Geology Reference
In-Depth Information
Fig. 15.20
Bar graphs of
foreset sandstone laminae
thicknesses in a large-scale
sandwave deposit, Moodies
Group, Eureka Syncline;
(
a
) complete data set; (
b
) data
set with inferred diurnal
subordinate laminae removed.
Note rhythmic thickening and
thinning of laminae best
expressed after removal of
subordinates and interpreted
as possible neap-spring-neap
cycles. Neap-spring-neap
cycles are alternately thicker
and thinner and are
interpreted as perigee and
apogee cycles, respectively
successions (e.g. Hagadorn and Bottjer
1999
; Gehling
2000
; Noffke et al.
2002
) and from Archean sedi-
mentary intervals in South Africa (Noffke et al.
2003b,
2006, 2008
; Heubeck
2009
). Wrinkle structures that
resemble runzel marks are a common MISS and may
imply that all such structures described from the rock
record are microbial in origin.
MISS in the Moodies Group are developed in tidal
channel and tidal flat facies in the Dycedale and
Saddleback synclines (Fig.
15.17
; Noffke et al.
2006
;
Heubeck
2009
). In the Saddleback Syncline, wrinkle
structures are preserved on bedding planes of fine-
grained sandstone. The wrinkles are 5 mm in wave-
length and about 3 mm in height, and in one example
define a 5-10 cm-wide sinuous belt. Wrinkle structures
record crinkling and dewatering of a microbial mat
during burial by freshly deposited sand (Gehling
2000
;
Noffke et al.
2002
), possibly implying a syneresis
origin. Wrinkle structures in the Moodies Group are
covered by a pattern of cracks, which indicate that the
loose grains of the ancient sandy surface must have been
bound together by a cohesive medium before cracking
occurred. Desiccated mudstone drapes are common in
the facies containing the wrinkles supporting a tidal flat
setting (Eriksson
1977
). Microbial binding of sediment
is indicated by roll-up mudstone flakes preserved in a
sandstone matrix (Fig.
15.23
). A roll-over structure is
preserved in a 2-3 cm-thick, fine-grained sandstone
