5-湖泊沉积体系

Principal factors
affecting lacustrine sedimentation
• Properties of lake’s water
• Chemistry of lake’s water • Fluctuations of shorelines • Relative abundance of river-derived clastics and autochthonous sediments
Various factors controlling lake sedimentation
Thermal stratifications of lake’s water
• Epilimnion (): an upper, warm, oxygenated and circulating layer • Hypolimnion(): a lower, cooler and relatively undisturbed region, and might be anoxic, favoring preservation of organic matters on lake floor. • Metalimnion (): the intervening zone where temperature decreases most rapidly with depth (called a thermocline)
TWO MAIN FEATURES OF LAKES
• Sensitivity to climate changes, and thus the best indicators of continental paleoclimate • Abrupt variations of sedimentary facies in vertical sequences due to shifting of shorelines and biochemical fluctuations in lake waters
CLASTIC SEDIMENTATION
• Siliciclastic sediments are mostly transported by rivers, either in suspension or as bedforms;
• Input of the sediments is primarily influenced by the nature and size of the surrounding drainage basins;
LAKE DELTAS
• Lacustrine deltas are limited to a range of coarse-grained deltas, particularly the Gilbert-type, found mainly in tectonically active basins, and a range of finer-grained fluvial and wave-modified deltas. • Underflows (hyperpycnal flows) tend to be predominant in fresh water lakes, responsible for distinct prodelta facies composed of a heterolithic assemblage of interlaminated mud, silt and fine sands. • Large sediment loads, due to progradation of delta front, can cause intense liquefaction, load structures, mud diapirs in prodelta area.
CURRENTS IN LAKES (2)
Driven by inflowing rivers
• Underflows: flowing along lake floors
• Overflows: flowing along lake surface
• Interflows: sediment-laden river water flow along the thermocline, greatly influenced by Coriolis force;
Schematic representation of open lake system with high terrigenous input into the basin and low input hard-water lake
Salt precipitations due to increasing evaporative concentration
Mixing of thermal-density stratifications
• Holomictic lakes: experience circulation completely down to the bottom at the time of winter cooling • Meromictic lakes: only undergo partially circulation, leaving a permanently stagnant bottom layer *Factors affecting the mixing: seasonal changes; wind strength, directions, and fetch (distance over which the wind is uninterrupted by land); and variations in salt concentration
either side of the deepest axis of the lake basin. Geostrophic effects (Coriolis force) are superimposed on these circulation pattern, causing a deflection to the right in the northern hemisphere and to the left in the southern hemisphere.
• Supply of the sediments is often seasonally controlled;
BEACH & NEARSHORE ZONE
• Siliciclastic sediments generally occur around river mouth;
• Shoreline is most marked by extensive swamps instead of beaches, in contrast with marine counterpart. • Extensive sand- or mudflat are particularly characteristic of saline lakes, also called playa or inland sabkha • There is no tidal influence
Various types of lacustrine lithofacies
END OF THIS SESSION
LACUSTRINE SYSTEM
CLASSIFICATION OF LAKES
• Hydrologically open lake: having an outlet
• Hydrologically closed lake: lacking an outlet
*Ho may pass through “open” and “closed” phases many time in their histories
OFFSHORE ZONES
• Sediments are supplied by pelagic fall-out, surge-like and semi-permanent density currents and mass flows. • Bottom-hugging density underflows can travel a long distance and for a long time period, being effective in distributing fine-grained sediments over large areas of deep lake basin. • Turbidite fan can be formed, with well developed channels and levees
Rhythmite types shown by lithologies and colors
Various Orders of Cyclicity
(1-3 order cycles are of tectonic origin, 4-5 order climatically controlled)
CURRENTS IN LAKES (1)
Wind-driven circulation:
• In the nearshore, currents are strong, and directed parallel to the coast
• In deep-water region, currents tend to be weak, and lack preferred direction *Typical circulation pattern is one of the closed gyres lying to
RHYTHMITES
Rhythmites are consequences of finely laminated , regular alternations of two or more contrasting sediment types;