There are two main idealized models for deposition during cycles of sea level rise and fall:
Type 1 sequences
A type 1 sequence is bounded below by a Type 1 sequence boundary, which is an unconformity marked by subaerial exposure and significant subaerial erosion associated with stream rejuvenation (increase in stream gradient and rate of flow caused by a lowering of base level). Type 1 boundaries are also marked by an abrupt basinward shift in facies so that nonmarine or shallow marine facies above the boundary overlie deep marine rocks below the boundary. Basically, a type 1 sequence begins with relative sea level at its lowest point.
Stage 1: Lowstand systems tract (LST): During a relative fall in sea level, valleys are eroded into the coastal plain and shelf and submarine canyons are eroded into the slope. Sediment bypasses the shelf and slope and is deposited as turbidity currents in submarine fans on the basin floor. Sediment fans can also be deposited on the slope. As relative sea level stops falling sediments may begin to fill the valleys carved on the shelf, creating a lowstand wedge. Together, the lowstand wedge, slope fan, and basin-floor fan deposits form the LST.
Transgressive Systems Tract (TST): As sea level starts to rise, base level increases and fluvial deposits form in incised valleys. The shelf becomes flooded again, creating a transgressive surface. The TST is characterized by one or more retrogradational parasequence sets, meaning that each subsequent cycle of lower order sea level change results in the shoreline stepping landward. Landward of the initial shoreline, where there are no deposits from the lowstand wedge or the transgressive surface, these parasequences will onlap directly onto the sequence boundary.
Seaward of the initial shoreline the TST deposits are often thin due to sediment starvation as clastic sediments become trapped in estuaries. This causes the deposition of a condensed section, which includes the maximum flooding surface – stratigraphic level of maximum relative sea level. Above the MFS parasequences change from retrogradational to aggradational.
Highstand Systems Tract (HST): This systems tract is characterized by aggradation and then progradation of parasequences as the rate of sea level rise slows, stops, and then reverses. Often this is the thickest part of the sequence because clastics stored in estuaries during sea level rise a flushed out onto the shelf during early sea level fall. The upper boundary of the HST is a sequence boundary, formed as sea level fall accelerates and begins to expose the coastal plain and shelf to erosion once again.
Note that during early HST relative sea level may still be rising on the shelf, but at a slow rate compared to sediment supply. At the same time, transgression may stop at the shoreline if sediment supply begins to exceed the creation of accomodation space.
There are two main idealized models for deposition during cycles of sea level rise and fall:
Type 1 sequences
A type 1 sequence is bounded below by a Type 1 sequence boundary, which is an unconformity marked by subaerial exposure and significant subaerial erosion associated with stream rejuvenation (increase in stream gradient and rate of flow caused by a lowering of base level). Type 1 boundaries are also marked by an abrupt basinward shift in facies so that nonmarine or shallow marine facies above the boundary overlie deep marine rocks below the boundary. Basically, a type 1 sequence begins with relative sea level at its lowest point.
Stage 1: Lowstand systems tract (LST): During a relative fall in sea level, valleys are eroded into the coastal plain and shelf and submarine canyons are eroded into the slope. Sediment bypasses the shelf and slope and is deposited as turbidity currents in submarine fans on the basin floor. Sediment fans can also be deposited on the slope. As relative sea level stops falling sediments may begin to fill the valleys carved on the shelf, creating a lowstand wedge.ด้วยกัน lowstand ลิ่ม , พัดลมพัดลมตั้งพื้นลาด และลุ่มน้ำเงินฝากรูปแบบ LST
transgressive ระบบทางเดินปัสสาวะ ( TST ) เป็นระดับน้ำทะเลเริ่มสูงขึ้น ระดับที่เพิ่มขึ้นและรูปแบบฐานเงินฝากชนิดแกะสลักหุบเขา ชั้นจะท่วมอีกครั้ง สร้างพื้นผิว transgressive . ซึ่งทั้งหมดเป็นลักษณะหนึ่งหรือมากกว่าหนึ่งชุด retrogradational parasequence ,ความหมายว่า แต่ละรอบ ต่ำกว่าระดับน้ำทะเล ตามมาเพื่อเปลี่ยนผลลัพธ์ในชายฝั่งก้าวเข้าหาแผ่นดิน . หันไปทางแผ่นดินของชายฝั่งเริ่มต้นที่ ไม่มี เงินฝาก จาก lowstand ลิ่มหรือพื้นผิว transgressive parasequences เหล่านี้จะ onlap , โดยตรงบนลำดับ
ขอบหันไปสู่ทะเลเริ่มต้นของชายฝั่งที่ TST เงินฝากมักจะผอมเพราะอดอยาก เป็น clastic ตะกอนตะกอนกลายเป็นติดอยู่ในบริเวณปากแม่น้ำ . ซึ่งทำให้เกิดการสะสมของส่วนย่อ ซึ่งรวมถึงพื้นผิวและระดับของน้ำท่วมสูงสุดของระดับน้ำทะเลสัมพัทธ์สูงสุด เหนือ MFS parasequences เปลี่ยนจาก retrogradational เพื่อ aggradational
ระบบ highstand ทางเดิน ( HST ) This systems tract is characterized by aggradation and then progradation of parasequences as the rate of sea level rise slows, stops, and then reverses. Often this is the thickest part of the sequence because clastics stored in estuaries during sea level rise a flushed out onto the shelf during early sea level fall. The upper boundary of the HST is a sequence boundary, formed as sea level fall accelerates and begins to expose the coastal plain and shelf to erosion once again.
Note that during early HST relative sea level may still be rising on the shelf, but at a slow rate compared to sediment supply. At the same time, transgression may stop at the shoreline if sediment supply begins to exceed the creation of accomodation space.
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