vergent thrust sheets in the southern part of the buried thrust belt. The observed structural features make it possible to consider the Dagestan buried thrust belt front as an initial phase of triangle zones. Ongoing subsurface thrusting and consequent uplift within the study area may eventually lead to deeper erosion and outcropping of the deep-seated forethrusts. IMPLICATION FOR PETROLEUM EXPLORATION

2KM gas condensate

361

pool

Fig. 10. Geological cross-section through the ShamkhaI-Bulak gas condensate field, western part of the Dagestan thrust belt. Note opposite vergence of the surface and subsurface thrusts. Refer to Figure 2 for location.

South

The identification of structurally stacked thrust sheets, composed of hydrocarbon-bearing Mesozoic carbonates concealed under the frontal molasse monocline, provides new targets for petroleum exploration. This interpretation suggests that many wells were drilled off potential traps.

North

5kmI

TWT(s)

Fig. 11. Migrated seismic reflection profile through the leading edge of the Dagestan thrust belt. Note merging of the upper and lower detachments within the Oligocene - Lower Miocene Maykop Formation (from Sobornov, 1993). Refer to Figure 2 for location.

362

K.O. SOBORNOV

NORTH

SOUTH

SALATAU RIDGE

+2 +1 0 1 2 3 4 5 6 7 8

KHADUM D O M E

SHAMKHAL-BULAK FIELD

9.

krn 10knn ~]

L.-M.Jurassie

~ U . JEocene . . . . . ic-

D D

Maykop Fm. (Olig.-L.Mio.) U.Mi . . . . .

~

M.Miocene

[]

Plioc . . . . Quaternary

Fig. 12. Geological cross-section through the Dagestan thrust belt showing wedge-shape thrusts injected into the base of foredeep clastic section. Refer to Figure 2 for location. Structural mapping constrained by newly obtained seismic data suggests new promising targets for future exploration. (Sobornov, 1988), which are represented mostly by anticlines associated with the frontal parts of the Mesozoic carbonate thrust sheets. The potential reservoir consists of fractured Upper Jurassic - Upper Cretaceous carbonates sealed by the Oligocene (Maykop) mudstones at depths from 3 - 6 km. The large size of these anticlines suggests that they may be able to hold significant reserves of hydrocarbons. One of the proposed targets is the Severniy (North) Shamkhal-Bulak anticline, which is associated with a thrust sheet overthrusted by the Shamkhal-Bulak anticline (Fig. 10). A wildcat well drilled in this area to the west of the crest of the proposed anticline showed the presence of hydrocarbons (Sokolov, et el., 1990). Recent geochemical studies show that the main source of hydrocarbons in this area is within the Oligocene - Lower Miocene mudstone-siltstone section (Sokolov et el., 1990). Average total organic carbon content in these rocks is 1.0 - 1.5 %. The wedging of the thrust sheets into the source rocks situated in the oil window created favorable conditions for oil and gas migration and accumulation in the frontal anticlines. It is also possible that hydrocarbons within the thrust sheets may be sourced from Aptian - Albian and Jurassic marine shales which are also enriched in organic matter and are situated under conditions favorable for generation of hydrocarbons (Sokolov, et el., 1990). So far, only a few wells have been drilled to test the buried anticlines. Some of them have been very successful. This implies that exploration of this play is promising and that the petroleum industry in this old oil- and gas-producing region could have a second life. CONCLUSION

Although the Dagestan thrust belt is a mature area of petroleum exploration, re-interpretation of its subsurface structure suggests new, untested targets. They are represented by folded thrust sheets inserted into the foredeep section. A skeleton

of the allochthonous assemblage is formed by Jurassic and Cretaceous rocks that are oil- and gas-bearing elsewhere in the region. The Mesozoic-involved thrust sheets are stacked in front of the disturbed belt. Structural culminations of the allochthonous assemblage do not have an obvious surface manifestation since the thrusts do not crop out but merge with the upper detachment at the base of foredeep clastic fill. The development of subsurface thrusting is a result of significant rheological stratification of the sedimentary section. Most thrusting and shortening occurred only recently, in Pliocene Quaternary time, as deposits of this age are involved in the deformation along the thrust front. The wedge-shaped thrust zones of the Dagestan thrust belt indicate a resemblance with classical examples of the triangle zones. A considerable difference between well-known examples of the triangle zones and the structural pattern described in this paper is the absence of mapped basinward thrusts in the study area. This specific feature accounts for the young age of formation of the Dagestan thrust belt and its present lower erosion level. This buried thrust belt can be regarded as analogous to other examples at the initial stage of structural evolution. ACKNOWLEDGMENTS

I am indebted to Dagneft and Grozneftegeofizica companies which provided a great deal of geological and geophysical information on the structural and petroleum geology of the northeastern Caucasus. My special thanks to A.W. Belly, M. Barazangi, M.O. Dzhabrailov, P.B. Jones, V.E. Khain, N.V. Koronovsky, D.A. Mirzoev, J. Morgan, M.U. Nikitin, Ya.A. Roytmann, F.G. Sharafutdinov, V.F., Sharafutdinov, Yu. P. Smirnov, and B.A. Sokolov all of whom discussed the concept presented in this paper. I am grateful to F.A. Cook,~P.B. Jones, T.E. Kubly, F.A. Montandon, A. Newson, D.A. Sprgtt, and K. Vasudevan who provided me with an unique oppj0rtunity to study the geology of the triangle zones in the southern Canadian Cordillera. I also thank reviewers for their constructive critical remarks which resulted in substantial improvements in this paper.

S T R U C T U R E A N D P E T R O L E U M P O T E N T I A L O F T H E D A G E S T A N T H R U S T BELT, N O R T H E A S T E R N

363

CAUCASUS, RUSSIA

A ............................ . [ i ].l [ l I . . ] l ] l l ] ]' l ] l' l l 'l "l l l l•l l I I]1111111111]

.--.~-"~.,._ 2=,,... . . . . . . . . , I i I.l l.t I.t I . . . . . . I L l ~ I , I 7":I 1,,., . .] . ~. . .~] [ .] . .] . .I . .] . I ]

i

:ii!i.'.'ii::i::i:i:i:i:i.i-!-i-::::::::-:-:-:.:-:-':':':::: "

~

"~ "

15.. ]

I

I

I

~

1

..... I

I

I

I

........ I

J

1

' ....

1

]*t]

->L-

"'

~-~-C77"----TY:~T"i'L-:~"';-,-:~,-7-:7-7, ]

] ~

J

Y

I

I

]¢~.|

~

~

i

]

t~

t

i

~ .i

~

,

..1' 1

, , ,~

13

' " : " ~";I~5:i!::~1:i+1".~':(.£'1::;1 i : !. :11::!:i:::::': : : :!1G::!!11~'9;>:" ::i 1:!: !5::::11(';;'.: !:.'i :/:i::iDi :!i:(:.~i ¢1"ii;!:::!11!]!:::1i::% !i!:!i:::(f.(:iii:i £11:~!£}!}1('i('!](.~{~:]':1{~i!i!!!ii(1:~i??~!i!;

[~> ~:~:~:~:~:~:~:~:~:~:~:~:::::::::!:iii!iiii?!:i::...~.:~~.~:::[:~:~'~::~`~:~'~'~:~;~:2~j~::;:~4~)~(:~;:.::.~.;1:.~:J.::..L:..::~ C

D

~]

L.-M.Jurassic

[~

U. J u r a s s i c Eocene Maykop Fm. (Olig.-L.Mio.) M.MioceneQuaternary

[~3

E

50 KM D I S P L A C E M E N T 40% S H O R T E N I N G

>

Fig. 13. Schematic sequence of deformation in the Dagestan thrust belt, based on cross-section shown in Figure 12. A - undisturbed section showing trajectory of future thrusts, B to D - progressive stages of thrusting and tectonic wedging, E - present structure of the thrust belt. REFERENCES Amirkhanov, Kh.I., Suetnov, V.V., Levkovich, R.A., and Gairbekov, Kh.A. 1972. Thermal regime of sedimentary rocks. Makhachkala, p. 133-164, (in Russian). Averbukh, A.G., Broytmann, A.R., and Bulgakova I.A. 1974. Regional geological structure of the eastern Pre-Caucasus constrained by integrated geophysical surveys. VIEMS review. Series 9. Regional exploration and reconnaissance geophysics. 34p. Brod, I.O., (ed.). 1960. Geological structure of the eastern part of the Northern slope of the C a u c a s u s . P r o c e e d i n g s of K U G E , G o s t o p t e k h i z d a t , Leningrad, v. 2, p. 164-272, (in Russian). Dotduyev, S.I. 1990. Structure and origin of the Shatskiy nappe packet in the Dagestan foothills. Geotektonika (Geotectonics), no. 3, p. 59-69, (in Russian). Gordy, P.L., and Frey, F.R. 1977. Geological Guide for the C.S.P.G. 1977 Waterton - Glacier Park Field conference, Calgary. Canadian Society of Petroleum Geologists, 93p. Gushchin, A.I., Dotduyev, S.I., Koronovskiy, N.V. 1986. Structure of the Varanda anticline in the Calcareous Dagestan. Vestnik Moskovskogo Universiteta. Geology (Moscow University Geology Bulletin), no. 4, p. 24-30, (in Russian). Jones, P.B. 1982. Oil and gas beneath east-dipping underthrust faults in the Alberta Foothills. In: Geological studies of the Cordilleran Thrust Belt, R.B. Powers (ed.). Rocky Mountain Association of Geologists, Denver, v. 1, p.61-74.

1987. Quantitative geometry of thrust and fold belt structures. American Association of Petroleum Geologists, Methods in Exploration Series, no. 6, 26p. Krasnopevtseva, G.V. 1984. Deep structure of the Caucasus seismically active region. Nauka, Moscow, 107p., (in Russian). Kunin, N.Ya., Kosova, S.S., and Blokhina, G.Yu. 1989. Seismic-stratigraphic analysis of the sedimentary section of the Eastern Pre-Caucasus. Lithology and mineral resources, no. 6, p. 54-68, (in Russian). Kuprin, P.N. 1959. Oil and gas potential of the Eastern anticlinal zone of Dagestan. In: Geologiya i n e f t e g a z o n o s n o s t yuga SSSR, Dagestan (Geology and oil and gas potential of the southern part of the USSR, Dagestan). Proceedings of KUGE, Gostoptekhizdat, Leningrad, v. 4, p. 149-323, (in Russian). Levkovich, R.A., and Asmanov, O.A. 1985. Seismically active area "the Dagestan salient". Proceedings of IG DagFAN SSSR, Makhachkala, v. 33, p. 5-17, (in Russian). Markus, M.A., and Sharafutdinov, V.F. 1989. Oligocene olistostromes in the East C a u c a s u s and Late A l p i n e t e c t o n o g e n e s i s . G e o t e k t o n i k a (Geotectonics), no. 4, p. 87-98, (in Russian). Mirzoev, D.A., and Sharafutdinov, EG. 1986. Geology of oil and gas fields of Dagestan. Dagestan Knizhn. Izd., Makhachkala, 312p., (in Russian). Muller M., Nieberding, F., and Wanninger, A. 1988. Tectonic styles and pressure distribution at the northern margin of the Alps between Lake Constance and the river Inn. Geologishe Rundschau, v. 77, p. 787-796.

3~

Mussaev, S.E. 1968. Once more on the Main Derbent Fault. Proceedings of IG DagFAN SSSR, Makhachkala, v. 7, p. 68-72, (in Russian). Nikitin, M.U. 1987. Neotectonics of the Eastern Caucasus. MOIP Bulletin, v. 62, no. 3, p. 21-36, (in Russian). Philip, H., Cisternas, A., Gvishiani, A., and Gorshkov, A. 1989. The Caucasus: an actual example of the initial stage of continental collision. Tectonophysics, v. 161, no. 1, p. 1-21. Sanderson, D.A. and D.A. Spratt. 1992. Triangle zone and Displacement transfer structures in the Eastern Front Ranges, Southern Canadian Rocky Mountains. AAPG Bulletin, v.76, p. 828-839. Sarkisian, S.G., and Kotel'nikov, D.D. 1980. Clay minerals and problems of the petroleum geology. Nedra, Moscow, 232 p., (in Russian). Sharafutdinov, F.G. 1975. On the tectonics of the Sulak salient in connection with oil and gas potential of the Mesozoic deposits. Proceedings of IG DagFAN SSSR, Makhachkala, v. 9, p. 103-106, (in Russian). Shatskiy, N.S. 1929. Geological structure of the eastern Chernykh Gor and oil fields Miatly and Dylym in northern Dagestan. Izd. Nauchno-Tekhn. Upravleniya VSNKh, Moscow, p. 31-95, (in Russian). Sholpo, V.N. 1978. Alpine geodynamics of the Greater Caucasus. Nedra, Moscow, 395 p., (in Russian). Smirnova, M.N., Brazhnik, V.M., and Tchupfin, V.V. 1985. Geophysical fields and deep structure of the focal zone of the Tchernogorsk earthquake. Proceedings of IG Dag. FAN SSSR, Makhachkala, v. 33, p. 77-88, (in Russian). Sobornov, K.O. 1988. Subthrust oil and gas accumulation zones - new exploration target in Pidmont Dagestan. Geologiya nefti i gaza (Geology of oil and gas), no. 2, p. 8-12, (in Russian). 1991a. Formation of the fold-and-fault structure of the Dagestan salient. Geotektonika (Geotectonics), no. 3, p. 34-46, (in Russian). _

~ O . SOBORNOV

1991b. Wedge-shape structure of Duzlak anticline, Southern Dagestan. MOIP Bulletin, v. 66, no. 6, p. 44-50, (in Russian). _ _ 1992. Blind duplex structure of the North Urals Thrust belt front: J. Geodynamics, v. 15, no. 1/2, p. 1-11. 1993. Structure of the transition zone between the Caucasus and Terek-Caspian foredeep. Dokl. Russ. Acad. of Sci., v. 330, no. 3, p. 492496, (in Russian). Sokolov, B.A. and Sobornov, K.O. 1986. Evaluation of petroleum prospects of Dagestan. In: Sovremennie problemi geologii i geohimii goruchikh iskopaemikh (Contemporary problems of petroleum geology and geochemistry), Publications of Moscow State University, Moscow, p. 130136, (in Russian). , Korchagina, U.I., Mirzoev, D.A., Sergeeva, V.N., Sobornov, K.O., and Fadeeva, N.P. 1990. Oil and gas generation and accumulation in the Eastern Pre-Caucasus. Moscow, Nauka, 206p, (in Russian). Yuan, T., Spence, G.D., and Hyndman, R.D. 1993. Seismic velocity analysis of the accretionary wedge sediments at the Cascadia margin with the Landmark/IT&A insight system. LSPF Newsletter, vol. 6, no. 1, p. 29-33. Zakharova, A.I., Moskvina, A.G., and Chepkunas, L.S. 1990. Focal parameters of the Dagestan earthquake May 14, 1970. Fizika zemli (Physics of the Earth), no. 2, p. 30~41, (in Russian). Zangiev, Sh.D. 1979. On structure and petroleum potential of the Mesozoic deposits of the Sulak salient. Proceedings of IG Dag. FAN SSSR, Makhachkala, v. 4, p. 57-65, (in Russian). Zonenshain, L.P., Kuzmin, M.I., and Natapov, L.M. 1990. Plate tectonics of the territory of the US SR. Nedra, Moscow, v, 2, p. 168-182, (in Russian). _

_

Manuscript received: August 24, 1993.

_

Revised manuscript accepted: February 16, 1994.