SHIPILOV E.V., SHKARUBO S.I. , KARIAKIN J.V.
ММBI Kola SC RAS, Russia, MAGE, Russia, GIN RAS, Russia

The Jurassic--Cretaceous stage in geological evolution of the Barents Sea region was characterized by sudden changes in tectonogeodynamic and sedimentation settings that resulted in substantial transformations of the Earth's crust and accumulation of variable (in thickness and lithology) sediments.

Based on marine geological--geophysical data, this paper analyzes geological events and tectonogeodynamic processes in the Barents Sea versus Amerasian regions during the Jurassic--Cretaceous.

The geodynamic evolution of the Arctic basin remains debatable, although most researchers consider it a Late Mesozoic structure related to the initial destructive (riftogenic) activation that produced the first generation of young oceans (Canada and Amerasian Basins). This is evident from different materials indicating that, at least in the Jurassic (when the northern part of the Central Atlantic already existed), all transgressions toward the present-day North Sea and West Arctic margin advanced from the north, where a vast deep ocean-type basin was probably located.

The existence of such an oceanic basin during the Mesozoic (since the Triassic) is also evidenced by a specific (highly diverse) marine biota.
Available paleotectonic reconstructions show the longitudinal position of the spreading axis in the Amerasinan Basin. The extinct spreading axis was reconstructed based on the anomalous magnetic field in the Canada Basin.

Several models have been proposed to explaining the formation of the region. The rotation mechanism of the Amerasian Basin formation is the most substantiated model confirmed by geological--geophysical data and paleogeodynamic reconstructions. According to this model, the Amerasian Basin developed in two phases.

The first phase of tectonic activation corresponds to an "unsuccessful" (or incomplete) riftogenic event during the Aalenian--Bathonian-Tithonian time that resulted in the formation of wide semigrabens along the Arctic margin of Alaska. This inference is supported by the stratigraphic and structural analysis of sediments and drilling data on the continental margin of Alaska and neighboring areas.
In the Barents Sea region, this phase commenced with subsidence of the area and transgression of a northern sea basin that reached the Pechora Plain in the terminal Bathonian.

Lacustrine-alluvial sedimentation gave way to coastal--marine and marine ones. Consequently, typical Early and Middle Jurassic sandy sequences were replaced by sandy-clayey sediments since the Callovian and mainly clayey sediments in the Late Jurassic.

In the South Barents Basin, which was not as yet separated by the Luddlov Ridge, the studied period was marked by more drastic changes. In the Middle Jurassic, the basin acquired a new structural appearance as a result of the formation of rises and tectonic scarps mainly in peripheral zones. This is evident from the presence of gritstone and conglomerate units in borehole section located along the periphery of the basin. Judging from the occurrence of these rocks in the interval from the uppermost Lower Jurassic to the uppermost Middle Jurassic, differentiation and formation of provenance was a prolonged process. The transportation paths of detrital material changed as well. Therefore, the Middle Jurassic is locally separated from the Upper Jurassic by an unconformity (presumably angular) related to change in the inclination of some paleofloor areas. It should be noted that no such features are observed in the depocenter section: sandstones in the Upper Jurassic sequence are replaced by clayey sediments in Borehole Arkticheskaya.

According to our observations (summer of 2002), gritstones of the Kanin Peninsula are presumably Middle Jurassic rocks. Abundant ammonites, belemnites, and bivalves are most likely pre-Late and Late Jurassic forms similar to those from boreholes drilled at the offshore continuation of the Pechora Plate. Sea transgression and basin deepening during the studied period was accompanied by endogenic activity (basic plutons). The K-Ar age of basalt flows recovered in the Luddlov Rise, which separating the South and North Barents basins, and Borehole Nagurskaya (Frantz Josef Land) is estimated at 159 Ma and 151 Ma , respectively. The close age of basalts (145 Ma) is also established in Spitsbergen, its eastern shelf area, and the Sverdrup Basin 152±6 Ma). These values mainly correspond to the Late Jurassic (Volgian) interval of the geological scale, when the Barents Sea region, particularly the East Barents megabasin, underwent the maximum transgression and accumulated black shales of the Kimmeridgian - Volgian clayey complex. The seismic prospecting and drilling materials reveal wide distribution of these sediments that are diachronously replaced along both lateral and vertical directions by clinofroms of the Neocomian slope facies (clayey-sandy sediments). Both seismic complexes are easily recognizable in most time records from the Pechora Sea (in the south) to the Saint Anna Trough (in the north).

The second phase of the Amerasian Basin development was related to riftogenesis and subsequent spreading from the Hauterivian-early Aptian to the early Cenomanian. During this period, the Canadian Arctic Archipelago (on the one hand) and Alaska with Chukotka (on the other) started to move away from each other and rotate relative to the center located in the Mackenzie River delta area. Simultaneously, the Sverdrup-New Siberian transform fault zone with the sinistral kinematics was developed along the Barents--Kara paleomargin that incorporated a block subsequently transformed into the Lomonosov Ridge.

The newly forming mid-oceanic spreading center influenced the Barents--Kara paleomargin via this transform fault zone. In particular, it is similar to the Spitsbergen-North Greenland and recently defined Khatanga-Lomonosov zones. They separate different (in structure, age, and geodynamic features) oceanic segments and continental and oceanic blocks. At the same time, continental margins along aforementioned transform fault zones are marked by areas of basaltoid magmatism corresponding to phases of the most active development of deep-sea basins.

In the Barents Sea region, transition to the second (main) phase in development of the Amerasian Basin was marked by the replacement of shales (bazhenites) by Lower Cretaceous (Veldian) coarse-grained regressive facies. The transition migrated in time and space over almost all continental margins. Activation of tectonogeodynamic processes with powerful regressive events in the Early Cretaceous was caused by global processes (breakup of Pangea II and sharp sea level fall). This resulted in erosion of rises surrounding the East Barents system of troughs during the pre-Neocomian and early Aptian regression. As a consequence, both Upper Jurassic and Neocomian sediments were eroded from rises. The Ludlov Rise was formed in a strike-slip fault zone located between the South and North Barents depocenters. Here, Borehole Luddlovskaya penetrated the layered gabbrodiabase pluton with a K-Ar age of 131-139 Ma. The pluton is located above another pluton mentioned above and related to the ascending mantle diapir. Hence, riftogenesis was well developed in the East Barents megabasin in the Cretaceous. The system of diagonal dip-slip and oblique faults, which reactivated in the paleomargin, governed trends of the majority of newly developing structures in the region. This is well recognizable in the seafloor topography.

Simultaneously, intense basaltoid magmatism occupied the Frantz Josef Land, where it lasted until the Cenomanian (the K-Ar age of basalts in Borehole Nagurskaya is 103±7 Ma, Saint Anna Trough, and eastern shelf of Spitsbergen (upper age limit is 105 Ma). Seismic records show the abundance of coeval sills in sedimentary cover of the Barents margin, particularly in the East Barents megabasin.

According to the majority of isotopic datings, the period of riftogenic magmatism in the Sverdrup Basin lasted from 131 to 100 Ma (to 92 Ma, according to biostratigraphic data). Trap basalts from De Long Island and igneous rocks from the Alpha Rise are dated back to the Hauterivian--Albian.
Thus, we can conclude that all Hauterivian-Albian (locally, up to the Cenomanian) sills, dikes, and sheets described above point to the opening of the Amerasian Basin.

The evolution of oceanic basins in the Arctic-Atlantic segment of the Earth probably took place according to the following scenario.
The Central Atlantic began to open at the beginning of the Middle Jurassic (about 170 Ma ago), which corresponds to the continental rifting phase in the Amerasian Basin (beginning of the Aalenian--Bathonian). The South Atlantic began to open about 150 Ma ago. This is marked by Jurassic magmatism in the Barents Sea, sea transgression from the northern side, deepening of basins in the Barents region, and accumulation of Kimmeridgian--Volgian black shales in the depressions. The main phase of the Amerasian Basin opening lasted from the Hauterivian to Albian-Senomanian. Riftogenic processes accompanied by magmatism were developing in the Barents Sea and Sverdrup Basin from (135-130 to 95-92 Ma). In the North Atlantic, spreading commenced 100-80 Ma, when the spreading center in the Amerasian Basin virtually disappeared (95-80 Ma).

It should be noted that the Jurassic-Cretaceous generation of ocean formation in the Arctic region was related to opening and development of the Amerasian Basin. This is reflected in tectonogeodynamic reorganizations and paleofacies sedimentation settings at the Barents Sea margin. The along-strike migration of the axial spreading zone of the Amerasian Basin, which is marked by fractures and deformations, toward the opening North Atlantic was provoked by a system of deep riftogenic troughs (Saint Anna, North Barents, and subordinate South Barents, Nordkapp, Hammerfest, Medvezhinskii, Olga, and others).

OIL AND GAS OF ARCTIC SHELF 2008