Argillaceous rocks(or argillite)can be subdivided into two categories

mudstone and shale

which differ significantly in genetic mechanism

content of organic matters and mode of occurrence

constituent minerals

etc.A study on distribution patterns of lacustrine mudstone and shale as well as comparisons of their distinguished characteristics would be of great help in building up an organic-rich shale depositional model.The Chang 7 Member of the Yachang Formation is represented by a large set of mudstone and shale

which was deposited in the period of maximum lake depth of the Ordos Basin during the Triassic.Their differences

which are ascribed into five aspects (distribution pattern

geochemical cha-racteristics

rock composition

fabric

and main and trace elements)

have been disclosed by typical vertical profile analysis

core-thin section observation

geochemical analysis

etc.That is

(1)distribution pattern:mudstone and shale are complimentarily deposited

with mudstone mainly deposited in delta front

deep-lacustrine-turbidite facies

and semi-deep lacustrine facies

while shale mainly in deep lacustrine facies;(2)fabric feature:shale is characterized by laminae and organic matters occurring along layers

while mudstone exhibits no laminae and its organic matters scattered;(3)rock composition:shale is rich in pyrite

eleven times the content of mudstone pyrite

whereas mudstone holds more dolomite and si-derite;(4)organic geochemistry indices:the TOC of shale with an average of 6.6%

is three times as much as that of mudstone

and the value of S

1

averaging at 3.12 mg/g

is 2.5 times that of mudstone;(5)main and trace elements:shale is significantly higher in trace elements content like U

Th

Mo

Co

Ni

and in ratios like U/Th

V/Cr and P/Al than those in mudstone

while Ti/Al and K/Al ratios are higher in mudstone.In a word

the formation of mudstone and shale in the Chang 7 Member is mainly controlled by sedimentary facies

lake depth

and lake basin productivity.Shale is deposited further away from sediment source area with higher productivity and less oxygen in water.