The coast of Oyogos Yar

Stratigraphic and geological-geomorphological studies of the northern margin of the Yana-Indigirka lowland were carried out along the south coast of the Dmitrii Laptev Strait named Oyogos Yar in August 2007. The main camp site of the field team was located at the margin of an alas depression 19.4 km east of the mouth of the Rebro River (see Figure 4.6.1-1). Detailed studies were realized 5.5 km east of the camp site, where one of the rare remnants of the Late Pleistocene Ice Complex deposits from the Oyogos Yar coast are preserved. In addition, mapping survey was carried out along the entire about 125 km long Oyogos Yar coast from the Cape Svyatoy Nos (west) to the mouth of the Konechnoy River (east). Objectives and methods of these studies were similar to those along the coast of Bol’shoy Lyakhovsky Island. Besides of stratigraphical, palaeogeographical, micropalaeontological, and stable isotope studies additional tasks were the stratigraphical correlation and comparison of Quaternary sequences of the northern and the southern coast of the Dmitrii Laptev Strait.

Stratigraphical studies of Quaternary deposits from the Oyogos Yar coast started in the middle of the 50ies of the last century (see chapter 4.4.1). Actually an official local stratigraphical scheme for the area of the Dmitrii Laptev Strait does not exist. The decision of the stratigraphical commission in 1982 (Decision, 1982) did not recommend to use parts of the former orders of Ivanov (1970, 1972) (e.g. Kuchchugui Suite) although there was no new proposal for

this area. Because of the missing scheme we use the nonofficial proposal of O.A. Ivanov, which is well applicable in the exposures and for comparison.

The mapping survey along the entire coast of Oyogos Yar allows to characterize the general features of their structure.

The elevated segments of the coast that belong to the western part of Oyogos Yar and the border of the western area of the mainland coast, the Syatoy Nos Peninsula, the rock massive of Svyatoy Nos are located in the axis of the submerdial Chokhchuro-Chokhurdakhsky uplift that continues further north to the side of the Kigilyakh Peninsula in the western part of Bol’shoy Lyakhovsky Island. The top of Svyatoy Nos is characterized by 300-400 m height and by outcrops of basement rocks (granites) in most of the coastal exposures. Further southwest of Cape Svyatoy Nos marine near shore deposits of the Serkinskoy Suite were described by Ivanov (1970). At the north coast of Svyatoy Nos Peninsula above the basement rocks gravely debris deposits of 6 m visible thickness crop out (Nikolsky et al., 1999), which are however not easy comparable with deposits of the Serkinskoy Suite. Further upwards they are covered by ice-rich deposits with syngenetic ice wedges, which belong to the Yukargirsky Suite. The thickness is limited to < 3 m. According to all peculiarities including the 0.5 m cover layer on top of the Yukagirsky Suite these sediments are similar of those studied west of the Zimov’e River mouth on the Bol’shoy Lyakhovsky Island (see chapter 4.4.2).

Deposits of the Yukagirsky Suite on Svyatoy Nos are covered by characteristic brownish silty fine-sand of low ice content determined as Kuchchugui Suite.

Their stratotype is located a little bit to the east, in the western part of Oyogos Yar in the area of the Kuchchugui Creek. Further east of Cape Svyatoy Nos the base of Quaternary deposits continues below sea level and already in western part of the Oyogos Yar coast the Yukagirsky Suite does not outcrop. Loams with intrapolygonal peat lenses are exposed at the western flank of the Syatoy Nos Peninsula in the area of the Ulakhan Tala Creek below the base of the Kuchchugui Suite, which is situated here not higher than 6 m a.s.l. (Konishev and Koselnikov 1981). These are deposits, which were initially ice-rich due to simultaneous accumulation and freezing but later they thawed and were strongly compressed. Probably it was happen before the accumulation of the Kuchchugui Suite because these sediments were not transformed. They are similar to first type sediments of the Kuchchugui Suite already described on Bol’shoy Lyakhovsky Island (chapter 4.4.2). Maybe they are taberites of Ice Complex deposits of the Yukagirsky Suite or of still older formations.

East of the Ulakhan Tala Creek ice wedge casts were found on top of Kuchchugui Suite deposits that belong to the Krest Yuryakh Suite. They are covered by Late Pleistocene Ice Complex deposits of the Yedoma Suite, which were already found on Svyatoy Nos Peninsula, where they were deposited directly on the Kuchchugui Suite. Preserved fragments of the Bychchagy Suite were not found on the western part of the Oyogos Yar coast; most likely because of limited studies in this area. The stratigraphical highest deposits are

the widely distributed Holocene alas complexes and alluvial deposits of small streams and terraces of larger rivers

In eastern direction the base of the Kuchchugui Suite descends and in the area of the Rebrov and Kondrat’ev River mouths such sediments were exposed only fragmentarily up to 3-4 m a.s.l.. In the valley of the Kondrat’ev River the base of the Yedoma Suite is also situated below sea level. However, further to the west the upper part of Krest-Yuryakh Suite is exposed at the base of the outcrops covered by Ice Complex remains or by deposits of the Holocene alas complex.

Westwards of the Kondrat’ev River, in the central part of the preserved large Ice Complex massive we found a small fragments of non-taberated loamy deposits with ice wedges, which were situated above the Kuchchugui Suite. The presence of two horizons of characteristic intrapolygonal peat layers in a vertical distance of 2-3 m suggested that these could be preserved fragments of the Bychchagy Suite as described on Bol’shoy Lyakhovsky Island (chapter 4.4.3).

Further to the east the outcrop base gradually descends and in the area of the Konechnaya River at the easternmost end of the Oyogos Yar coast of the upper part of the Krest Yuryakh Suite is exposed at the beach level covered by Ice Complex deposits of the Yedoma Suite, which were strongly intersected by alas deposits. The height of the Yedoma remains and of alas level decrease in the same direction.

Thus we can preliminarily conclude that all stratigraphic units found on the southern coast of Bol’shoy Lyakhovsky Island are present also along the Oyogos Yar coast and at Svyatoy Nos Peninsula. Similar stratigraphy and types of lateral contacts show that during the Middle and Late Pleistocene period the area of the Dmitrii Laptev Strait developed as one region

4.5 Palaeoenviromental studies on Bol’shoy Lyakhovsky