24
Chapter 2 Continental Sediments
UNDERFLOW
a MELTWATER STREAM
(DENSITY CURRENT) _ _ -=1TERFLOW _OV/RFLOW ~
___________ __ -- ------::t.-J'..,.
c _____________ ----------~
I
/ I
/
_ . _ . _ . -._._ ' _ . _ . _ . -~ /
I
b
UPPER DELTA
FORE SETS
E
....
ci
1
c
MID-DElTA
FORESETS
sand and gravel
(matrix-supported)
gravelly sand
d
l OWER
DELTA
I . . .. . ....... . . : : :
~;~, ~.~
ripple cross.:.J
bedding
multiplegraded beds _
01 line sand
and sill
MASS FLOW
LAKE BOTTOM
DISTAL
VARVES
e
VARVES AND
SURGE DEPOSITS
---t- -
:--1-1~~~rl/ ;:Ui ~~-~~r:~~~i diteg
year E
DISTAL VARVES
1_ ~l- -~f.l winter r
~
layer",:!~,
summer E
_~,." .. "" . ."..
layer
J_
lyear
r
grain
lIow
PROX. VARVES
ci
.1.
~~~~_winter layer
~
(suspension seitling)
load casts
summer layers
(seitling Irom overlIow, interflOw
underllow, and
surge currents)
Fig. 2.4. a Sedimentation in a proglacial (non-i ce
contact), meltwater-fed lake. Note the marked facies
change from the upper to the lower delta fore sets
(b,c,d), as weH as the different types of annual
varves (e,f,g) which are partiaHy obscured by event
deposits, i.e., distinct surge deposits (lake turbidites)
and successions of very thin, graded beds of silt and
fine sand. (Modified from Smith and Ashley 1985)
The irregular proximal rhythmites grade into thinner
«1 cm) and better developed rhythmites in the distal
lake basin (Fig. 2.4). These annual varves are more
regularly bedded and devoid of any current structures
(Fig. 2.4g). Their material is delivered by surface
currents (overtlows) or by currents crossing the lake
at middepth (intertlows). The varves are produced
mainly by vertical settling of fine grained particles
and therefore distinctly retlect summer (light
laminae) and winter (dark laminae) conditions.