Paleogeographic settings and geodynamics of the northeastern Pacific margin of Asia in the terminal Cretaceous-Early Paleogene

Abstract

Evolution of the ocean-continent transition zone in the northeastern Pacific margin of Asia in the con� text of general geodynamics continues to receive attention from geologists. This problem is considered in many circumstantial works (1-3). At the same time, these general interpretations largely concern its Meso� zoic history, while the Cenozoic remain insufficiently studied. In addition, these works ignored particular features of paleogeographic settings and variations in the geodynamic regime within short geological peri� ods marked by changes in the character of boundaries between domains with the oceanic crust and continen� tal margins. The purpose of this communication is analysis of the paleogeographic settings and geody� namics in the Late Cretaceous-Early Paleogene in the spacious ocean-continent transition zone between the western part of the presentday Bering Sea shelf and western Kamchatka (Fig. 1). This period is sand� wiched between two geological events: early (pre� Maastrichtian) formation of the main fold-thrust structures in the Koryak Highland and intermediate (Early Paleogene) reorganization related to accretion of the Olyutor-eastern Kamchatka island arc (2). In the northern Koryak region and Bering Sea shelf, this stage preceded the onset of formation of sedimentary basins during the second half of the Paleogene and Neogene in the presentday shelf and adjacent shore areas (Khatyrka, Navarin). In this connection, of significance is the problem of former spatial relationships between coeval (late Cam� panian-Paleocene) sediments of the continental mar� gin (shelf, coastalmarine, paralic, and continental) and deepwater flyschoid sequences that were likely formed on the oceanic crust of the adjacent basin. In the presentday structure, the uppermost Cretaceous- basal Paleogene sediments deposited on the continen� tal margin are known in the western part of the Bering Sea shelf (based on drilling and dredging data (4)) and on land, where they are traceable from the Navarin Peninsula via the southern and central parts of the Koryak Highland, Shelekhov Bay in the Kamchatka Isthmus southward to western Kamchatka. Onshore, coeval deepwater flyschoid sequences (known as the Ukelayat, Lesnaya, and Khozgon flysch sequences) are distributed from the Dezhnev and Anastasiaya bights in the east via the Ukelayat River basin to the upper reaches of the Apuka and Pakhacha rivers and farther southwestward to the Kamchatka Isthmus, where they outcrop from under tectonic nappes near the axial part of the Sredinnyi (Median) Range. In the central part of western Kamchatka, similar sequences constitute the western slopes of the Sredinnyi Range. According to seismic data, the lower part of the conti� nental slope in the western Bering Sea shelf is repre� sented by an accretionary wedge of turbidites (4), which are likely correlative with the onshore flyschoid sequences. It should be noted that deepwater flyschoid sequences are either characterized by tectonic rela� tionships with coeval sediments of the continental margin or occur near them with the contact being cov� ered by young sediments. No facies that could be interpreted as transitional between the two above� mentioned sediment types are recorded in these areas. Moreover, deepwater terrigenous sequences are uni� versally characterized by a quartz-feldspar composi� tion with insignificant admixture of volcanogenic detrital material against the background of the com� plete lack of tephra (5, 6). In the northeast (Ukelayat zone), the Campanian-Maastrichtian and Pale� ocene-Lower Eocene deepwater flyschoid sequences (Taven, Ayaon, Val'en, and Lyapganai formations), which form a continuous succession, contact along the thrust with the upper Campanian (Vachvayam