Geological Setting Of Superdeep Deposits

The deepest deposits occur within the Kura Intermontane Depression, which is located between mountainous uplifts of Greater and Lesser Caucasus mega-anticlinoria. Structurally, it is a mega-synclinorium that originated during the orogenic stage of Caucasus development. By its abyssal structure, the Kura Depression is divided into Upper, Middle, and Lower Kura troughs or subdepressions which demonstrate different mobility. The Middle Kura Trough with an extent of 300 km embraces the area from Tbilisi, Georgia, to the meridian of Kyurdamir, Azerbaijan. A wide, buried uplift extends toward Vandam from the region of Talysh foothills to the north. The Lower Kura Trough extends from the western Caspian abyssal fracture, located along the eastern slope of Talysh-Vandam uplift, to the western shore of the Caspian Sea. These geological features are separated by faults of the northwest extension (Figure 1-2).

The surface of the Middle Kura intermontane area is named the Mil-Mugan steppe and is composed of the Quaternary alluvial-deluvial deposits 800-m thick. The first indication of abyssal structure was revealed as a gravity maximum by a survey conducted in 1929-1931. The first investigator, V. V. Fedynskiy, named this gravity maximum as Talysh-Vandam. Detailed investigations of Talysh-Vandam gravity maximum were conducted by geologists and geophysicists of Azerbaijan who noted that the Saatly uplift region in latitudinal section is a block of shallow (about 8 km) "basalt" rocks with a velocity discontinuity of 6.7-6.8 km/sec. Different tectonic regimes caused a change in folding characteristics of the Middle Kura Trough. During Caledonian-Hercynic stages the trough was a part of the Transcaucasus anticline (the Median Masiff). Within this trough, the uplift, erosion, and formation of separate basement fault blocks predominate. The main structural elements of the Middle Kura subdepression originated during the earliest Alpine stage. These are the Geokchai-Saatly (or Kyurdamir-Saatly) uplift, Iori-Adzhinour, and Yevlakh-Agdzhabedy troughs (Figure 1-2). During the Liassic time, the region of the modern Kura Depression was occupied by a shallow sea where terrigenous sediments accumulated. During Middle and Late Jurassic, a 5,000-m volcanogenic sequence accumulated as a result of intensive volcanic activity. Carbonate reefs grew in the Late Jurassic-Early Cretaceous time. A second stage of volcanic activity occurred during the Late Cretaceous time when volcanogenic sequence accumulated in separate parts of the Talysh-Vandam gravity maximum. The end of Late Cretaceous is marked by the accumulation of Campanian-Maastrichtian carbonate sediments. Sedimentation occurred in the Iori-Adzhinour, Yevlakh-Agdzhabedy, and Lower Kura troughs.

The beginning of Oligocene-Miocene orogenesis altered the pre-existing geotectonic regime in the Kura Depression, and was dominated by warping with molasse accumulation. The Geokchai-Saatly zone of the buried uplifts is characterized by an elevated basement surface in the eastern part of Middle Kura Trough. The Saatly-Kyurdamir and Mil-Khaldan subzones (blocks) occur within the Geokchai-Saatly zone.

The Saatly-Kyurdamir subzone includes Karadzhaly, Sor-Sor, Dzharly, and Saatly local uplifts, whereas the Mil-Khaldan subzone experienced Muradkhanly, Zardob, and Mil uplifts.

It is hard to investigate Saatly-Kyurdamir buried uplift because there are no natural outcrops, and Cenozoic molasse deposits, overlapping Mesozoic sedimentary-volcanogenic strata, are very thick. Drilling on various parts of the uplift, however, has produced new data on Mesozoic magmatism. It was ascertained that the Mesozoic stage of uplift involved volcanogenic-sedimentary deposition with a volcanogenic-plutonic association.

Seismic, gravity and magnetic investigations of the Earth's crust along profile, which crosses the uplift in a latitudinal direction, show that a velocity model of the crust based on reflected waves is rather informative. Seismic observations were conducted by vertical seismic sounding by reflected waves. Observations were conducted only along the sublatitudinal profile that crosses the uplift area. As a result, a more detailed velocity model was obtained (Figure 1-3). According to these data, a high-velocity layer [V > (6.7-6.8) km/sec] is expected at a depth of 9 km.

It became evident that the Saatly-Kyurdamir gravity maximum is expressed as a nose of the most ancient (Pre-Baikal) complex (Figure 1-4). The nose is overlapped by a magmatic sequence of basic and intermediate composition, mainly of Mesozoic age. Its roots penetrate deep into the mantle to the west where the Zardob magnetic maximum is present.

These investigations show the development of Mesozoic magmatites as thick, highly-magnetic strata. To confirm these data, it was decided

Figure 1-3. Seismic density model along the line of deep seismic sounding (Modified after the Excursion Guide-Book for Azerbaijan SSR, Vol. II, 1984). (a) Observed and calculated plots of gravity field for section; (b) Seismic density model; Curves: 1—observed, 2—calculated, 3—Cenozoic sequence, 4—Mesozoic sequence, 5—sequence G (velocity analogous to that in "granitic" layer), 6—sequence B (velocity analogous to that in "basalt" layer), divided into two sub-sequences: Bu and Bl, 7—sequence B1 (supposed peridotite content), 8—boundary of velocity (density), 9—unconformities, 10—deep wells.

Figure 1-3. Seismic density model along the line of deep seismic sounding (Modified after the Excursion Guide-Book for Azerbaijan SSR, Vol. II, 1984). (a) Observed and calculated plots of gravity field for section; (b) Seismic density model; Curves: 1—observed, 2—calculated, 3—Cenozoic sequence, 4—Mesozoic sequence, 5—sequence G (velocity analogous to that in "granitic" layer), 6—sequence B (velocity analogous to that in "basalt" layer), divided into two sub-sequences: Bu and Bl, 7—sequence B1 (supposed peridotite content), 8—boundary of velocity (density), 9—unconformities, 10—deep wells.

Figure 1-4. Geologic and geophysical model of Saatly-Kyurdamir anticlinal trend (Modified after the Excursion Guide-Book for Azerbaijan SSR, Vol. II, 1984). 1—Cenozoic sequence-terrigenous deposits; 2—Mesozoic sequence: a. terrigenous-carbonate deposits, b. extrusive formations of basic and intermediate composition; 3—Baikal sequence, metamorphozed primary terrigenous formations; 4—Pre-Baikal sequence, gneiss and marl; 5—the oldest interval, gneiss and amphibolite; 6—intrusive formations of basic and intermediate composition; 7—undivided extrusive-intrusive interval; 8—low density rocks, serpentinites; 9—rocks of intermediate composition between crust and mantle; 10—position of upper mantle top; 11—zones of large faults; 12— deep wells.

to drill a super-deep well. It was expected that on reaching a depth of 9 km, the super-deep well would penetrate a volcanogenic section. Disturbed deposits, which are the source of the regional Talysh-Vandam gravity maximum, are expected lower. The authors suppose that as a whole, these are primarily sedimentary, metamorphosed, and consolidated deposits of the Upper Archean-Lower Proterozoic age.

It is possible that between the Mesozoic magmatic strata and the Pre-Baikal basement, there may occur somewhat thin intermediate deposits, which cannot be identified by common techniques.

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