The Late Quaternary Geomorphologic And Depositional Evolution Of Two Continental Margin Systems In The High Latitude And The Middle Latitude

Repetitions and cycles in stratigraphy are the cores of the Sequence Stratigraphy, which are induced by the cyclic climate change. Since the middle Pleistocene the world climate has evolved into the100-ka climatic cyclicity. In this thesis the evolution of the depositional systems on the continental margin in the last one climatic cycle was studied. Two depositional systems are chosen:one is the deltaic system of Changjiang in the middle latitude, and the other is the slope depositional system in Orphan Basin, western Atlantic in the high latitude. For the deltaic sequence stratigraphy of Changjiang, the postglacial stratigraphy has been well known, but little information has been known about the stratigraphy before LGM, specially the deltaic deposits in the previous-high-sea-level stage of MIS3(57～29ka Cal BP). One70-m core (07SR11) targeted to the stratum of MIS3was drilled on the tidal flat north of Changjiang estuary, and sediment structures, grain size and fossils and14C dates were acquired to interpret the sedimentary record. For the slope depositional systems in the Orphan Basin dominated by the ice stream, the integrated study about the glacial processes of Trinity ice stream and the depositional processes on the slope has been lacked, and this study filled this gap based on the newly and previously acquired data. The newly acquired3D industrial seismic volume and previous2D seismic profiles were used to study the stratigraphy and depositional process on the continental slope. The glacial history around Orphan Basin and the slope depositional processes in the late Quaternary, including sandy turbidity currents, muddy turbidity currents and other mass wasting processes, were inferred based on the sedimentary analysis of piston cores. Core photographs, X-ray photographs, physical properties assisted in the core analysis. The integrated depositional history on the two continental margins including the shallow shelf and the deep continental slope within the last glacial cycle since MIS3was brought forward as follows:In the MIS3stage three or four ice-rafted deposited layers formed during Heinrich events in Orphan Basin, which were sourced from two small ice sheets on the northern Canadian Shield. The melt-water sediments from Hudson Strait directly contributed little to the Orphan Basin. During this period muddy turbidity currents were not present but sandy turbidity currents occurred in the channels on the slope around Heinrich events (H5:45ka Cal BP; H4:38ka Cal BP; H3:31ka Cal BP). Sea level during the MIS3stayed at the depth of30-50meters below the present sea level in Yellow Sea and East Sea off China. In the early stage of the MIS3deltas and other coastal aggradational bodies developed in offshore water where the present water depth is shallower than40m. The sediments transported by the Changjiang River were deposited in the incised valley of MIS4or MIS6age, and built the paleo-Changjiang delta which extended to the water depth of around40m. Since the middle MIS3the main branch of Changjiang had migrated to the south and incised deeper accompanying with the decreasing sea level.In the MIS2stage, Laurentide ice sheet extended to the maximum and some ice streams arrived to the shelf edge. On the northwest Newfoundland shelf, Trinity ice streams moved to the shelf edge and transported the subglacial sediments to the upper slope of Orphan Basin. Glaciogenic debris flow on the trough mouth fan prevailed during29-20ka Cal BP when subglacial sediments were poured down by the Trinity ice stream. Sandy turbidity currents boomed after the advance of the ice stream because of the increase of ice rafted deposits and melting water. Sandy turbidity currents on the trough mouth fan favored to scour the lower relief between the glaciogenic-debris-flow lobes and produced some small channels and gullies. On the southern slope sandy turbidity current dominated in the well-drained canyon systems and built the channel-levee system on the lower slope. Muddy turbidity current occurred when the melting water plume from Trinity Trough burst out between Heinrich event2(24ka Cal BP) and Heinrich event1(16.8ka Cal BP). At the same stage in the present coastal zone the Changjiang River incised to the depth of70～85m below the present floor and the deltaic system moved to the shelf edge. At the stage of MIS2the sedimentation rate enhanced greatly in the Okinawa trough and the turbidity currents were more frequent on the slope of East Sea than in the stage of MIS3. During the period of20～15ka Cal BP, the Trinity Trough ice stream retreated to the modern coastline. On the slope off Trinity Trough, glaciogenic debris flow stopped without the direct supply of subglacial sediment. Sandy and muddy turbidity current still prevailed with the supply of glacial sediments from the local and farer ice sheets.After Heinrich Event1(16.8ka Cal BP), global sea level started to go up with two sea-level jumping events produced by two melting water pulses from Laurentide ice sheet and Antarctic ice sheet. In the Orphan Basin and the surrounding ocean, the second melting-water pulse is obvious by the presence of Heinrich carbonate bed (HO:11.3ka Cal BP) during the Younger Dryas event. After the Younger Dryas event, the sea water intruded to the incised Changjiang valley of the MIS2age. Fluvial sands over30m thick were unloaded in this old valley during16.8～11.3ka Cal BP. In the lower relief tidal flat started to develop after11ka Cal BP, while on the higher platform, north of the incised Changjiang Valley, tidal flat was present after10ka Cal BP. In the Holocene the sedimentation rate decreased substantially in the Okinawa trough and the turbidity currents were less active.Laurentide ice sheet completely disappeared after around5ka Cal BP. Small amount of rafted icebergs were still transported to the Orphan Basin before that. Around7ka Cal BP on the eastern plain of China the sea water intruded to the innermost and the paleo-Changjiang Estuary formed with the peak at Zhenjiang. At7ka Cal BP the tide-dominated deltaic systems started to develop at the bay-head of paleo-Changjiang Estuary and then migrated toward the southeast accompanying the growth of shoals along the south bank and the merging of the sand bars into the north coast. The postglacial stratigraphy around the Shigang Bay showed that the natural evolution of the modern tide-dominated deltaic systems may partly be influenced by the paleo-relief of the base. Some sandbars flanking the Shigang Bay were built on the old knoll. The abandoned channel, the ancestor of the Shigang Bay, followed one branch of the Changjiang Valley in the Last Glacial Maximum.