Geology Reference
In-Depth Information
Fig. 9.34 Cyclic variations in sand-lamina thickness of the
estuarine facies in the Changjiang Delta (depth of 47.12-
48.49 cm along the borehole CM-97) deciphering neap-spring
cycles. ( a ) Core photos; ( b and c ) plots of original data and
smoothed data (using 5-point adjacent averaging method) of
lamina thickness over lamina number; ( d ) FFT amplitude-fre-
quency plot of the lamina-thickness data showing two major
peak periods at 28.4 and 11.5 laminae. N , S , and MC in ( a ) are
shortened for neap tide, spring tide, and mud couplets, respec-
tively (After Fan 2001 , photos courtesy of Yoshiki Saito)
of the Wenchang Formation, a shallowing-upward pro-
gradational succession from shallow marine to open
coastal settings (Fig. 9.35 ). Tonglu tidalites exhibit
three orders of periodicities in terms of sandstone and
mudstone layer thickness. Millimeter- to centimeter-
thick alternations of sandstone and mudstone laminae
were ascribed to be deposited by single tidal cycles.
Centimeter- to decimeter-thick alternations of sand-
dominated layers (SDLs) and mud-dominated layers
(MDLs) were interpreted to be formed by seasonal
alternations of storm- and calm-wave climates. The
storm-genesis interpretation of each single SDLs was
convincingly based on the abundance of wave and
storm action products, like intraformational mud peb-
bles, symmetrical wave ripples, and the asymmetrical
small successions of thinning-upward trends which
began with an erosion surface and overlain thick sand-
stone bed with abundant shell debris and mud pebbles,
similar to modern storm-generated SDLs in the
Changjiang Delta. The megacycle of several meters
thick, composed of a lower half coarsening-upward
succession and an upper half fining-upward succes-
sion, was interpreted as a vertical regressive succes-
sion produced by gradual shoaling from the lower
subtidal zone to the upper intertidal zone with the
coarsest and thickest sand layers at the middle, similar
with that of modern open-coast tidal-flat depositional
succession in Fig. 9.32a . Other evidence like general
lack of tidal-channel filling deposits and abundance of
wave-generated structures and small depositional suc-
cessions also supports the open-coast tidal-flat envi-
ronmental interpretation (Fan et al. 2004b ).
Search WWH ::




Custom Search