Geoscience Reference
In-Depth Information
Table 2.
Sandy lithofacies; their characteristics and inferred depositional processes. Numbers in coloured boxes in left
column refer to lithofacies coding in Fig. 6.
Facies
type
Characteristics
Log motif
Depositional process
Depositional geometry
Fine-grained,
normally-
graded
sandstones
Sharp-based, normally graded,
laminated to ripple cross-
laminated, fine to very fine,
thin-bedded sandstones and
siltstones. Gradational tops -
often bioturbated. Few cm to
dm-thick lamina and beds.
Fall-out from
suspension from
turbulent flow, fol-
lowed by upper to
lower flow regime,
unidirectional
tractional flows.
Tabular beds tapering
out downflow.
3
Normally-
graded,
massive to
stratified
sandstones
Sharp erosively based, normally
graded, laminated to ripple
cross-laminated, medium to
fine, thin to medium-bedded
sandstones. Sharp or thin,
gradational tops. 10+ cm to
dm-thick beds.
Fall-out from
suspension from
erosive turbulent
flow, followed by
tractional, waning
depletive flow.
Tabular beds thinning
downflow.
4
Massive to
dewatered
sandstones
Sharp, locally erosively-based
medium to thick-bedded,
massive, occasionally
dewatered, medium to coarse,
granule sandstones. Basal
shear zones and internal
small-scale normal faulting
locally present. Dish struc-
tures and sandstones pipes/
pillars common in some beds.
Dm to few m-thick beds to
amalgamated units several
metres thick.
Rapid fall-out from
high-density sedi-
ment gravity flows,
followed by post-
depositional dewa-
tering, fluidisation
and remobilisation.
Tabular beds to
elongate tongues;
central to peripheral
part of lobate beds.
5
Ungraded,
stratified
sandstones
Sharp, erosively-based, medi-
um-bedded to thick-bedded,
inversely graded, ungraded,
and normally graded, vaguely
stratified, medium to fine
grained sandstones alternating
with massive intervals. Plane-
parallel to wavy stratifications
and low-angle truncations.
Beds are amalgamated or
have sharp tops. Metres-thick
amalgamated intervals.
Fall-out from
sustained
high-density
sediment gravity
flows as aggrading
traction carpets
(space stratifica-
tion) and shear
lamina.
Inner to central part
of lobes and as scour
fills.
6
Cross-
stratified
plough &
fill sand-
stones
Erosively-based, alternating
cross-stratified and mas-
sive, coarse to fine grained,
sometimes pebbly sand-
stones. Cross-stratification
normally defined by crude
mineral alignments. Basal and
internal scours are claystone
draped.
Repeated scouring
and ploughing
followed by fall-out
from sustained
high-density
sediment grav-
ity flows with or
without tractional
movement as upper
flow regime migrat-
ing megaripples.
Scour fills of channel-
to-lobe transition
area and central
lobes.
7
Cross-
stratified
sandstones
Sharp, erosively based, inversely
to normally graded, medium
to thick-bedded, vaguely to
clearly planar and trough cross-
stratified, coarse to fine grained,
sometimes pebbly sandstones.
Beds/sets/cosets are amalgam-
ated or have sharp tops. Dm-
thick beds and bedsets, cosets
up to few metres thick.
Fall-out from
sustained
high-density
sediment gravity
flows followed
by tractional
movement as upper
flow regime migrat-
ing megaripples.
Outer distributary
channel, channel-
to-lobe transition
megaripples to inner
to central lobe scour
fills.
8
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