Title: Basic building blocks and process variability of a mixed-influenced
ancient river delta
Running title: Building blocks of an ancient delta
Authors:
First author: M. Royhan Gani
Second author: Janok P. Bhattacharya
Address of corresponding author:
M. Royhan Gani
Geosciences Department
University of Texas at Dallas
P.O. Box, 830688
Richardson, TX, 75083-0688
USA
Phone: 972-883-2401 (off); Fax: 972-883-2537
E-mail: [email protected]
Manuscript submitted to:
Sedimentology
The journal of the international Association of Sedimentologists
Publisher: Blackwell
Basic building blocks and process variability of a mixed-influenced
ancient river delta
M. ROYHAN GANI and JANOK P. BHATTACHARYA
University of Texas at Dallas, Geosciences Department, P.O. Box, 830688, Richardson,
TX, 75083-0688, USA
(E-mail: [email protected])
ABSTRACT
Architectural-element analysis is applied to determine the basic building-blocks of a
Cretaceous delta at the top of the upper Turonian Wall Creek Member exposed along the
western flanks of the Powder River Basin, Wyoming. Cliff sections show an upwardcoarsening facies succession displaying basinward dipping clinoform strata and
interpreted to represent a prograding delta deposit. Five orders of bounding surfaces bind
six facies architectural elements within prodelta and delta front deposits. These elements
are prodelta fines (PF), frontal splay (FS), channel (CH), storm sheet (SS), tidally
modulated deposit (TM), and bar accretion (BA). The most conspicuous element is
channel (CH), which consists of gravites showing lateral accretion surfaces interpreted to
be produced by terminal/chute channels on the delta front. The depositional mechanism
of these channels is probably analogous to that of laterally migrating channels in a
submarine fan. Frontal splays (FS) are thought to be produced by unconfined sediment
gravity flows. Elements TM and BA consist of mostly bi-polar and uni-polar crossstratified sandstones, respectively. Seasonal to decadal river floods are thought to
represent the main building phases of the delta, producing elements CH, FS, and BA.
1
During intervening periods, the delta is reworked by waves, storms, and tides, producing
mainly elements SS and TM. Due to the complex interactions of river effluents with
waves, storms and tides the delta is interpreted as mixed-influenced. Marine transgression
has eroded away delta plain deposits leaving a distinct lag. Regional sandstone isolith,
facies, and paleocurrent maps help to reconstruct the paleogeography and show how the
various architectural elements vary across the delta front. Elements CH, FS, TM, and BA
are found only near the distributary mouth; whereas, extensive development of element
SS is observed away from the distributary mouth. The plan view morphology of this delta
would indicate wave-dominance, but the internal facies architecture suggests that the
delta was rapidly constructed during major river-floods. This matches similar
observations on several modern deltas, including the Burdekin in Australia, Brazos in the
Gulf of Mexico, and Baram/Trusan in Borneo.
Keywords River delta, architectural element, bounding surface, facies and process
variability, Wall Creek Member
INTRODUCTION
The tripartite classification of deltas into wave-, tide- and fluvial-dominated end members
was largely based on compilations about the plan-view morphology of modern deltas
with the assumption that plan-view morphology faithfully reflects internal framework
facies (Galloway, 1975). The modern Brazos, Baram/Trusan, and Burdekin deltas show
smooth-fronted plan view shapes, traditionally classified as wave-dominated end
members, but recent detailed internal facies analyses of these deltas (Rodriguez et al.,
2000; Lambiase et al., 2003; Fielding et al., in press) suggest dominance of river and
2
tidal processes. Therefore, it is extremely important to study the internal facies
architecture of deltas to appreciate complex delta-building processes. Because deltaic
depositional systems mark the crucial link between continental fluvial and fully marine
depositional systems, they show greater facies architectural complexity and process
variability. However, detailed facies architectural studies of deltaic deposits in the
outcrop have been attempted only recently and are few (Willis et al., 1999; Willis &
Gabel, 2001; Willis, in press). On the other hand, the internal architecture of fluvial (e.g.
Mail, 1996; Bridge, 2003) and deep-marine (e.g. Bouma & Stone, 2000; Mutti et al.,
2003) deposits are comparatively well studied.
Since the first architectural studies of deltas (Gilbert, 1885) there have been a
number of key studies generalizing delta building processes (Barrell, 1912; Rich, 1951;
Scruton, 1960; Coleman & Wright, 1975; Galloway, 1975). Large-scale sequence
stratigraphic architecture of Quaternary deltas has also been well studied (Anderson &
Fillon, 2004; Sidi et al., 2004). Still, there are two major holes in our understanding of
deltaic systems. Firstly, the basic depositional building-blocks and their linkage in 3D
space are poorly understood. Secondly, the temporal and spatial variability of a deltaic
system during its lifetime (not modern snapshot) has been largely ignored; so that there is
a general tendency to force-fit an ancient delta into one of the end member types within
the classic tripartite delta classification (cf. Bhattacharya & Giosan, 2003). Without
understanding these two factors, incremental growth and decay of deltas, crucial to
society, are difficult of comprehend. Bed-scale compartmentalization of deltaic
reservoir/aquifer also largely depends on these two factors.
3
The objectives of this paper are to identify the basic architectural-elements
(building-blocks) and the bounding surface hierarchies of an ancient delta; to question the
validity of the underlying assumption of tripartite delta classification by reconstructing
the paleogeography of this delta; and to compare it with modern deltas to understand the
process variability. The availability of subsurface data adjacent to the outcrops as well as
complex cliff exposures led us to focus on the Wall Creek Member (Late Cretaceous) in
central Wyoming (Fig. 1).
REGIONAL GEOLOGY
The Cretaceous Western Interior Seaway (WIS) stretched roughly north-south in central
North America, and developed on an eastward migrating and asymmetric retroarc
foreland basin generated between the Cordilleran volcanic arc in the west and hinterland
region in the east (Fig. 1A; Dickinson, 1981; Lawton 1994). This seaway contains one of
the most studied sedimentary successions in the world. A pronounced and widespread
Cenomanian-Turonian transgression led to the joining of an epeiric seaway with the
Tethyan Sea towards the south (Leckie et al. 1998). Shortly after, the clastic wedge of the
Wall Creek Member (Fig. 2A) of Late Turonian age (based on Ammonites and
Inoceramid biozones; Cobban, 1990; Gardner, 1995a, 1995b) was deposited in Wyoming
in a ramp setting. The Wall Creek Member was sourced from uplifted strata in the west
due to thrusting associated with the Sevier orogeny (Fig. 1A), and later itself was
deformed into a gently tilted series of anticlines during the Maestrichtian-Tertiary
Laramide orogeny (Wiltschko & Dorr, 1983; Cobban et al., 1994). Numerical paleooceanographic modeling of the seaway (Ericksen & Slingerland, 1990; Slingerland et al.,
4
1996), particularly during Turonian, indicates the following – 1) the seaway was
generally wave-dominated with mainly southward longshore drift at the western margin;
2) both winter storms and summer storms (hurricanes) affected the seaway in such a way
that storm-driven currents were shore-parallel, predominately to the south and
occasionally to the north; 3) most parts of the seaway were in a microtidal setting, but
tides along the western shores were enhanced locally.
Seven major sand bodies (Fig. 2) deposited in shallow marine conditions under
cyclic regressions and transgressions were identified in the Wall Creek Member (Howell
& Bhattacharya, 2004). Because much of the Cretaceous is thought to represent ice-free
green-house conditions, the origin of these stratigraphic cycles has variously been
assigned to Milankovitch climatic cycles (Scott et al. 1998), foreland basin tectonics
related to thrust loading, and forebulge migration (Gardner 1995b, Bhattacharya & Willis
2001). However, Miller et al. (2004) recently claimed that continental ice sheets paced
sea-level changes during the Late Cretaceous. Accordingly, the stratigraphic cycles of the
Cretaceous seaway may represent a complex origin of in-phase and/or out-of-phase
interaction of glacio-eustatic and tectonic cycles.
Each of the seven sand bodies of the Wall Creek Member shows one or more
upward-coarsening facies successions, called parasequences, bounded by a marine
flooding surface commonly marked by a chert-pebble and shark-tooth lag (Fig. 2; Howell
& Bhattacharya, 2004). Although the Wall Creek Member was previously interpreted as
classic “wave-dominated” shoreface deposits (e.g. Winn, 1991), these parasequences
(Fig. 2B) show a mixture of top-truncated deltaic and shoreface deposits with varying
amounts of wave, storm, tide, and river influence. Subaerial deposits are notably absent
5
in these deposits because of the top truncation associated with marine transgression. The
present study deals with Sandstone-6, and focuses on a particular gully, here named
“Raptor Ridge”, located at the northeast of Natrona County, Wyoming (Fig. 1B).
DATA AND METHODS
A series of gullies dissect the Wall Creek outcrop belt and allow detailed study of
Sandstone-6 at and around Raptor Ridge. Strata show 60 southeast structural dip and there
is no abnormal structural deformation within the strata. The bentonite horizons below the
Wall Creek Member were used as a datum to map the sand bodies within the Wall Creek
Member using well logs and cores (Sadeque & Bhattacharya, 2004). The base of the
Cody Shale and conglomerate lag within the Emigrant Gap Member stand out as
regionally traceable distinct well log signatures (Sadeque & Bhattacharya, 2004). In this
study, 30 of these well logs (Fig. 1B) were used to map the limit of Sandstone-6 in
subsurface. 28 outcrop logs were measured along and around Raptor Ridge, documenting
standard sedimentological data like grain size, sedimentary structures, ichnology, and
paleocurrents. The Seilacherian approach (e.g., Pemberton 1992, Pemberton et al., 2001)
is taken in grouping identified ichnotaxa into different ichnofacies. The Bioturbation
Index (BI) of Taylor & Goldring (1993) is used to semi-quantitatively measure the degree
of bioturbation in the sediments (BI 0 = 0% bioturbation, BI 1 = 1-4% bioturbation, BI 2
= 5-30% bioturbation, BI 3 = 31-60% bioturbation, BI 4 = 61-90% bioturbation, BI 5 =
91-99% bioturbation, and BI 6 = 100% bioturbation). A portable scintillometer was used
to collect gamma ray profiles for each of the measured sections.
6
The concept of facies architecture and architectural-element analysis, following
Jackson (1975), Brookfield (1977), Allen (1983), and Miall (1985, 1996), was followed
in this study. However, whether an established scheme of bounding surface hierarchies
and architectural elements (e.g., Miall, 1985, 1996) should be used in every faciesarchitectural analysis is debatable (e.g. Bridge, 1993; Fielding, 1993). Miall (1996)
defined ‘architectural elements’ as “units enclosed by bounding surfaces of third- to fifthorder rank”. However, in this study we used the term ‘architectural element’ in a more
general sense defined as one or package of stratum that form the building-blocks of
depositional systems. Moreover, an independent scheme of ordering and numbering of
bounding surfaces was applied in this study. Tracing of bounding surfaces simply
followed the principle of superposition and the principle of cross-cutting relationships. In
ranking bounding surfaces, we started with the lowest (zero) order and systematically
went up to the highest order, following the general notion that no surfaces truncate
against surfaces of lower rank. Following these basic rules, one would have the flexibility
in ranking bounding surfaces and identifying architectural elements.
At Raptor Ridge, high-resolution (cm-scale) cliff maps (bedding, facies, shale,
and cement maps; Willis et al., 1999) for a 250-meter-long depositional dip-oriented cliff
face and an adjacent 125-meter-long depositional strike-oriented cliff face were generated
on photomosaic overlays in front of the outcrop. Within the study area, these two cliff
sections show the most complex bedding geometry and the most diverse facies
variability. Ten well cores were taken behind the cliffs (Fig. 1C) to link with a related 2D
and 3D ground penetrating radar study (Lee et al., 2004). To reconstruct the
paleogeography of Sandstone-6, paleocurrent and facies change were documented along
7
the Wall Creek outcrop belt for about 20 km around Raptor Ridge. Every measurement
was plotted on a map with the aid of a hand-held GPS machine with 5 m accuracy.
ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) images
(15 m resolution) of the Burdekin Delta, thought to be a close modern analog, were
digitally processed using standard remote sensing techniques (Jensen, 1996) to study
morphological elements of this delta.
SEDIMENTOLOGY
Sandstone isolith map
Using subsurface well logs and outcrop measured sections a sandstone isolith map of
Sandstone-6 at Raptor Ridge has been generated that shows a lobate geometry with a
slight shore-parallel elongation (Fig. 3). The shape of this sand body suggests a deltaic
origin (e.g. Coleman & Wright, 1975; Bhattacharya & Walker, 1992). The sand body,
henceforth referred to as the Raptor Delta, shows two distinct bulges indicating two
possible sediment input-points.
Facies
Six depositional facies were recognized in Sandstone-6.
Facies 1: Thinly laminated sandy mudstones
Facies 1 (Fig. 4A) is sandy mudstones with <25% very fine grained sands. The thickness
ranges from 0.5 m to <2 m. This facies is mostly homogenized by bioturbation with
partially preserved paper-thin laminations and sporadic marcasite (FeS2) nodules (1-4
8
mm diameters). BI (bioturbation index) ranges from 4 to 5 with a distal to archetypal
Cruziana ichnofacies showing diverse ichnogenera including Chondrites, Helminthopsis,
Phycosiphon, Zoophycos, Asterosoma, Teichichnus, Planolites, Terebellina (sensu lato),
Thalassinoides, Palaeophycus heberti, Rosselia, and Cylindrichnus, in order of
decreasing abundance.
The diverse Cruziana ichnofacies and high BI (Gani et al., 2004) suggest an open
marine environment of deposition (e.g. Pemberton et al., 2001). A fair amount (<25%) of
disseminated sand grains within this facies suggests possible frequent river input, but at
low enough rates to be homogenized by bioturbation. This is typical of a distal prodelta to
offshore environment (e.g. MacEachern et al., in press).
Facies 2: heterolithics
Facies 2 (Fig. 4B) comprises interbedded mudstones and thick (<5 cm) very fine to fine
grained, rippled sandstone beds (very fine to fine grained). Ripples are asymmetrical with
subordinate symmetrical forms. Mud chips and coal chips (few mm long) occur locally
within sandstone beds. BI ranges from 2 to 3 with archetypal Cruziana ichnofacies with
ichnogenera
Helminthopsis,
Palaeophycus
tubularis,
Planolites,
Chondrites,
Thalassinoides, Terebellina (sensu lato), Skolithos, and Asterosoma, in order of
decreasing abundance.
Interbedded mudstones and sandstones suggest fluctuating flow conditions.
Asymmetrical ripples are probably indicative of river currents carrying sands farther
seaward. The rare symmetrical ripples suggest some wave activity. Coal chips indicate a
non-marine source of the sediments delivered to the sea via a possible distributary
9
channel. The trace fossil assemblage, moderate BI (e.g. Gani et al., 2004; MacEachern et
al., in press), and other observations suggest that facies 2 was deposited in a proximal
prodelta to distal delta front environment.
Facies 3: massive to parallel laminated sandstones
Facies 3 (Fig. 4C) comprises sharp to erosional based and fine to medium grained
sandstones. Sandstone beds range from 5-40 cm thick and are non-graded massive with
floating clasts to parallel laminated. BI ranges from 0 to 2 with distal Skolithos
ichnofacies including Ophiomorpha irregularire, Palaeophycus tubularis, Planolites,
Chondrites, Helminthopsis, Skolithos, and fugichnia.
The sharp to erosional base, floating clasts, and structureless to parallel laminated
structures developed within a single bed suggest deposition from waning sediment
gravity flows (Collinson & Thompson, 1989; Gani, 2004; and references therein); hence
the deposits are interpreted as gravites (deposits of sediment gravity flows, sensu Gani,
2004). These sediment gravity flows could originate from high sediment discharge during
river floods, slump failures at a river mouth, and/or storm re-suspension of bottom
sediments.
Suspension-feeding
structures
Ophiomorpha
and
Skolithos
indicate
opportunistic colonization of event beds, followed by the reestablishment of a lowenergy, fairweather community generating dwelling/deposit-feeding structures Planolites,
Chondrites, and Helminthopsis (e.g. Pemberton et al., 2001; MacEachern et al., in press).
Based on the relatively low BI and its occurrence above facies 1, Facies 3 is interpreted
to be deposited in middle to lower delta front environments.
10
Facies 4: channelized sandstones
Facies 4 (Figs. 4D, 4E, and 4F) comprises medium-grained sandstones showing erosional
channel bases and mostly sharp upper boundaries. Localized zones of concentrated to
scattered mud clasts contain allochthonus Asterosoma. A few are shell clasts, and coal
clasts with allochthonus Teredolites. This facies show low-angle (~100) accretion
surfaces, which are mostly indistinct. Convolute bedding is observed locally. Average
bed thickness is 1 m. BI is mostly 0 and rarely 1, with Skolithos ichnofacies including
Skolithos, and Arenicolites.
Erosional U-shape depressions and thin lateral wings indicate channelized flow.
Dip directions of accretion surfaces (Fig. 4E) are roughly 900 from inferred channel flow
directions, suggesting lateral migration. Abundant floating clasts (Fig. 4F) indicate high
(probably >30% by volume; Gani, 2004) sediment concentration of the generating flows,
and convolute bedding indicates dewatering shortly after deposition (Collinson &
Thompson, 1989; Gani, 2004). A single bed (i.e. a single channel-fill deposit) is almost
devoid of burrows; only a very few penetrate into the bed from the top surface. Individual
channels are typically devoid of internal bedding surfaces. These probably indicate that
each channel was rapidly deposited from one sediment gravity flow event. Based on these
observations, Facies 4 is interpreted as channelized gravites deposited in a lower to
middle delta front environment.
Facies 5: trough cross-stratified sandstones
Facies 5 (Figs. 4G, 4H) is fine to medium grained trough cross-stratified sandstones with
rare ripple cross-laminations and very rare parallel laminations. Set thickness ranges from
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5 cm to 100 cm. Facies 5 is further subdivided into facies 5a and facies 5b. Facies 5a
comprises mostly seaward-directed and occasionally landward-directed trough crossstratified sandstones with scattered clasts (mostly mudstones, rarely shells and coals).
Facies 5b comprises mostly landward-directed or alternating seaward-directed and
landward-directed trough cross-stratified sandstones with mud and shell clasts, mostly
along foresets. Locally, hair-thin siltstones and paired mudstones drapes are observed
along foresets (Fig. 4H). Both facies 5a and 5b show BI 0-1 (rarely 2-3 on bedding
planes), and an archetypal Skolithos ichnofacies including mostly Ophiomorpha
irregularire,
Palaeophycus
tubularis,
Skolithos,
and
Asterosoma,
and
rarely
Thalassinoides, Planolites, Cylindrichnus, Chondrites, Macaronichnus, Palaeophycus
heberti, Bergaueria, Diplocraterion habichi, Lockeia, and fugichnia.
The trough cross-stratification suggests deposition due to migration of simple
and/or compound (i.e. superposed) dunes of variable sizes. Rare ripple cross-lamination
and parallel lamination reflects local decrease or increase in flow velocity, respectively.
The mud clasts indicate strong currents capable of eroding underlying beds. Current
reversal is reflected in both facies 5a and 5b. Herringbone cross-bedding and double mud
drapes of facies 5b suggest tidal periodicities (e.g. Visser, 1980; Nio & Yang, 1991).
Very low BI is interpreted to indicate proximity to a river source (e.g. Gani et al., 2004;
MacEachern et al., in press). Dominance of the suspension feeding structures
Ophiomorpha and Skolithos indicates strong bottom currents (e.g. MacEachern et al., in
press). Based on these observations, facies 5 has been interpreted to be deposited in a
lower to upper delta front environment. Facies 5a indicates mostly riverborne sediments
12
and currents during periods of active bar growth, whereas facies 5b indicates tidal
modulation of bar fronts with deposition mostly from tidal currents.
Facies 6: Hummocky cross-stratified sandstones (HCS)
Facies 6 (Fig. 4I) comprises hummocky cross-stratified fine to medium grained sandstone
beds ranging in thickness from a few centimeters to few decimeters. Locally, wavy to
parallel laminations, gutter casts, and symmetrical wave ripples (at bed tops) are
observed. Sandstones contain rare mud and coal (with allochthonus Teredolites) clasts.
Facies 6 shows a wide range of BI (0-5) with a mixed Cruziana and Skolithos ichnofacies
including Ophiomorpha irregularire, Thalassinoides, Cylindrichnus, Asterosoma,
Skolithos,
Palaeophycus
tubularis,
Chondrites,
Planolites,
Helminthopsis,
Monocraterion, Gyrochorte, and fugichnia.
HCS has been interpreted to be generated by the combined effect of strong
oscillatory flows and unidirectional currents during storms (e.g. Myrow & Southard,
1991). Gutter casts, and mud and coal clasts indicate strong basal scouring. Wave ripples
at tops of HCS beds suggest waning storm waves. The wide range of BI is related to the
occurrence of this facies distal (BI 3-5) or proximal (BI 0-2) to river source in the
direction of both depositional dip and strike. Based on ichnofacies assemblage and other
observations, facies 6 is interpreted to be deposited in distal to middle delta
front/shoreface environment. Rarely, facies 6 was found sandwiched within facies 1.
Facies architecture
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Cliff mapping at Raptor Ridge shows consistently seaward inclined bedding in
depositional dip section (Fig. 5A), and parallel to sub-parallel bedding in strike section
(Fig. 5B). Five orders of bounding surfaces have been established (Fig. 5). Zero order
surface includes sedimentary structures representing ripple cross-lamination, crossstratification, parallel lamination, and hummocky cross-stratification. First order surface
includes cross set boundaries, hummocky-stratification set boundaries, and accretion
stratification within channels. Second order surfaces bind sets of first order surfaces
(except accretion stratification), and sets of parallel lamination. Third order surfaces bind
channels, and sets of second order surfaces. Most of the second order surfaces downlap
and/or onlap on third order surfaces. The fourth order surface is the top marine
ravinement surface that truncates lower order surfaces and eroded delta plain deposits.
This is a regionally traceable surface strewn with granules and pebbles consisting of
chert, shark-teeth, shell fragments, mudstone rip-ups, and sandstone. With the help of
high-resolution cliff mapping, six basic architectural elements (building-blocks) have
been identified within the Raptor Delta at Raptor Ridge.
Element PF: prodelta fines
This element (Fig. 5) consists of mainly facies 1 (laminated sandy mudstones) and
subordinately facies 2 (heterolithics). Element PF is bounded by second order surfaces,
and forms areally extensive sheets. Transition into overlying delta front facies is sharp to
gradational with locally developed Glossifungites ichnofacies. Element PF is interpreted
to be deposited in relatively low energy conditions (see interpretation of facies 1 and
facies 2) with repetitive development of suspension-deposited strata with rare starved-
14
rippled strata. Local sharp facies change into the delta front is indicative of occasional
high sediment supply and/or decrease in local accommodation.
Element FS: frontal splay
This element consists of facies 3 (gravites) originating from non-channelized sediment
gravity flows. The majority of FS contains only one bed and occurs in the lower part of
the sand body (Fig. 5). Unless truncated, individual beds are mostly continuous across the
studied cliff sections, but thin basinward (Fig. 5). These elements are bound by second
order surfaces. Individual elements are probably lobate-shaped (e.g. Galloway & Hobday,
1996) indicative of a point source. However, they can form wedge-shaped aprons if
sediment gravity flows originate by spilling over subaqueous levees of distributary
channels.
Element SS: storm sheet
This element consists of facies 6 (hummocky cross-stratified sandstones). In the studied
cliff sections, SS is very rare, patchy, and contains only one bed (Fig. 5). However, in the
rest of the area, SS contains stacked HCS beds and develops as extensive sheets (Fig. 6).
These elements are bound by second or third order surfaces. Element SS records deposits
of seasonal and/or rare storms in the sea.
Element TM: tidally modulated deposit
This element consists of tidal facies 5b (bipolar dune cross-stratification), and occurs
mostly in the lower part and subordinately in the upper part of the sand body (Fig. 5).
15
Lower boundaries of TM are mostly erosional and define third order surfaces, whereas
upper boundaries are mostly non-erosional and represent second or third order surfaces.
Internal second order surfaces can dip both landward and seaward. TM pinches out in
both seaward and landward directions and includes both onlap and downlap surfaces.
This element is the product of tidal modulation of delta front sediments. Basal erosion
suggests periods of elevated tidal energy. Periodic occurrence of TM, particularly in the
upper part of the sand body (Fig. 5), probably indicates periodic enhancement of tidal
activity during overall delta front progradation. This periodicity should be greater than
neap-spring cycles, and may indicate semiannual tidal maxima (Kvale et al., 1998),
although not necessarily every maximum is preserved.
Element CH: channel
This element consists of facies 4 (channelized gravites) and occurs in the lower to middle
part of the sand body (Fig. 5). Both lower and upper boundaries are third order surfaces.
Internally they show indistinct lateral accretion-stratification forming first order surfaces.
Most of these accretion surfaces dip westward, but some dip eastward. Element CH is
tabular in shape, resulting from the lateral migration of U-shaped channels. In strike
section, channels show lateral wings. In dip section, channels pinch out seaward and
show a gradual transition into other elements in a landward direction (Fig. 5A). Locally,
they form two-story amalgamated sand bodies reaching 4 m thickness. Dimensions of
rare fossil-channels suggest that these channels were probably <3 m deep and <15 m
wide.
16
Deposition within CH is considered analogous to that of channels on a submarine
fan (e.g. Peakall et al., 2000; Abreu et al., 2003), although our examples are much
smaller in scale compared to the submarine examples. Channelized sediment gravity
flows develop unit bars within sinuous channels, probably due to the decreased shear
stress within inner banks. These bars migrate and produce bar-accretion surfaces. The
channels described here are probably chute channels, similar to those described in the
delta front region of fan deltas (Prior et al., 1981; Nemec, 1990). If these channels are
linked upstream to distributary channels, they should be called ‘terminal distributary’
channels (Olariu and Bhattacharya, in press). The relatively steep slopes (40) of the delta
front indicate that these channels should show low sinuosity (e.g. Hart et al., 1992, their
Fig. 2). These channels probably originated during high sediment discharge as bottomhugging undercurrents. Periodic alternation of CH with other elements indicates a
probable periodicity in channel occurrence, which could be the annual to decadal (ENSOtype; Rodriguez et al., 2000) high water discharge associated with an upstream river
flood. Lack of intra-element bedding surfaces indicates that individual CH elements were
probably rapidly filled within a few weeks, probably during the short-lived peak
discharge of a trunk distributary (e.g. similar to Burdekin River discharge; Amos et al.,
2004; Fielding et al., in press).
Element BA: bar accretion
This element consists of mostly seaward-directed dune cross-stratified sandstones of
facies 5a, and occurs mostly within the upper part of the sand body (Fig. 5). Both lower
and upper boundaries of BA represent second or third order surfaces. In general, internal
17
second order surfaces within BA dip seaward with landward truncation and seaward
downlap. Proximally, elements BA superpose on each other, whereas distally, they
alternate with other elements. In dip section, BAs are wedge-shaped and pinch out
seaward (Fig. 5A). In strike section, they are continuous across the outcrop (Fig. 5B), but
should pinch out laterally at a distance.
Facies 5a has been interpreted to be deposited due to migration (mostly seaward)
of simple and/or mainly compound (i.e. superposed) dunes of variable sizes. Because the
sets (in case of simple dunes) or cosets (in case of superposed dunes) boundaries dip at an
angle (mostly 10-100) toward the sea, they represent accreting bar front surfaces. These
elements are extensively developed in the upper delta front region closer to distributary
mouths. This suggests that BAs are produced mainly by riverborne unidirectional
currents and sediments. A minor tidal activity can be inferred from locally observed
landward-directed cross-stratification. Individual BA probably indicates annual to
decadal growth of bars.
Strike variability of the Raptor Delta: facies and paleocurrent mapping
The last accessible outcrop of the Raptor Delta (Sandstone-6) towards the southwest was
measured in a gully 1 km westward from Raptor Ridge (Fig. 6A, location a). At this site,
tidal facies (i.e. facies 5b and element TM) and channel facies (i.e. facies 4 and element
CH) are conspicuously absent (Fig. 6B, log a). Upward-coarsening facies successions
shows a relatively high BI (3-4) in prodelta and lower delta front deposits, and a low BI
(rarely exceeds 1) in middle to upper delta front deposits. Dominant paleocurrents are
towards the south (Fig. 6B). Clinoforms are as steep as in Raptor Ridge (40). In contrast,
18
the entire northeastern flank of the Raptor Delta, from Raptor Ridge northward (Fig. 6),
shows a predominance of HCS facies (i.e. facies 6 and element SS) alternating with
parallel-laminated facies (facies 3) (Fig. 6; logs c, d). Again, tidal facies (facies 5b) and
channel facies (facies 4) are absent. Upward-coarsening facies successions show a
relatively high BI from bottom to top (BI 4 at prodelta to lower delta front, and BI 2-3 at
upper delta front). Although the dominant paleocurrent is 1620, a shore parallel trend
(northeast-southwest) was also documented (Fig. 6B). Clinoforms are less steep (<20)
than in Raptor Ridge (40). These dramatic lateral variations of facies, paleocurrents, and
BI of the Raptor Delta are discussed in the next section.
DISCUSSION
Concept of facies architecture
Stratigraphic architecture describes the stacking patterns of stratigraphic units in a
regional scale (e.g. sequence stratigraphy), whereas facies architecture demonstrates the
stacking patterns of facies units within a depositional system at a local scale (e.g.
architectural-element analysis). The concept of facies architecture and architecturalelement analysis was originally developed for fluvial rocks (Jackson, 1975; Brookfield,
1977; Allen, 1983; Miall, 1985, 1996). Architectural elements are the 3D building-blocks
of a depositional system. These architectural elements commonly represent a, or part of a
‘morphological
element’.
In
surficial
modern
environments
important
sandy
morphological elements include channels, bars, large bedwaves, and splays. These
morphological elements migrate and/or grow to produce architectural elements in the
rock record. The 3D shape of a morphological element could be significantly different
19
than that of a corresponding architectural element, and depends on how a morphological
element evolves, deposits, and is modified through time. For example, migrating
hummocky bedwaves may produce a storm sheet. In some cases fossil morphological
elements may be preserved in the rock record. Although architectural elements can vary
in type from one system (e.g. delta) to another (e.g. fluvial), or even within the same
system in time and space, there should be finite types of architectural elements in any
given depositional system. How far down we choose to break a given deposit to
determine its basic building blocks is a balance of how meaningfully the architecture of a
deposit can be described. Building-blocks (architectural elements) bind with each other in
3D space via bounding surfaces. Permutation and combination (both linear and nonlinear) of building-blocks can give rise to an entire depositional system.
The concept of bedforms is crucial to understand the development of architectural
elements in clastic systems. Bedforms were defined loosely as any deviation from a flat
bed, generated by the flow on the bed (Middleton, 1965). In the sedimentologic literature,
there is no consistency in terminology and classification of bedforms (cf. Ashley, 1990).
In some accounts, the term bedform was applied to ripples, dunes, sandwaves, and bars
(e.g. Collinson & Thompson, 1989), and in others, the term bedform was restricted only
to bedwaves (e.g. ripples, dunes, low amplitude bedwaves, antidunes; Collinson, 1996).
Furthermore, ‘large subaqueous dunes’ (sensu Ashley, 1990) are still being referred to as
bars by some researchers (e.g. Galloway, 2002). We felt that it is essential to be
consistent with these terminologies while doing facies architectural analysis. Although
our understanding of bedform mechanics are far from completion (Mazumder, 2000), we
suggest that bedforms are of two main types – bedwaves and bars, which are clearly
20
different from each other (Table 1). We support the view of Ashley (1990) that large
bedwaves (e.g. “sand wave”, “shelf sand ridge”) are simply large forms of dunes. Bars
are defined as within-channel depositional features (Lunt & Bridge, 2004). We extend
this definition to suggest that ‘bars are depositional features developed within a channel
(e.g. point-bar, mid-bar, tidal bar) and/or at the end of a channel (e.g. mouth bar)’. In
general, within-channel bars tend to fine upward and end-channel bars tend to coarsen
upward.
Delta building processes and deltaic architectural-elements (building blocks)
In the Raptor Delta, six types of architectural elements were identified within prodelta
and delta front deposits (Fig. 5). Figure 7 shows these deltaic architectural-elements in
idealized forms, which should have general applicability. These elements bonded with
each other via bounding surfaces (five orders were labeled; Fig. 5) to build the entire
delta. Prodelta deposits mainly consist of element PF of sandy mudstones. Storm sheets
(SS) and frontal splays (FS), produced by event deposition, may be found locally
sandwiched between two PF elements. It may not be easy to separate deposits of fluid
muds (event deposition) from background suspension muds, especially if fluid mud
deposits are thin enough to be obliterated by trace makers, which is suspected to be the
case for the Raptor Delta. Otherwise, suggestions made by Gani (2004) can be applied to
identify fluid mud deposits, and treat them as separate architectural element. Prodelta
deposits may consist of series of stacked elements PF, SS, and FS. There could be three
scenarios of stacking – vertical stacking, basinward shift and landward shift,
corresponding to aggradation, progradation, and retrogradation, respectively. In a normal
21
progradational succession and at the transition between delta front and prodelta region,
intercalation of delta front and prodelta elements give rise to facies interfingering (cf.
Gani & Bhattacharya, in press; Fig. 7, element PF).
Delta front deposits show alternation of elements FS, SS, TM (tidally modulated
deposit), CH (channel), and BA (bar accretion). Processes related with each of these
elements have been described previously in the ‘sedimentology’ section. Broadly,
interaction between river and basinal (tide and wave) processes develops these buildingblocks that fill the accommodation. Both FS and SS represent event deposits that
generally smooth out delta front topography. For example, Kostic et al. (2002) showed
that sediment gravity flows can reduce the angle of delta front clinoforms by 20%.
A conspicuous and geologically significant building-block found within delta
front deposits of Raptor Delta is the element CH (Figs. 5 and 7) interpreted as chute or
terminal channels. Terminal distributary channels (i.e. subaqueous distributary channels)
have only recently been recognized in ancient delta front deposits (Olariu &
Bhattacharya, in press). The present study demonstrates that distributary channels in
deltas do not necessarily stop at the delta plain; rather, they can continue subaqueously
down the delta front region as terminal channels or chute channels. Element CH of the
Raptor Delta were rapidly filled with gravites. Because slump scars, slide and slump
deposits, and synsedimentary faults are largely absent, the origin of these channelized
sediment gravity flows are likely extreme river floods, like those seen in the modern
Burdekin River (Alexander et al., 1999) where much of the annual water and sediment
discharge to the delta is delivered in less than five days, and which produces coarsegrained, sharp-based, 1-3 m thick sand beds (cf. Fielding et al., in press). Therefore, the
22
thick delta front gravites of the type found in Raptor Delta may be used as proxy for
paleoclimate prone to produce extreme and short-lived river floods.
Elements BA and TM consist mainly of packages of bedwave deposits
originating from river and tidal currents at the delta front region. BA corresponds to
incremental growth of individual mouth bars. The size of the mouth bar developed at the
end of a distributary channel should be proportional to the width of the channel and to the
horizontal spreading-angle of river effluent. As the distributary channels bifurcate
seaward their widths get smaller. Therefore, deltas with high orders of distributaries tend
to develop small bars, which may coalesce together to form bar assemblages. Element
TM indicates periodic reworking of riverborne sediments by strong tidal forces that
normally slows down seaward growth of bars. TM can develop both within and outside
of a bar. An additional architectural element, not described above, is BM (bedwave
migration), which is a discrete building-block produced by bedwave migration that are
not part of any previously described elements. For example, BM can develop within
inter-bar region, and on shorefaces away from distributary mouths. The dune-scale crossstratified sandstones of log c and d (Fig. 6B) developed at the upper part and in the
eastern flank of the Raptor Delta probably represent element BM.
The most prominent feature of deltaic deposits is seaward dipping inclined beds
know as clinoforms (Bhattacharya & Walker, 1992; Gani & Bhattacharya, in press). In
the Raptor Delta, clinoforms are characteristically observed in the cliff sections (Fig. 8).
Depositional dip section shows persistent seaward dipping clinoforms (Fig. 8A), whereas
strike section shows parallel to sub-parallel clinoforms (Fig. 8B). The height (from top
ravinement surface to prodelta mudstones) of a single clinoform is, on average, 9 m. If it
23
is assumed that one third of the preserved height (i.e. 3 m) was removed by erosion due
to marine transgression, then pre-erosion clinoform height was 12 m. Using a compaction
factor of 80% (normal for sandstones) the pre-compaction height of a clinoform would be
15 m, which is interpreted to be the water depth the Raptor Delta prograded into.
Paleogeographic reconstruction
The shape of the sandstone isolith map of the Raptor Delta (Fig. 3) reflects its general
paleogeography. A slight shore-parallel elongation of this lobate sand body may be
interpreted to indicate an overall wave-influence (e.g. Coleman & Wright, 1975). There
are two bulges in this lobate sand body. Outcrops of the northeastern bulge (Raptor
Ridge), particularly the identification of elements CH and BA, clearly indicates the
presence of a major, subaerial distributary channel in this region. Although no outcrop
data is available for the southwest bulge, it can be logically concluded that this region
was also close to a major distributary channel. Therefore, these two bulges represent two
depositional loci of the Raptor Delta. The sand isolith map of the Holocene Burdekin
delta in NE Australia (Fielding et al., in press) and the Rhone delta in France (Galloway
& Hobday, 1996, p. 111) are smooth-fronted like that of the Cretaceous Raptor Delta.
The Rhone Delta is being built by two main distributary channels of the Rhone River, as
has been inferred for the Raptor Delta. However, whether the two main distributaries of
the Raptor River co-existed, or one is slightly younger than the other is not certain.
The regional outcrop facies map of the Raptor Delta (Fig. 6) shows predominance
of element SS developed as an extensive sheet, along with element FS in the northeast
flank (Fig. 6, log c and d). In this region, comparatively high BI and a shore-parallel
24
component of paleocurrents may suggest that the northeast flank of Raptor Delta had less
river-influence, and hence may lie on the updrift side of the distributary mouth (c.f.
Bhattacharya & Giosan, 2003).
Incorporating the above observations, a true-scale paleogeographic reconstruction
of the Raptor Delta (Fig. 9) is made. Fig. 9 suggests that the Western Interior Seaway, at
least in the studied region, was mainly wave- and storm-dominated, had shore-parallel
currents dominantly towards the south and occasionally towards the north, and had very
localized tides. All of these observations support the findings of paleo-oceanographic
modeling of the seaway (Ericksen & Slingerland, 1990; Slingerland et al., 1996)
mentioned in the ‘regional geology’ section. Like other parasequences of the Wall Creek
(Fig. 2), a through-going marine ravinement surface was observed at the top of
Sandstone-6. A relative rise in sea-level is thought to produce this transgressive erosion
surface that eroded delta plain deposits leaving the Raptor Delta top-truncated.
Limitation of tripartite classification of deltas
Tripartite classification of deltas into wave-, tide- and fluvial-dominated end members
assumes that plan-view morphology faithfully reflects internal framework facies
(Galloway, 1975). Although this classification remains a powerful framework for
understanding deltas, several recent studies have shown some serious limitations. The
Brazos delta in the Gulf of Mexico, for example, has long been cited as a classic example
of a wave-dominated delta, based on its plan view morphology; but recent coring studies
show that internally, much of the delta was built during major river-floods, and it is now
classified as a river-flood dominated delta, with some wave-reworking of the surficial
25
sediments (Rodriguez et al., 2000). The Trusan River delta in NW Borneo has also been
historically classified as wave-dominated, based on its smooth-fronted plan view shape,
but recent studies of the internal facies show tide-dominance (Lambiase et al., 2003). The
Burdekin delta also shows a smooth-fronted morphology that has caused it to be
classified as “wave-dominated”, but detailed internal facies analysis show sharp-based
fluvial-dominated mouth bars that record river-flood processes (Fielding et al., in press).
Although the sandstone isolith map of the Burdekin delta (Fig. 10A) shows a shoreparallel elongation, a considerably higher fluvial-influence was documented in cores and
outcrops of this delta (Fielding et al., in press). The plan-view morphology of any given
modern delta reflects dominant surficial process but may not necessarily reflect the shortlived constructional processes of river floods that dominate the internal facies
architecture.
Similarly, the shape of the Raptor Delta may be seen as overally controlled by
waves (Fig. 10), but the internal facies architecture reveal a complex interaction of river,
waves, storms, and tidal processes. Rather than force-fitting the Raptor Delta into an end
member type of the tripartite delta classification (Galloway, 1975), it is described as a
‘mixed-influenced delta’ to indicate the complex delta building processes of variable
energies. Table 2 of Galloway (1975) suggested different framework facies for each of
the three types of delta. However, these framework facies are not mutually exclusive for a
single delta; hence, mixed-influenced deltas may be the norm rather than the exception
for ancient deltas. Like the modern Brazos Delta (Rodriguez et al., 2000) and the modern
Burdekin Delta (Fielding et al., in press), the main building phases of the Raptor Delta
probably was seasonal to decadal river floods producing elements CH, FS, and BA.
26
During intervening periods (e.g. between two floods) the delta was reworked by waves,
storms, and tides producing mainly elements SS, TM, and BM, which are reflected in the
plan-form morphology (Fig. 9).
Modern analog and morphological elements
The Raptor Delta was already compared with the modern Brazos, Baram/Trusan,
Burdekin (Fig. 10), and Rhone delta. A sharp lateral facies change between tidal and
wave regime of the Raptor Delta (Fig. 6) can be compared with that of the modern
Orinoco delta in NE Venezuela (Warne et al., 2002), although the latter is almost an
order of magnitude larger than the former.
Of the above modern deltas, the Burdekin Delta is thought to be the closest scaled
analog for the ancient Raptor Delta, particularly, in terms of delta morphology and river
mouth processes. Like the Raptor Delta, wave and storm produced beach ridges lie on the
flank of the Burdekin Delta away from the mouth of two main distributary channels (Fig.
11). Subaerial distributary channels extend subaqueously as terminal distributary
channels. These terminal channels (width <100m) gently meander on the delta front and
taper basinward. Both mouth bars and in-channel bars coalesce to form bar assemblages
close to the main distributary mouths. Tides produce a series of large dunes on bar fronts.
The migration and/or growth of these morphological elements should produce
architectural elements like those described in the Raptor Delta.
In fact, in all deltaic systems, major morphological elements include channels,
bars, large bedwaves, beach-ridge/chenier, and splays. Channel types include: trunk
distributary, above first avulsion point; subaerial distributary; terminal distributary, below
27
mean sea level; delta front chute channel; and tidal creek and channel, both subaerial and
subaqueous. Bars are associated with each of these channels. Number and size of mouth
bars are directly proportional to the number and size of distributary channels,
respectively. Beach-ridge/chenier develops when riverborne sediments accrete to the
coast away from a river mouth, but within the same river delta system. In the rock record,
these morphological elements produce architectural elements, similar to those described
here.
CONCLUSIONS
Architectural-element analysis was used to study the Cretaceous Raptor Delta in the Wall
Creek Member, Wyoming. High resolution (cm-scale) cliff mapping of bedding and
facies reveals six architectural elements within the prodelta and delta front deposits – PF
(prodelta fines), FS (frontal splay), CH (channel), SS (storm sheet), TM (tidally
modulated deposit), and BA (bar accretion). Element FS is produced by unconfined
sediment gravity flows occasionally generated at the delta front region. Element CH
represent subaqueous terminal/chute channels rapidly filled with gravites showing lateral
accretion surfaces. The depositional mechanism of these channels is probably analogous
to that of laterally migrating channels in a submarine fan. The main building phases of
the Raptor Delta were probably seasonal to decadal river floods producing elements CH,
FS, and BA. During intervening periods, the delta was reworked by waves, storms, and
tides producing mainly elements SS, and TM. It is suggested that these elements are the
basic building blocks of all deltas, although the relative proportions, combinations, and
bonding styles of these elements may vary. A comparison with the morphologic elements
28
of the modern Burdekin delta suggests that individual morphologic elements evolve
through time and produce corresponding building blocks in the rock record.
The sandstone isolith map of the Raptor Delta shows a lobate geometry with a
slight shore-parallel elongation suggesting some wave-influence. The smooth plan-form
morphology vs. the internal facies complexity of the Raptor Delta violates the basic
assumption of the tripartite classification of deltas. The well studied modern deltas of the
Brazos, Baram/Trusan, and Burdekin show similar observations that the main
constructional processes are not reflected in the external morphology of deltas. Therefore,
only a detailed facies architectural analysis may reveal the complexity of interactive delta
building processes. Rather than force-fitting into an end member type, the Raptor delta is
classified as mixed-influenced delta to indicate its inherent complexity.
ACKNOWLEDGEMENTS
Funding for this research was provided by the U.S. Department of Energy DOE Grant #
DE-FG0301ER15166. Additional support was provided by the University of Texas
Quantitative Sedimentology Research Consortium, of which BP and Chevron/Texaco are
members. Nahid Sultana helped greatly in the field and in the lab. Thanks are also due to
Randy Griffin and Chuck Howell for helping in the field. This is contribution number
_____ of the Geosciences Department, University of Texas at Dallas.
29
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Table 1: Differences between bedwave and bar.
Bedwave
Bar
1. Bedwaves (e.g. ripples, low-amplitude
bedwaves, dunes, antidunes) develop due to the
interaction of vortex and eddy of fluid
turbulence with sediment particles.
1. Bars (e.g. point bar, mid bar, mouth bar)
develop when decrease in flow velocity induces
deposition.
2. Bedwaves can be analyzed with classical
wave theory.
3. In a given condition, bedwaves occur
repetitively with a constant wave length
(spacing).
4. Bedwaves are scaled to the thickness of
turbulent boundary layer. Therefore, there is a
natural limit on the upward growth of bedwaves
(e.g. Allen, 1985).
5. Bedwaves are produced in both confined and
unconfined flow.
6. Generally, bedwaves produce angle-of-repose
foresets.
2. Bars are not bed waves, hence can not be
analyzed with wave theory.
3. Bar does not occur repetitively with a
constant wave length.
4. As bars do not follow wave theory, they can
grow up to the upper boundary of flow.
5. Generally, bars produce within or at the end
(mouth) of a confined (channelized) flow.
6. Generally, bars produce gentler foresets, but
can produce foresets of any angle up to angle-ofrepose.
40
Fig. 1. (A) Cretaceous paleogeography of the Western Interior Seaway during middle
Turonian. During this time the Wall Creek Member was deposited into a foreland basin
sourced from the Sevier belt in the west (modified from Bhattacharya & Willis, 2001;
and others). (B) Location map showing the positions of outcrop and subsurface data, and
the Wall Creek outcrop belt at the western margin of the Powder River Basin, Wyoming.
The present study focuses at Raptor Ridge. (C) Raptor Ridge site showing studied cliff
sections and well locations behind the cliffs.
41
Fig. 2. (A) Cretaceous stratigraphy of the Powder River Basin, Wyoming (modified from
Willis et al., 1999). (B) Outcrop type section of the Wall Creek Member at Raptor Ridge
site. This study deals with Sandstone-6. (C) Lithologic and ichnologic symbols used in
logs.
42
Fig. 3. Sandstone isolith map shows that Sandstone-6 is lobate with slight shore-parallel
elongation. Paleocurrent diagram includes all outcrop data (see text for discussion).
43
44
Fig. 4. Facies photos at Raptor Ridge. (A) Facies 1: sandy mudstones with BI 4-5
including trace fossils Phycosiphon (Phy), Helminthopsis (He), Terebellina (Ter),
Planolites (Pl), and Zoophycos (Zo). (B) Facies 2: heterolithics comprising interbedded
mudstones and rippled sandstones. (C) Facies 3: massive to parallel laminated sandstones
with erosional base. (D) Facies 4: channelized sandstones with erosional U-shape
depression. (E) Facies 4: channelized sandstones with lateral accretion surface. (F) Facies
4: mudstone clasts within channel. (G) Facies 5a: Dominantly seaward-directed (to the
right) trough cross-stratified sandstones. (H) Facies 5b: Landward-directed (to the left)
trough cross-stratified sandstones with double mud drapes and mud clasts. (I) Facies 6:
hummocky cross-stratified sandstones (HCS) overlying facies 1. (J) A coarsening-upward
succession showing vertical association of facies 1 to 5 prograding towards sea (right).
45
46
Fig. 5. Cliff maps (bedding, facies, shale, and cement maps) at Raptor Ridge. For location of cliff sections see Fig. 1. (A) Depositional
dip-oriented section. Note the bounding surface hierarchies and architectural elements in the bedding diagram. (B) Depositional strikeoriented section. Legend is same as Fig. 5A. See text for discussion.
47
Fig. 6. Lateral variability of facies and paleocurrents of the Raptor Delta. (A) Locations
of regional cross section (Y-Z, depositional strike-oriented) and measured representative
lithologs on the isolith map. (B) Regional cross section with gross facies distribution and
paleocurrent data. Architectural elements are PF (prodelta fines), SS (storm sheet), FS
(frontal splay), CH (channel), TM (tidally modulated), BA (bar accretion), and BM
(bedwave migration). Note the overall facies percentages, and that both river- and wavedominated facies can vary laterally from 0% to 100%.
48
Fig. 7. Summary diagram of six basic architectural-elements (building-blocks) of
prodelta and delta front deposits.
49
Fig. 8. Characteristic development of clinoforms in the cliff sections of the Raptor Delta.
(A) Depositional dip section show persistent seaward dipping clinoforms. The section is
extended 100 m updip and northwestward from Fig. 5A. (B) Strike section shows parallel
to sub-parallel clinoforms. This is the same cliff section as in Fig. 5B.
50
Fig. 9. 3D reconstruction of the Raptor Delta based on mapping of sandstone isolith,
facies, and paleocurrents. Compare this figure with Figs. 3 and 6.
51
Fig. 10. Comparison of modern Burdekin delta (A) and the Cretaceous Raptor Delta (B)
in terms of sand/sandstones isolith map. Both deltas, presented in the same scale, are
lobate with a slight shore-parallel elongation. For location of the Burdekin delta see Fig.
11. (Fig. 10A is modified from Fielding et al., in press).
52
Fig. 11. ASTER images of modern Burdekin Delta (NE Australia) showing major
morphological elements that produce architectural elements in the rock records (see text
for discussion).
53