Geology Reference
In-Depth Information
Fig. 21.17
Desiccation cracks in laminated supratidal sediment:
(
a
) Sediment fi lled mud crack in a peloid wackestone lamina cov-
ered by an intraclast grainstone transgressive lag deposit; Cave
Hill Member of the Mississippian Kinkaid Formation, southern
Illinois. (
b
) Quartz sand-fi lled 'V' shaped mud crack in a mixed
carbonate-siliciclastic tidal fl at sequence. Note the irregular wall
of the crack and a small quartz sand-fi lled lenticular bed (
dark
gray
) in thickly-laminated dolomudstone toward the
middle
of
the
left side
of the photograph. Fine-scale laminations in the
upper part
comprise dark crinkly stromatolite and dolomudstone
laminae. (
c
) Plan view of mud cracks in the upper surface of a
supratidal deposit; Mississippian lower St. Louis Limestone,
Columbia Quarry, St. Clair County, southwest Illinois. (
d
)
Dolomudstone-fi lled mud crack in tan to
pink laminated
deposits
of the supratidal facies (Photo courtesy of Dr. M. Ghomashi,
Sistan-Baluchistan University, Zahedan, Iran); Lower Triassic
Sorkh Shale Formation, Tabas failed rift basin, east central Iran
stacked complete and incomplete meter-scale succes-
sions (see the illustrative examples). Progradation to
develop stacked peritidal meter-scale cycles can be
generated by intrabasinal autocyclic processes and
extrabasinal allocyclic mechanisms including eustatic
sea level fl uctuation and tectonic subsidence (e.g.,
Hardie
1986
; Pratt et al.
1992
; Pratt
2010
) .
fl at shoreline and island progradation and lateral migra-
tion of tidal channels. Tidal fl at progradation is the
dominate process during greenhouse periods (small polar
ice volume) due to lower-amplitude high-frequency
sea level changes (Lehrmann and Goldhammer
1999
;
Burgess
2006
) . Tidal fl at shoreline progradation to
generate stacked shallowing-upward peritidal cycles
(Ginsburg
1971
; Hardie
1986
) assumes gradual sub-
sidence, slow sea level rise or stillstand and changes in
sedimentation rate during deposition. High sedimen-
tation rate in tidal fl at areas results in progradation
21.6.1.1 Autocyclicity
Autocycles form in response to processes operating
within the environment of deposition and include tidal
