Geology Reference
In-Depth Information
Fig. 9.24
Schematic model
for the genesis of plasmic
fabric layers (~0.01-0.1 mm
thick) on the muddy banks
along the coast of Guianas.
Because of the viscosity
difference, shear is produced
at the boundary of fluid mud
and sediment with passage of
solitary wave crests (
upper
panel
). The shear is
considered to break the
surface flocs and orient platy
mineral grains, creating a
plasmic layer. Between
weaves, floc deposition is
possible, producing
unoriented interlaminations
(After Allison et al.
1995a
)
in grain size and combined wave-current energy
consequently determine the sedimentary character
changing in a spectrum from the wave-influenced,
tide-dominated on the muddy (fine silt domination)
open-coast tidal flats, through the wave- and tide-
dominated on the muddy (coarse silt domination) open-
coast tidal flats, to wave-dominated on the sandy
open-coast tidal flats. The spectrum change is also evident
in the strata with increasing volume of storm deposits
from the muddy to sandy open-coast tidal flats.
The mudbanks along the Guiana coast are muddy
open-coast tidal flats, predominantly consisting of fine
silt and clay. The prevalent bedding is massive beds, or
parallel to subparallel laminations of a few microme-
ters to millimeters thick (Table
9.3
; Rine and Ginsburg
1985
; Allison et al.
1995b
; Allison and Lee
2004
). The
millimeter-scale laminations of silt enrichment are
commonly layered structures in X-radiographs. The
micrometer scale of the structures should be exam-
ined in thin sections under microscopes, named plas-
mic or unistrial fabric (Kuehl et al.
1988
; Allison et al.
1995a, b
). The plasmic fabric is composed of alternat-
ing layers (about 0.01 mm thick) of oriented and unori-
ented clay and mica with the oriented layers showing
uniform extinction under polarized light. The formation
of plasmic fabric was presumed to result from in situ
shearing by surface gravity waves in sediments being
rapidly deposited from a fluid-mud suspension
(Fig.
9.24
, Rine and Ginsburg
1985
; Allison et al.
1995b
). Solitary waves have almost unidirectional flow
approaching the shore (Wells and Coleman
1978
).
Because of the viscosity difference, passage of solitary
waves induces shear along the fluid mud/sediment
boundary. This shear is postulated to break flocs and
orient platy particles (micas and clays) in the direction
of shear. Floc deposition takes place between waves,
producing unoriented interlaminations (Allison et al.
1995a
). The plasmic fabric is the finest scale of sedi-
mentary structures having wave imprints as known so
far, but it is difficult to be distinguished in fossil rocks
and linked to wave generation.
Most of the muddy open-coast tidal flats are pre-
dominantly composed of silt, with small fractions of
clay and fine sand (Table
9.3
). The preferred surface
structures are small ripples on the muddy flats if no
presence of storm waves. On the Chongming Eastern
Flat in the Changjiang Delta, small symmetrical wave
ripples and combined flow ripples are the commonest
