Geology Reference
In-Depth Information
Fig. 21.11
Photographs of hand specimens showing millime-
ter- to centimeter-scale interlayering of quartz sand (
gray
) and
mud-sized carbonate (
tan
) (heterolithic stratifi cation) in the
lower part of the Mississippian Bayport Formation, Bayport,
Michigan. (
a
) Lenticular and wavy bedding of quartz sand in
carbonate mud laminae in the lower part, changing upward to
fi ner scale lamination. Note rain drop impressions in the carbon-
ate lamina on the top of the specimen (
coin
diameter is 2.5 cm).
(
b
) Interlayered planar- to wavy-bedded and rippled quartz
sandstone (
dark gray
) and carbonate mud laminae (
light gray
to
tan). The ripple in the
lower part
of the photograph and its
internal trough cross-lamination indicate tidal current direction
(possibly fl ood tide) to the right. Note also that the smaller current
ripples in the upper right indicate tidal current reversal (ebb
tide). In both ripples carbonate mud fi lls the ripple troughs forming
fl aser bedding
21.6
Peritidal Cycles and Sequence
Stratigraphy
Other features common in supratidal setting
include very thin lenticular bedding (Fig.
21.17b
),
root casts (Fig.
21.15b
), rain drop impressions
(Fig.
21.11a
) and tepee structures (see the previous
section). The supratidal facies may be capped by
wind-blown bimodal and super-mature quartz sand-
stone (Fig.
21.19d
) during seaward progradation (see
the Lower Triassic tidalites in Sect.
21.7.5.1
). The
supratidal facies of beach ridges and channel levees
(Fig.
21.2b
) are laterally discontinuous and have a
very low preservation potential. They consist of very
thin bedded, interlaminated mud cracked fenestral
mudstone laminae with a thin microbial coating and
crinkly stromatolite boundstone (Shinn
1986
) .
The peritidal facies commonly are arranged into
meter scale, shallowing-upward successions of sub-
tidal- to tidal fl at facies (e.g. Wilson
1975
; James
1984
; Hardie and Shinn
1986
; Grotzinger
1986b,
1989
; Pratt et al.
1992
; Pratt
2010
). In a single shallow-
ing-upward cycle, facies boundaries are transitional
in accord with Walther's Law (for an example see
fi gure
21.21a
), and the contact between the shallow-
water and deeper facies of the overlying cycle is
erosional or sharp due to non-deposition or prolong
exposure (Fig.
21.21a, b
).
