Geology Reference
In-Depth Information
Fig. 14.9
Interpreted tidalites in the deep-water Cretaceous
Wheeler Gorge, California. (
a
) Location and geologic map,
showing the location of the measured section in the thin-bed-
ded 'Zebra-Striped' succession. (
b
) Outcrop photograph of
the measured section. Despite the color difference, there is
not much difference in mean grain-size between the
dark
layers and the
light
layers, but the
light
layers are much
cleaner with more evident sedimentary structures, primarily
ripple cross-strata. (
c
) Graph of lamina thicknesses versus
lamina number counted. The laminae are generally arranged
in thick-thin couplets, and there is a cyclicity that approxi-
mates 20-30 laminae
by a relatively thin-bedded succession dominated by
both in-place and remobilized (slumped) turbidite
sandstones and siltstones interpreted to be of channel
overbank origin (Fig.
14.11
). The remobilized sec-
tions alternate laterally and vertically with in-place
beds (Fig.
14.12
), and consist of slumped thin-beds
intercalated with and often injected by pebble-cobble
conglomerates similar to those of the channel axis.
The remobilized intervals usually have a topographi-
cally complex upper surface that formed highs and
lows on the paleo-seafl oor (Fig.
14.12
). The in-place
sediments are commonly partially 'ponded' or later-
ally confi ned by this seafl oor topography. Interbedded
with the in-place turbidites are numerous intervals
5-40 cm thick, that I have interpreted as deep-water
tidalites; these are present only in areas overlying
mass-failure generated seafl oor topography. Common
sedimentary structures in these tidalite beds include
plane-parallel and wavy lamination, fl aser lamination,
and ripple cross-lamination which sometimes is
bi-directional, and is commonly encased in mud
drapes (Fig.
14.13
). Detailed analysis of one of these
intervals (32 cm thick) shows that groups of laminae
preserved in this short succession are arranged into
thick-thin couplets and into thickening and thinning
successions exhibiting inverse to normal grading
(Figs.
14.13
and
14.14a
). A power-spectral analysis of
the laminae thickness data (Fig.
14.14b
) shows a very
clear signal with a frequency of 2, equivalent to a
semi-diurnal frequency, as well as frequency peaks
