Geoscience Reference
In-Depth Information
SUBENV .
(FACIES
ASSOCIATION)
GRAIN SIZE
AND STRUCTURES
LITHOFACIES
GROSS
ENVIRON.
LITH
% Sand
DESCRIPTION
clay
silt
vf
f
mc
vc
gp
c
MSb,
MSs(b),
Mb
Dark grey sandy siltstones with local cm-scale event beds of moderately
to well sorted very fine to fine sandstone. The sandstone may be rippled
or highly bioturbated. Occasional coarse grained lag deposits, sometimes
with sideritised mudstone clasts. Planolites , Teichichnus , Phycosiphon ,
Palaeophycus and bivalve burrows.
SMb2
Distal transition
zone
(FA 18)
Sx, Sr,
Shcs,
SMs
(b,I,p,r),
SMb1,
SMb2
Moderate to well sorted v.f. to f.-grained silty sandstones with wave- and
current-generated ripple scale structures. Highly bioturbated versions
which may be poorly sorted where lag deposits have been biogenically
reworked. Planolites and Palaeophycus are the main ichnotaxa,
Asterosoma , Teichichnus , Skolithos , bivalve burrows and Diplocraterion
are also present.
Moderate to well sorted silty v.f. to f.-grained sandstones. Small- to
medium-scale cross-stratification generated by wave activity. Occasional
event beds. Locally high levels of bioturbation, with Palaeophycus and
bivalve burrows prominent, but also including Asterosoma ,
Rhizocorallium and Skolithos .
Sg, Sp,
MSs(p),
MSs(I),
MSb
Proximal
transition
zone
(FA 17)
Sx, Sp,
Sr, Stb,
SMs
(b,I,p,r),
SMb1
Sm,
MSs(r)
Lower
shoreface
(FA 16)
Sx, Sr
Sp,
SMs(r)
Moderate to well sorted f.-grained sandstones exhibiting medium scale
cross-stratification, hummocky/swaley laminae-sets and wave and
current ripple cross-lamination. Sporadically bioturbated, with occasional
resting, escape and excavation traces, plus Planolites , Diplocraterion
and Skolithos .
Upper
shoreface
(FA 15)
Fining-upward profile with moderately to poorly sorted, locally bimodally
sorted, m. to c.-grained sandstones. Lags of mudstone clasts, quartz
and coaly debris are present. Higher beds are moderately to well
sorted, finer grained and locally heterolithic. They mainly show current
ripple cross-lamination, along with sporadic bioturbation in the form of
Planolites and escape traces.
Sx, Sr
SMs(r),
MSs(r)
Tide-influenced
distributary
channel
(FA 14)
Stb, Sp
Sx, Sr,
SMs(r,b)
Moderately to well sorted fine grained sandstones. Micaceous, with up
to around 20% mud. Current and wave ripple cross-lamination.
Occasional bipolar bedding and paired mud drapes. Generally weak
bioturbation with Planolites , excavation, resting and escape traces, plus
occasional bivalve burrows and Skolithos .
Sandflat
(subtidal)
(FA 13)
SMs
(b,I,r),
MSs(b,I)
Sp, Sx,
MI, Mr
Heterolithic sandstone-mudstone deposits containing ~30-80%
moderately to well sorted v.f to f. sandstone. Micaceous and showing an
intricate fabric of wave- and current-generated structures. Prominent
paired mud drapes and local bipolar foresets. Generally weak bioturbation
with Planolites P . montanus , Arenicolites carbonarius , Skolithos ,
excavation and resting traces.
Dark grey micaceous siltstones that are well laminated, with pin-stripes
and lenticules of coarse silt/very fine sandstone. Both wave- and
current-generated small-scale structures. Prominent fine carbonaceous
debris. Weak bioturbation with mainly Planolites , Skolithos and
excavation traces.
'Mixed flat'
(subtidal)
(FA 12)
SMs(r)
MI, Mp,
MSs(I)
Mudflat
(subtidal)
(FA 11 )
Moderately sorted m.-grained sandstones pass upwards into to v.f. to f.-
grained sandstones and heteroliths. Medium-scale cross-stratification
grades vertically to current-rippled fabrics which may exhibit paired mud
drapes. Upward increase in bioturbation intensity with Planolites
dominant, but may also include bivalve burrows and Skolithos , plus
some A. carbonarious and excavation traces.
Sx,
SMs(r,b),
MSs(r,b),
Stb, MI
MSs(p),
MI, MSb,
SMb2,
SMped,
Mped,
Mv, V
Tidal channel
(FA 10)
Fig. 19. Facies association scheme of the estuarine-fill and transgressive shoreface depositional system (see Figs. 6A for
legend, and 6B for the lithofacies scheme).
(FA 10); mudflat (FA 11); mixed flat (FA 12); sand-
flat (FA 13) and tide-influenced distributary chan-
nel (FA 14) (Figs 9 and 19). Typical tide-influenced
facies associations recognised within the Åre 5
and 6 reservoir zones are composed of bimodally
sorted, medium-grained to coarse-grained, broadly
upward-fining sandstone units (1.5 m to 6 m thick)
showing frequent single and paired mud drapes
and mud clast lags (FA 14; Figs  19 and 20).
Åre 5 and 6 Zones: Mixed wave-influenced
and tide-influenced estuary
The Åre 5 and 6 reservoir zones (70 m to 115 m in
combined thickness; Fig.  4) comprise a diverse
mix of wave-influenced and tide-influenced
deposits including the following facies association
types: bay-floor mud (FA 7); wave-influenced bay-
fill (FA 8); bay-margin mud (FA 9); tidal channel
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