Since the late Pleistocene, the sediments in bransfield strait, Antarctica have obvious signs of ice-sea environment and are typical glacial sediments. Based on the comprehensive analysis of grain size, composition, structure, micropaleontology and surface structure of seasonal particles of surface and columnar sediments from 43 stations, the types, zoning and sedimentary environment of glacial sediments in the study area were preliminarily studied, and the sedimentary models of interglacial/high water level and glacial/low water level were discussed. Glacial sediments can be divided into two types: residual moraine and mixed moraine. Different types of glacial sediments have different media conditions and corresponding biological assemblages, reflecting a certain sedimentary environment. The types of modern glacial sediments in the study area can be divided into seven regions, and the characteristics of each region are obviously different. The sedimentary models of interglacial/high water level and glacial/low water level have obviously different characteristics, which are controlled by environmental factors such as global climate evolution, coastline contour, seabed topography and hydrological conditions.
Glacier marine sedimentary environment; Sedimentary model; Brasfield Strait; South Pole
1 preface
One of the most important characteristics of Antarctic continental margin sediments is that they are strongly influenced by glaciers. Glaciation not only directly shapes the sedimentary characteristics, but also continuously transforms the seabed sediments through ocean currents. Glacial sediments are the products of melting and sinking of moraines transported into the sea by glacial rafts, which are mixed with marine sediments and transformed to varying degrees by contemporaneous or epigenetic ocean forces. Anderson et al. (1980) defined glacial sediments as sediments accumulated on the seabed under the action of glaciers and oceans, and transported by glaciers, ice shelves or related currents, which contained rock materials with different particle sizes brought by glaciers and ice rafts. Therefore, glacier sediment is a kind of mixed sediment, which has the dual genetic characteristics of glacier and ocean in particle size, composition, structure and structure.
The Bronsfield Strait is located between the Antarctic south shetland islands and the Antarctic Peninsula, with a geographical location of 6130 ′ ~ 64 00 ′ s, 62 00 ′ ~ 54 30 ′ w, which belongs to the high-latitude sub-Antarctic region and is close to the Antarctic continent, and is a typical glacial marine sedimentary environment (Jeffers,1988; Jeffers et al., 199 1), is one of the ideal places to study glacial deposits and glacial and interglacial sedimentary models in tectonic active basins.
1990 ~ 199 1 in the southern ocean summer, Ocean No.4, the former Ministry of Geology and Mineral Resources of China, conducted geological and geophysical surveys in bransfield strait, Antarctica, and collected surface and columnar sediment samples from 43 stations of different geomorphic units (island shelf, island slope, trough, continental slope and continental shelf) in the Strait. Based on the comprehensive analysis of grain size type, composition, structure, microfauna and surface structure of seasonal grains of surface and columnar sediments, the types, sedimentary environment and zoning of glacier sediments in the Bronsfield Strait are preliminarily studied, and the sedimentary models of high water level/interglacial period and low water level/glacial period are discussed.
2 background of modern sedimentology
The Bronsfield Strait is covered with ice and snow for six months every year (June-165438+1October, Antarctic winter), and the other six months (June-February, 65438+May, Antarctic summer) are the melting season of ice and snow, which is the active period of sediment transport and deposition. In fact, the study area is a complex sea area controlled by different water masses and with diverse topography in the northwest waters of the Antarctic Peninsula.
Due to the influence of islands and topography, the water mass distribution and circulation in the Bronsfield Strait in summer are extremely complicated. In addition to local action, there are three kinds of water intrusion in the strait, which affect the hydrological conditions of the strait. They are the surface water of Weddell Sea with lower temperature and slightly higher salinity, the surface water from Billings King's Sea in summer and the low temperature Billings King's Sea near the Antarctic Peninsula (Yang Tianzhu et al., 1989). Among them, the first two kinds of water masses are the main components that affect the hydrological conditions in the Strait. The current in the Strait is mainly northeast. The Antarctic circumpolar deep water can partially reach the western part of the strait, and after entering the strait, it becomes denatured deep warm water with higher temperature and slightly higher salinity. However, the analysis of lateral circulation shows that this culture does not seem to change the general trend of strait flow, but only the size of local regional flow. According to the sea area, the Strait can be divided into three parts: the northern part of the Strait, the southern part of the Strait and the central part of the Strait. The temperature and salinity characteristics of the three parts are quite different. The northern part of the Strait is mainly composed of the surface water of the Billings King's High Sea, which is characterized by high temperature and low salt. The southern part of the Strait is dominated by Weddell sea water, which is characterized by low temperature and high salinity. The bottom water in the center of the strait is a cold water mass with slightly higher salinity.
The Bronsfield Strait is a long and narrow submarine trough, which extends in the northeast, and its northeast end turns to the northeast, with a length of about 400km and a maximum width of 100km (from Maxwell Bay to Antarctic Peninsula). The Antarctic Peninsula on the south side is an S-shaped peninsula extending northward from the Antarctic continent. South shetland islands in the north is composed of islands arranged in chains in the northeast, such as Smith Island, Rhode Island, Si Nuo Island, Deception Island, Livingston Island, Greenwich Island, Robert Island, Nelson Island and king george island. There are many natural fjords in the Strait, such as Livingston Bay and Maxwell Bay. The seabed slopes from northeast and southwest to the middle. Narrow island shelves and slopes are distributed in the north, wider in the south, with a trough in the middle. The seabed topography on the north and south sides and at the east and west ends is very different, showing asymmetry, steep in the north and gentle in the south, high in the west and low in the east. The northern 1000m isobath is near the shore, the island shelf is less than 5km wide, the southern part is relatively gentle and shallow, and the Antarctic Peninsula shelf is 45km wide. Taking the 60 W meridian as the boundary, the study is divided into two parts: northeast and southwest. The terrain of the northeast section runs from northeast to southwest, with a length of about 360km and a maximum water depth of 2784m, which is the main part of the trough, and the seabed gradually descends from the northeast and southwest sides to the middle. The terrain on the north and south sides is obviously asymmetrical, steep in the north and gentle in the south. The average slope of the northwest slope is (84 ~192) ×10-3 (4 50 ′ ~10 54 ′); The average gradient of the southeast slope is (38 ~ 81) ×10-3 (210 ′ ~ 4 40 ′). The secondary topography of this section is arranged in NNW-South-South-East direction, such as small waterways and submarine canyons between islands on both sides of the trough slope. The water depth in the southwest section is shallow, most of which is less than 1000m, and the terrain changes are complicated. The terrain trend is still dominated by northeast-southwest, followed by northwest-southeast. Island shelf and continental shelf, island slope and continental slope, and even bottom trough all intersect in a grid shape. Generally speaking, the terrain in this area is stepped from shallow to deep, that is, from the continental shelf and island slopes with different widths to the wider platform, and then to the edge of the platform, the terrain becomes steep again, and finally becomes a gentle depression at the deepest starting point. According to the topographical features, the Bronsfield Strait is actually a trough, which can be divided into three sub-troughs: the northern sub-trough, the central sub-trough and the southern sub-trough. The water in the northern trough is the deepest, reaching 2784 meters. The water depth of the southern sub-trough is the shallowest, less than 1000 m.
Three types of glacial sediments
According to the study on the characteristics of surface and columnar sediments and the regional geological background data, most of the terrigenous detritus and volcanic materials in the Bronsfield Strait sediments come from the Antarctic Peninsula and South shetland islands (Wang Guangyu et al., 1996), and there is no surface runoff. The bedrock lithology of Antarctic Peninsula and South shetland islands is complex, mainly composed of Mesozoic and Cenozoic volcanic rocks and metamorphic rocks. The bedrock is constantly frozen, etched and ground by glaciers, and ice rafts carry a lot of debris into the sea, forming glacial deposits in the Bronsfield Strait. According to the comprehensive analysis of the characteristics of surface and columnar sediments, such as type, granularity, composition, micropaleontology and surface structure of timely particles, the sediments in Bronsfield Strait have obvious ice-sea environmental signs since the late Pleistocene, and are typical ice-sea sedimentary products.
Due to the difference of environmental factors, glacier sediments with different characteristics will appear in different regions. Harland( 1966) systematically studied the glacial sediments in the Southern Ocean, and divided the glacial sediments into positive moraine and secondary moraine. The former is the product of deposition after the melting of stranded ice shelf, which is characterized by lack of sorting, bedding, marine fossils and almost bottomless transformation; The latter refers to the sediments formed by the melting of ice shelves or icebergs and floating ice. The main feature is that sedimentary particles have undergone different degrees of ocean current transformation and are rich in marine fossils. Anderson et al. (1977, 1980) further divided the secondary moraine into mixed secondary moraine (complex) mainly composed of fine-grained mud and silty sand according to the changes of gravel, sand and mud content, sediment granularity parameters, bedding and relative abundance of foraminifera.
According to the characteristics of surface sediments and columnar sediments, and referring to the standards put forward by Harland( 1966) and Anderson et al. (1977, 1980), the glacial sediments in bransfield Strait can be divided into two types: residual moraine and mixed moraine. Table 1 shows the classification characteristics of glacier sediments in the study area. Residual and accessory moraines are mainly gravel, followed by silty sand, with low silt content, good coarse and fine composition and poor fine composition. Mainly distributed in the southern shelf-upper slope, the eastern shelf and the northern shelf-island slope, with relatively rich diatom content, low foraminifera and radiolaria content, and foraminifera are mainly siliceous shells; Mixed moraine is mainly silt and silt, with a small amount of gravel. Choose fine ingredients, while coarse ingredients are the opposite. Mainly distributed in the central trough-southern low slope, south shetland islands Bay and the southwest shelf area of the Strait, with relatively rich diatom content. It can be seen from the table 1 that the grain size composition and characteristics of residual moraines and mixed moraines in the Bronsfield Strait are slightly different from those discussed by Anderson et al. (1980). According to the grain size composition characteristics of surface and columnar sediments in the study area, residual moraines can be further divided into two categories: silty sand, argillaceous sand and silty sand; Mixed moraines can be divided into two types: gravel-containing and basically gravel-free.
Table 1 Classification Characteristics of Glacier Sediments in Bronsfield Strait
Classification of modern glacial marine sediments and sedimentary environment
The classification of glacial sediments is mainly restricted by environmental factors such as coastline contour, seabed topography and hydrological conditions. Different types of glacial sediments have different media conditions and corresponding biological assemblages, which reflect the specific sedimentary environment. Figure 1 shows the distribution of modern glacial sediments in the Bronsfield Strait.
Figure 1 distribution map of glacial sediment types in Bronsfield Strait
Figure 1 Distribution of Ice-Sea Sediment Types in bransfield strait, Antarctica
According to the characteristics of surface sediments, the classification, basic characteristics and sedimentary environment of modern glacial sediments in the Bronsfield Strait are briefly described as follows.
Residual moraine sedimentary area (Ⅰ)
Area ia: located in the southern shelf-upper slope environment. The sediment is mainly gravel, with a small amount of silt and silt, which is poor in sorting. The composition contains high content of basalt chips and low content of volcanic glass. Heavy minerals are mainly olivine, pyroxene, amphibole and magnetite, and clay minerals are characterized by high illite content and low montmorillonite content, indicating that terrigenous clastic materials mainly come from the western Antarctic Peninsula. The timely particles have formed scratches and impact pits, some of which have been obviously worn and rounded. Diatoms and siliceous shell foraminifera develop. This area is close to the Antarctic Peninsula, and the transport medium is obviously glaciers and ice rafts. At the same time, it is strongly influenced by the cold water tributary of Weddell Sea, and the hydrodynamic conditions are relatively strong.
Zone Ⅰ B: the area between the island shelf-island slope belt in the northwest of Taiwan Province Strait and Nelson Island and Robert Island, with an open bay on the north side, which is an environment with no or weak barriers. The sediments are mainly gravel, basically lacking silt and silt, and the sorting is poor. The composition of rock debris and volcanic glass are high, heavy minerals are mainly olivine, pyroxene, amphibole and magnetite, clay minerals are high in montmorillonite, diatoms are mainly in deep-water assemblage, foraminifera are characterized by mixed assemblage of siliceous shells, calcareous shells and cemented shells, and the mechanical action structure of timely particles is clear, indicating that terrigenous debris mainly comes from its northern island, with high hydrodynamic energy and strong ocean current transformation in the later stage.
Area Ⅰ C: Located in the eastern part of the Strait, it is an open marine environment with no barriers. The sediments are mainly gravels, which are basically short of mud and silt. The surface sediments are thin or missing, with poor sorting, high cuttings content, complex composition, complex heavy minerals, and more garnet, rutile and zircon besides common unstable minerals, indicating that the provenance is complex, and the ocean current has strong transformation, which is related to the fact that this area may be the place where Antarctic underflow and Vedel current flow pass, or the high-density cold water mass.
Mixed Paramoraine Sedimentary Area (Ⅱ)
Zone Ⅱ A: Located in the central trough of the Strait and the low slope environment in the south. The sediments are mainly diatom mud or siliceous mud silt, without gravel, mainly composed of biogenic siliceous components, terrigenous quartz silt and volcanic materials. The mass fraction of diatom is more than 30%, and the highest is 70%, indicating that the scouring effect of ocean current is relatively weak, the sedimentary environment is relatively stable, and a small amount of terrestrial materials come from Antarctic Peninsula and South shetland islands.
Ⅱ B area: Island shelf-island slope area south of king george island-Nelson Island Greenwich Island Line, with semi-enclosed bays and fjords on the north. The sediments are mainly silt and silt, with a small amount of gravel. Heavy minerals are mainly olivine, pyroxene, amphibole and magnetite, diatoms are mainly composed of deep-water assemblages, and foraminifera with calcareous shells have a certain position, indicating that glaciers are the main factor controlling sedimentation, followed by water flow. According to the research of Kin et al. (199 1), Maxwell Bay is a typical U-shaped glacier valley, with a depth of 500m in the middle of the valley bottom, in which Holocene sediments have seasonal mud layers, which is a typical glacial deposit.
Ⅱ c area: the shelf slope area located in the southwest of the Strait. According to the research of et al. (1989) and Lin Qing et al. (1989), the sediments in this area are mainly mixed deposits of silt and silt.
Volcanic ejecta Sedimentary Area (Ⅲ)
Located near Deception Island on the west side of the Strait, it is an island shelf environment. The sediments are mainly composed of pyroclastic materials (including gravel, sand and silt), and the mass fraction of volcanic glass is as high as 72% ~ 80%. The provenance is obviously related to the submarine volcanic activity in 1969 ~ 197 1 year. Of course, we can't rule out the source of south shetland islands pyroclastic materials, which have been transformed by ice rafts and ocean currents.
The boundary between residual moraine and mixed moraine in Brownsfield Strait is roughly equivalent to the boundary between the upper and lower slopes in the southern part of the strait, with a water depth of 700 ~ 800 m; However, its northern border has changed greatly. In the area without open bay, the boundary line is equivalent to the foreshore to offshore area of sea ice zone, and in the area with open bay island shelf-island slope, the boundary line is close to the lower edge of island slope.
Discussion on sedimentary model of 5
5. 1 high water level/interglacial sedimentary model
According to the classification and sedimentary environment of modern glacial sediments in bransfield Strait, the comprehensive sedimentary model of high water level/interglacial period is shown in Table 2 and described as follows:
During the interglacial period, the water scoured the continental shelf-upper slope area (shoal and platform) with water depth less than about 250 meters, resulting in residual moraine deposition mainly composed of coarse ice raft debris and volcanic gravel; In the deep water area, the influence of ocean currents on sedimentation is obviously weakened, mainly due to the sediment deposition caused by the erosion of shallows and platforms. Sediments are usually argillaceous sand and sandy mud, which gradually become thinner with the increase of water depth (Figure 1, Table 2). Fine-medium gravel produced by ice rafts still exists in deep-water sediments, but its composition ratio is much lower than that in shallow water. On steep slopes and island slopes, gravity flow deposits generally exist in sediments, forming near-source sedimentary facies; Coarse clastic particles redeposit and recombine into gravel and coarse sand (Jeffers,1988; Jeffers et al., 199 1).
Table 2 High Water Level/Interglacial Sedimentary Model in Bronsfield Strait
In the central trough and the southern slope area (usually called the bottom of the Blancsfield basin), sediments are mainly composed of three components: the most important one is siliceous biological components, usually diatoms; The terrigenous clastic components are mainly seasonal silt, which is the most abundant at the bottom of continental slope. Pyroclastic components (including volcanic ash) often come from volcanic eruptions near the seabed and land, and appear in sediments as disseminated layers and independent volcanic ash layers (sometimes several centimeters thick). Typical sediments at the bottom of the basin are siliceous mud and soft mud containing volcanic ash. Volcanic islands, seamounts and axial fire ridges (which may be back-arc spreading centers) produce secondary slope migration of sediments and accumulate pyroclastic sediments (for example, volcanic ejecta Sedimentary Area III near Deception Island). The axial fire ridge at the bottom of the basin can act as a barrier for the lateral migration of sediments in the basin (Jeffers et al., 199 1), so the pyroclastic components from south shetland islands are often separated from the pyroclastic and siliceous components at the bottom of the basin.
The bays, headlands and submarine canyons in southern south shetland islands transport a large amount of pyroclastic sediments from south shetland islands to the adjacent basin bottom, forming a fan-shaped sedimentary area. On the contrary, it seems that the grooves on the shelf of the Antarctic Peninsula did not transport a large number of terrigenous clastic sediments to the bottom of the basin (Jeffers,1988; Jeffers et al., 199 1), silica mud and ooze are also deposited on the lower slope of the Antarctic Peninsula. The coarse clastic components of interglacial period appeared in columnar sediments, which may be caused by the collapse and accumulation of channel sediments on the slope of continental shelf. The residual moraine on the southern shelf-upper slope shows that the glaciers on the Antarctic Peninsula do not seem to produce a large number of fine-grained sediments or meltwater, and transport fine-grained components to the seabed sediments on its north side.
5.2 Low Water Level/Ice Age Sedimentary Model
Due to the limitation of sampling quantity and test analysis data of columnar sediments, this paper can't get the classification of glacial marine sediments during the ice age. According to the characteristics of columnar sediments and seismic profile data, the sedimentary model of low water level/ice age is shown in Table 3 and briefly discussed as follows:
Table 3 Low Water Level/Glacial Depositional Model in Bronsfield Strait
During the ice age, the Antarctic Peninsula shelf and the South shetland islands shelf (platform) were covered by ice sheets. On the shelf of south shetland islands Island, the horizon of the ice sheet is roughly 200-375m below the modern sea level (Anderson,1989); It is speculated that there may be a thicker ice sheet on the shelf of the Antarctic Peninsula, the floor line may reach the outer edge of the shelf, and the grooves on the shelf may be covered by floor ice sheets. Seismic records show that there is a sedimentary wedge in the front of the Antarctic Peninsula shelf trench, which may reach a water depth of 750 meters, forming a platform with greater water depth. If the continental shelf of the Antarctic Peninsula has been covered by ice sheets, these sedimentary systems may continue to deposit. It can be seen that in the shallow platform area of continental shelf and island shelf, glacial erosion and coarse debris deposition such as gravel mainly occur, while the bays and headlands of south shetland islands are strongly eroded by glacial action, and coarse debris deposits migrate to the sea; In the continental slope area, the sediments eroded from the continental shelf area are characterized by progradation in areas lacking ditches and troughs; In the central trough and the southern low slope area (basin bottom), the deep-sea biological sedimentation is obviously weakened, and turbidite exists locally. Seismic data show that there is a set of superimposed sequences under the modern deep-sea sedimentary strata at the bottom of the basin, which is interpreted as turbidite, which is a notch progradation, and its genesis may begin with slump, landslide and debris flow.
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Preliminary discussion on sedimentary environment and model of Bronsfield Strait, Antarctica
Wu Nengyou, Duan, Cai Qiurong
(Guangzhou Marine Geological Survey, Ministry of Land and Resources, Guangzhou 5 10760)
abstract
Bransfield Strait is located in the high latitude area of sub-Antarctica. It is a back-arc basin with active expansion, and it is an ideal place to study Antarctic ocean-glacier deposition and sedimentary environment. Based on the data of HY4-9065438+ 0 and 1990~ 199 1 voyage, the types and distribution of marine-glacial sediments in Blashfield Strait, Antarctica since the late Pleistocene are inferred. According to the above characteristics, the sedimentary environment and model of high water level (interglacial period) and low water level (glacial period) are preliminarily discussed. Marine-glacial sediments can be divided into two types: residual accessory sediments and composite accessory sediments. It shows that different types of marine-glacial deposits in different environments have different media conditions, lithologic compositions and microbial communities. At present, marine-glacial sediments can be divided into seven regions. These data provide us with an opportunity to test the ocean-glacier deposition model of tectonic active basins during glaciers and interglacial periods. In addition to obvious structural and glacial effects, the sedimentary model emphasizes the influence of sea level change on sedimentation.
Key words: ocean-glacier sedimentary environment, sedimentary model, bransfield strait, Antarctica.
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