1. Trang chủ
  2. » Thể loại khác

Encyclopedia of geology, five volume set, volume 1 5 (encyclopedia of geology series) ( PDFDrive ) 2397

1 0 0

Đang tải... (xem toàn văn)

THÔNG TIN TÀI LIỆU

Thông tin cơ bản

Định dạng
Số trang 1
Dung lượng 78,19 KB

Nội dung

554 SEDIMENTARY ENVIRONMENTS/Lake Processes and Deposits Figure Depositional features formed along the shoreline of a wave prone lake (see Figure 5B for an example of a gravelly beach deposit) Reproduced with permission from Adams KD and Wesnousky SG (1998) Shoreline processes and the age of the Lake Lahontan highstand in the Jessup embayment, Nevada Geological Society of America Bulletin 110: 1318 1332 some lake basins, stranded beach ridges provide important evidence of major changes in lake level related to regional climate change Surface Currents Horizontal currents occur in all lakes, and in large lakes they can be a major feature of the water’s circulation Wind stress is the commonest cause of surface currents, but they can also occur in association with river inflow Strong persistent winds can set in motion longshore currents of sufficient strength to transport significant volumes of sediment, leading to the formation of spits (Figure 6) Where surface flow is restricted by topographical obstacles such as islands, the resulting intensification of the current can also affect deeper waters – sandy current-reworked deposits have been observed at depths in excess of 200 m in Lake Superior, for example Subsurface Currents As indicated above, cooler or sediment-laden river water may plunge beneath the surface of a lake to become an interflow or underflow; in this way riverderived sediment can be spread over large areas of a lake It is also not uncommon for significant quantities of suspended sediment to be temporarily trapped at the equilibrium depth of the interflow (which is often the thermocline), to be released gradually through simple settling or rapidly as a result of a breakdown in thermal stratification In both cases, large areas of the lake floor can be blanketed with silt- and mud-rich sediment (Figure 3) Persistent sediment-laden underflows can lead to the development of sublacustrine canyons and deep-water fans with well-developed channels and levees (Figures and 7) and can deposit beds of graded sand over large areas of the deep lake floor The accumulation of subaqueous fans from underflows and turbidity currents is a characteristic feature of deep-water sedimentation in lakes at high and low latitudes Underflows can also play an important role in the formation of one of the most characteristic lacustrine deposits, varves (Figure 8) These are composed of sand rhythmically interbedded with clay or silt and, as originally defined, preserve one annual cycle of sediment accumulation on a lake floor Each sand lamina represents summer deposition, when abundant sediment is transported into the lake, and the cold sediment-laden river water forms an underflow that transports the sand to the lake floor The clay or silt lamina accumulates in winter, when rivers are frozen and the lake is covered by ice However, detailed studies of rhythmically laminated sand and clay or silt deposits have shown that by no means all such couplets are the product of an annual cycle of deposition Some may be graded beds produced by turbidity currents during minutes to hours of sedimentation; others are the product of an individual flood event (the sand) and its waning flow (the clay or silt) with a duration of hours to days Given that varve counting is widely used for chronological purposes, it is clearly essential to exercise care in the interpretation of varve or varvelike deposits Ideally, the inferred annual cyclicity should be confirmed by an independent dating method A variety of rhythmically interlaminated deposits reflecting annual climatic variations other than those related to a freeze–thaw cycle are known from lakes Some of these are described below Mass Failure As indicated above, the foreset slopes of lacustrine deltas may be prone to collapse In addition, many lake

Ngày đăng: 26/10/2022, 11:01