Abstract:

Characteristics of lacustrine fan-deltas generally are as follows:1)Fan-deltas have steeper slope(average gradient of 0.6-15‰),but cover-ing smaller area;2)Texturally and mineralogically,fan-deltaic deposits are less mature and more coarse-grained,reflecting proximity to source areas;3)Fan-deltas have well-developed subaqueous distributary channels,but with little or poor-developed subaerial alluvial plain;4)Fan-deltaic deposits are often associated with the turbiditic deposits.According to features of ancient lacustrine fan-deltas in the eastern part of China,fan-deltaic system can be divided into seven sub-facies.1)Distributary channel sediments:they are developed within the upper part of fan-deltaic sequence.The depositional conditions are changed frequently because of the progradation of intermittent river-borne sediments directly into standing lake basin.These sediments are similar to those of braided rivers,with poor-sorted and immature sediments dominated by gravel and gravelly sands,cemented bymud,and grading upward into normal rhythms of fine-grained material.2)Interfan-mush sediments:they are poorly developed,comprised predomi-nately of silts and mud,sometimes intercalated with fine-grained gypsum and salts,poor bedding for bioturbation.3)Subaqueous distributary channel sediments:these constitute an important element in the fan-deltaic system.Their features of deposition are similar to those of braided rivers,but the uppermost mud is often absent because of later current erosion or reworking by waves.The probability cumulative curves of these sedi-ments show three populations,of which the traction population occupies 0-30%,saltation population 40-93%,while the suspension population occupies 10-20%.4)Subaqueous inter-distributary channel sediments:these sediments are dis-tributed aside from the subdistributary channels often reworked by waves or re-moved by currents along shore area.They are composed generally of fine-grained sands and silts,interbeded with mud characterized by flaser and convolute bed-dings,with normal or reverse rhythms.5)River mouth bars:sand bodies are mainly composed of well-sorted medium-grained sands,fine-grained sands and silts characterized by cross,hori-zontal,flaser and lenticular beddings with reverse rhythms.Sometimes deforma-tion layering and bioturbation structures can be seen in silty clays.6)Delta-front sheet sands.7)Prodeltaic mud.Fan-deltaic depositional models may serve as valuable tools for searching economic mineral deposits and fossil fuels.