Downloaded from http://sp.lyellcollection.org/ at Oregon State University on June 8, 2012
Geological Society, London, Special Publications
Sedimentary facies associations within subduction
complexes
Michael B. Underwood and Steven B. Bachman
Geological Society, London, Special Publications 1982, v.10;
p537-550.
doi: 10.1144/GSL.SP.1982.010.01.35
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Notes
© The Geological Society of London 2012
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Sedimentary facies associations within subduction complexes
Michael B. Underwood & Steven B. Bachman
S U M M A R Y : Sedimentation patterns within modern subduction zones are complex and
variable, and do not necessarily follow models of submarine fan sedimentation. Environmental reconstructions within ancient subduction complexes should follow modern
analogues as closely as possible and consider several criteria, including turbidite facies
associations, vertical depositional cycles, regional palaeocurrent patterns, and in many
cases, structural style and sandstone petrology.
Important variables in trench sedimentation include the volume and texture of sediment
entering the trench and the distribution of major sediment sources, especially large
submarine canyons. Sediment transport in the trench is commonly longitudinal, although
locally, such as at the mouth of a submarine canyon, flow may be at a high angle to the
continental margin.
Patterns of trench-slope sedimentation depend largely upon the topography of the
slope. In general, coarse sediment is either trapped behind tectonic ridges (within slope
basins) or bypasses the slope via submarine canyons. Background sedimentation is
dominated by hemipelagic settling. Current directions are commonly at a high angle to the
margin, but longitudinal flow may occur within large elongate slope basins. Major facies
associations include submarine-canyon, slope, mature-slope-basin, and immature-slopebasin.
Classification schemes for turbidites and related
resedimented deposits (e.g. Walker & Mutti
1973; Mutti & Ricci-Lucchi 1975; Ricci-Lucchi
1975) are applicable to deep-sea deposits regardless of the geometry of the depositional
system. Turbidite facies are commonly used to
describe specific facies associations, such as
slope, submarine fan, and basin plain (e.g.
Walker & Mutti 1973; Bouma & Nilsen 1978;
Ingersoll 1978a; Walker 1978; Pescadore 1978).
In these models, analogies are drawn between
stratigraphic sequences of turbidites and
modern depositional geometries. However,
submarine fan models (Normark 1970, 1978;
Nelson & Kulm 1973; Nelson & Nilsen 1974;
Nilsen 1980) are not applicable to all modern
sedimentary basins, particularly elongate
troughs (e.g. Hsii et al. 1980; Pilkey et al. 1980).
Moreover, turbidite facies associations, depositional cycles, and submarine-fan facies associations are not necessarily equivalent or interchangeable.
Meaningful environmental reconstructions
within ancient subduction complexes depend
upon well-established modern analogues, and
should, as closely as possible, be patterned after
the geometry of those analogues. For this
reason, we first discuss the diversity of
sedimentary processes and depositional systems
within modern subduction zones, where several
types of sediment bodies, including submarine
fans, are developed. The sedimentological
characteristics of several examples of ancient
trench and trench-slope deposits are also considered. We then model turbidite facies associations for the major depositional settings. Because many trench and trench-slope environments have not been adequately sampled, our
facies associations in some cases are largely
inferential, and designations are based upon the
types of deposits logically expected for a given
topography and sediment input. In this paper,
turbidite facies terminology is used in only the
most general sense (Table 1), following the
classifications of Walker & M u t t i (1973) and
Ricci-Lucchi (1975).
The deformation associated with subduction
and/or accretion commonly hampers the environmental reconstruction of ancient trench
and trench-slope deposits. However, the scale
of structural dismemberment, in many cases, is
such that identifiable depositional cycles and
facies changes are preserved. Nevertheless,
several criteria must generally be considered
before contrasting sedimentary environments
(e.g. trench floor versus slope basin) can be
recognized within structurally complex terranes
(e.g. Bachman 1978, 1981; Moore 1979; Moore
& Karig 1980; Moore et al. 1980).
Trench-floor deposits
Surface samples and Deep Sea Drilling Project
(DSDP) cores show that modern trench sediments generally consist of varying proportions
537
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M. B. U n d e r w o o d & S. B. B a c h m a n
538
TABLE 1. Classification o f turbidites and associated deposits, after Walker & Mutti (1973) and
Ricci-Lucchi (1975)
Facies
A - - thick to massive, lenticular beds of coarse sandstone, pebbly sandstone, and conglomerate
B - massive to thick-bedded, medium- to coarse-grained sandstone
C - - thick-bedded, classical sandy turbidites, with graded beds at base
D - - thinner-bedded and finer-grained turbidites, including lutite turbidites; basal graded beds absent
E - - t h i n - to medium-bedded, coarse- to fine-grained sandstone, with irregular wedge-shaped beds and
cross-stratification
F - - chaotic deposits formed by slumps and slides
G - - hemipelagic mudstone and shale
of sandy turbidites and interbeds of mud (Ross
1971; Kulm, von H u e n e et al. 1973; Piper et al.
1973; Kulm & Fowler 1974; von H u e n e 1974;
Karig, Ingle et al. 1975; J. C. Moore & Karig
1976; Moore, Watkins et al. 1979; von Huene,
Aubouin et al. 1980; McMillen & Haines 1981;
McMillen et al. 1981). Sedimentary sequences
underlying the trench turbidites commonly include abyssal pelagic clay or biogenic ooze and
hemipelagic sediments (Piper et al. 1973;
I06"
22 °
105 °
21 ~ 104 °
Schweller & Kulm 1978; Moore, Watkins et al.
1979). This sequence would not be expected,
however, if large volumes of terrigenous turbidites are deposited on the abyssal floor seaward
of the trench, as with the Bengal Fan (Curray &
Moore 1974), or if the trench floor is starved of
coarse terrigenous material, as is the Japan
Trench (Langseth, O k a d a et al. 1978; Arthur,
von H u e n e et al. 1981).
The geometry of trench fill is dependent
20°
103 °
19°iI02°-~
~,;o
~'\
i
8ta~derc
Monzanillo
~4000~
4500If
500C
5500
6000
FIc. 1. Bathymetric map and trench-axis profile of the Middle America Trench from Banderas Bay
to Rio Balsas, Mexico. General bathymetry (in metres) is modified from Fisher (1961). Axial profile
incorporates data from Fisher (1961), Ross & Shor (1965), and unpublished reflection records from
the following sources: Scripps Institution of Oceanography cruises Scan-11, Cocotow-4, Iguana-I,
Iguana-5, Lamont-Doherty Geological Observatory cruise Vema-28, and Glomar Challenger transit
to Deep Sea Drilling Project Leg 66 drilling sites. Axial-depth values are in corrected metres.
Sediment thickness was calculated using V = 2.0 km s -1, such that 0.5 s penetration = 500 m of
sediment. See Fig. 2 for explanation of symbols. Modified from Underwood & Karig (1980).
Downloaded from http://sp.lyellcollection.org/ at Oregon State University on June 8, 2012
Sedimentary facies associations within subduction complexes
upon the interaction between sediment supply
and convergence rate, the distribution and relative size of submarine canyons along the trench
slope, and the bathymetric relief on the downgoing oceanic plate (Schweller & Kulm 1978;
Underwood et aI. 1980). The great bulk of
terrigenous material reaches the trench floor
via large submarine canyons that are deeply
incised into the trench slope (Underwood &
Karig 1980). The sand/mud ratio of trench
deposits, in many cases, is thus dependent upon
the proximity of submarine canyons to the site
of deposition. After sediment reaches the floor
of the trench, axial transport involving distances of hundreds of kilometres is indicated
(Scholl 1974; Kulm et al. 1977; Schweller &
Kulm 1978).
Submarine canyons and sediment bypassing
Large submarine canyons are prominent features along the landward slope of the northern
Middle America Trench (Figs 1 & 2; Fisher
1961; Underwood & Karig 1980; Shipley et al.
1980; McMillen & Haines 1981; McMillen et al.
102 °
"~ 1(31o
~
---'~A
1981) as well as the Oregon-Washington continental margin (Carlson & Nelson 1969; Barnard 1978; Kulm & Scheidegger 1979) and the
southern Chile Trench (Scholl et al. 1970;
Hayes 1974; Prince et al. 1980). These canyons
have been responsible for funnelling terrigenous sediment from the continental shelf
directly to the trench floor, thereby bypassing
the trench slope. Sediment bypassing was particularly effective during low stands of sea-level
associated with Pleistocene glacial epochs (e.g.
Scholl et al. 1977; Barnard 1978). Data from
current-meter studies indicate that at least one
of the canyons off Mexico (Rio Balsas Canyon)
is still active (Reimnitz & Gutierrez-Estrada
1970; Shepard et al. 1979); by analogy, other
prominent canyon systems that head on the
narrow continental shelf (Figs 1 & 2) are probably active as well.
Sediment bypassing of the slfpe off Mexico is
also supported by an axial profile of the Middle
America Trench, which shows a strong correlation between the thickness of sediment and the
location of large submarine canyons (Figs 1 &
2; Underwood & Karig 1980). These data suggest that the submarine canyons act as the
I00 °
99 °
EXPLANATION
. . . . . .
/i) ~SUBMARINE CANYON
~----TRENCH AXIS
X)..~"
o DATA POINT FOR
.- 19o
103~
L
AXIAL
(,'~,o
~
.
~
I
97 °
~
a-
,
~
IN
.
METERS
..............
~
~
-";,~opu~o
(
"~ ~ ' ~
~.J,,~,ff
"-~
E] SEDIMENT THICKNESS __/.f~'~
~ -
170-"
.
(
PROFILE
~_-#£,~-o5.R'/c~:
/
98 °
~
/~
/
539
/~,~;
(,, ~ -
. . . . . _ . . . . . ~ e : '~
~__
(.~-
o
oooo~
-
I
,000 r
o-
,5 r
~
~/
~
Cocotow=4 Survey
~
~
t'
DSDP Leg 66
~r
"
F
,ooo 1-:, 11
F]o. 2. Bathymetric map and trench-axis profile of the Middle America Trench from Rio Balsas to
Tehuantepec Bay, Mexico. Depth calculations and bathymetry are the same as for Fig. 1. Data
sources, in addition to those cited in Fig. 1, include Karig et al. (1978), Shipley et al. (1980), and
unpublished reflection profiles from the University of Texas Marine Lab (DSDP Leg 66 site survey).
Modified from Underwood & Karig (1980).
/
q
I
Downloaded from http://sp.lyellcollection.org/ at Oregon State University on June 8, 2012
540
M. B. Underwood
primary conduits for sediment entering the
trench. Sediment thickness decreases in both
directions away from point sources at the
canyon mouths, indicating that bi-directional
transport probably occurs on the trench floor.
Seismic reflection data at the mouth of Rio
Balsas Canyon show a thick wedge of sediment
that is channellized along its upper surface;
these appear to be distributary channels of a
submarine fan (Underwood & Karig 1980).
Rates of sedimentation in the Middle America Trench (Ross 1971; Moore, Watkins et al.
1979) are not high enough to allow seaward
progradation of submarine fans over the outer
trench slope (Underwood et al. 1980). As a
result, individual channels initially orientated
perpendicular to the trench axis either shift
their courses in response to the axial depth
gradient or are deflected by the outer trench
slope. Channellized transport on the trench
floor appears to be very localized, as channels
are no longer evident within a distance of
approximately 30 km from the mouth of Rio
Balsas Canyon (Underwood & Karig 1980;
Underwood et al. 1980).
In trench systems with higher rates of sedimentation and lower convergence rates (e.g.
the Oregon-Washington margin), submarine
fans have built out over the seaward trench
slope, masking the trench as a bathymetric
feature (Kulm & Fowler 1974; Schweller &
Kulm 1978). The Astoria Fan off Oregon, for
example, has been used as a model of submarine-fan sedimentation (Nelson & Kulm
1973; Nelson & Nilsen 1974); in this case, the
effects of tectonics on trench sedimentation
appear to be minimal.
Large submarine canyons do not provide the
only source of sediment to reach the trench
floor. Secondary sources include background
settling of hemipelagic debris and locally-derived mass flows, including slumps and slides
(e.g. Piper et al. 1973; von Huene 1974; Moore
et al. 1976; Karig et al. 1981). High pore water
pressures resulting from tectonically induced
dewatering at the base of the trench slope,
combined in some cases with oversteepened
slopes, provides a mechanism for recurrent
slope failure (e.g. Carson 1980). Mass flows
thus initiated may transport reworked slope
sediments or accreted trench deposits to the
trench floor, commonly via small submarine
canyons that head on the lower slope (e.g.
Karig et al. 1981). In general, hemipelagic
deposition and locally-derived mass Ilows provide a significant source of trench sediment only
if the terrigenous supply is low. Moreover, once
slumps and debris flows reach the trench floor,
& S. B . B a c h m a n
redistribution of the sediment by longitudinal
flow mechanisms may be expected (Piper et al.
1973).
Axial transport
Axial channels are prominent features in the
eastern Aleutian Trench (Piper et al. 1973; von
Huene 1974) and the southern Chile Trench
(Scholl et al. 1970; von Huene 1974; Schweller
& Kulm 1978; Underwood et al. 1980), and are
locally developed in the Japan Trench (Arthur,
von Huene et al. 1981), the northern Sunda
Trench (G. F. Moore, pers. comm. 1980), and
the Middle America Trench off Oaxaca (Shipley et al. 1980; McMillen et al. 1981). The
spectacular channel off southern Chile is continuous for over 1000 km (Schweller & Kulm
1978); however, the extent of individual flow
units has not been documented. The longitudinal distance and continuity of axial transport is
thus poorly known.
Non-channellized longitudinal flow almost
certainly occurs along trench floors in many
cases. Axial channels are absent in the Middle
America Trench north of Acapulco, for example (Ross & Shor 1965; Karig et al. 1978), but
textural data and sedimentary structures suggest that turbidity currents transport sediment
away from the mouths of submarine canyons
(Ross 1971). Moreover, the continuity of the
trench wedge, which maintains a thickness of
over 300 m north of Rio Balsas Canyon (Figs 1
& 2), suggests that axial transport occurs for
distances of 200-300 km between major point
sources. Other trenches that do not display
axial channels include the central Aleutian
Trench (Scholl 1974), the Kuril-Kamchatka
Trench (Scholl 1974), northern California
(Silver 1971), Oregon-Washington margin
(Kulm & Fowler 1974; Barnard 1978), the Gulf
of Oman (White & Klitgord 1976), the Peru
Trench (Schweller & Kulm 1978), the Ecuador
Trench (Lonsdate 1978), and the Middle America Trench off Guatemala (Seely et al. 1974;
Ladd et al. 1978; Ibrahim et al. 1979).
The presence or absence of an axial channel
is probably dependent upon several variables,
such as the axial depth gradient, the texture of
sediment reaching the trench floor, the rate of
sedimentation, and the type of mass flow
mechanism involved in axial transport. Where
channels are lacking, coarse sediment is probably transported as sheet-like turbidites, in a
manner analogous to sand-layer deposition
within flat-floored basins along the Atlantic
margin (e.g. Bennetts & Pilkey 1976; Pilkey et
al. 1980). Individual sand layers in these abys-
Downloaded from http://sp.lyellcollection.org/ at Oregon State University on June 8, 2012
541
Sedimentary facies associations within s u b d u c t i o n c o m p l e x e s
and apparently isolated ponds of sediment (Fig.
2).
sal-plain basins are continuous for distances of
up to 500 km (Elmore et al. 1979). Sand-layer
thickness and the percentage of sand gradually
decreases away from the source (Pilkey et al.
1980). Similar sheet-like flows in trenches are
generally confined by the seaward and landward trench slopes, but in some circumstances,
sand bodies may migrate up or overtop the
outer trench slope (Damuth 1979; J. C. Moore
et al. 1981).
Most trenches appear to contain a continuous
wedge of turbidites. In some cases, however,
sediment supplies are not large enough for
sediment bodies to prograde over basement
highs on the downgoing oceanic plate, such as
seamounts, aseismic ridges, and fault blocks
(e.g. Scholl 1974; Kulm et al. 1977). In the
northern Middle America Trench, basement
topography has created silled basins south of
Rio Balsas Canyon (Fig. 3). Depth to the
trench floor increases by over 200 m from the
north side of an exposed basement ridge to the
south side (Figs 2 & 3). South-directed turbidity
currents emanating from Rio Balsas Canyon
apparently filled the trench virtually to the crest
of the basement ridge; subsequent flows could
then spill over the ridge and continue down the
trench axis. Farther south, near the DSDP Leg
66 drilling sites, closely spaced reflection profiles show that axial transport is locally blocked
by basement highs (Shipley et al. 1980); low
rates of sedimentation result in discontinuous
ii
~o
~rF
Ancient analogues
In spite of post-depositional deformation,
sedimentary sequences up to hundreds of
metres in thickness are preserved within ancient
subduction complexes, such as the Coastal Belt
Franciscan Complex of northern California
(Bachman 1978, 1981). Inferred trench sediments within the Coastal Belt include all turbidite facies (Table 1). However, massive channellized sandstone (facies B) is locally dominant
and closely associated with thin-bedded deposits of probable channel overspill origin.
Chaotic fine-grained deposits (facies F) may
represent hemipelagic material slumped off the
base of the trench slope. Turbidite facies associations are consistent with upper- and mid-fan
depositional settings, but outer-fan facies associations are generally lacking. Palaeocurrent
data indicate that flow directions were parallel
to the inferred continental margin; deposition
may therefore have occurred within an axial
channel on the trench floor. Elsewhere within
the Franciscan Complex, mid- to outer-fan
facies associations, consisting mostly of facies C
and D turbidites, occur together with inner-fan
deposits (Aalto 1976). These data suggest that
trench fans were at least locally developed
within the Franciscan trench.
I-z
c~
~:
~ -I"
I
0
I
,,~ w
w oF)
z
w
0
tmZ
w 0
c'r" - i
(.9<i[
rr"
0
._I
Z W o
,~rr ~
0
~-
It.
_I ~-iz)
Go
--6.0 U
4500 1
W
O9
:2
ILl
--6.5~
F-
;$L'
d
ILl
>
--7.0
~,~:: ~'~Q%-/~ !i
•
, " ~'~ ~ ' ~ " ~ " # ~
:~t',',- "~ - ! ~
~,-~~ ' , ~ : ~ - ' . ~ . .
~C=~ `
:~:
::::: :
<
S-,,: ~:
>-
5500
<
-- 7.,5 ~
I'
0
I
I
I0
KM
I
I
20
(VE : 12X)
FIG. 3. Single-channel seismic-reflection (air-gun) profile across the floor of the Middle America
Trench south of Rio Balsas Canyon. Location of trackline B-B' is shown in Fig. 2. Seismic profile is
from DSDP Leg 66 transit.
Downloaded from http://sp.lyellcollection.org/ at Oregon State University on June 8, 2012
542
M . B. U n d e r w o o d
& S. B. B a c h m a n
Sedimentological data from other ancient
subduction complexes demonstrate the diversity of possible transport processes and depositional geometries within trenches. Ordovician
greywackes in the Southern Uplands of Scotland, for example, are interpreted as accreted
trench deposits and consist primarily of facies B
and C sandstones (Leggett et al. 1979; Leggett
1980). Localized conglomeratic units may represent point sources within a largely nonchannellized trench-floor transport system
(Leggett 1980). Permian to Jurassic strata in
New Zealand include both m61anges and coherent packets of strongly channellized thickbedded sandstone and conglomerate (facies A
& B) with associated levee deposits and overbank turbidites (Carter et al. 1978). Some of the
channel-fill sequences probably accumulated in
channels orientated parallel to the trench axis;
localized point sources are also likely (Carter et
al. 1978). Tectonically juxtaposed packets of
accreted trench turbidites on Barbados are
laterally continuous and up to 500 m thick;
facies associations are consistent with deposition within middle and outer portions of one or
more submarine fans (Speed 1981; Pudsey &
Reading 1981). Inferred trench sediments on
Kodiak Island, Alaska, were probably deposited within a basin-plain environment; dominant facies include fine-grained facies D turbidites and interbeds of facies G hemipelagic
shale (Nilsen & Moore 1979). Regional facies
relations and palaeocurrent data suggest that
the basin-plain deposits represent long-distance
axial transport and distal sedimentation within
a major trench-floor system in SE Alaska
(Nilsen & Bouma 1977; Nilsen & Moore 1979;
Nilsen & Zuffa 1981).
Sedimentary facies model
Based upon the available data from both
modern and ancient trench deposits, we propose a conceptual model for trench sedimentation which includes four major types of
sediment bodies and corresponding facies
associations. The four trench-floor facies associations are trench-fan, axial-channel, nonchannellized, and starved-trench (Fig. 4).
Trench fans, in most cases, are not large
enough to prograde over the outer trench slope,
and the transition from more proximal to more
distal facies associations takes place in a direction that is parallel to the trench axis. Submarine fans distorted by restricted basin
geometries have been recognized within the
geological record (e.g. Pescadore 1978).
Trench-fan deposits include all turbidite facies
and submarine-fan facies associations, and
palaeocurrent patterns range from radial to
longitudinal (Fig. 4). Vertical cycles should
include typical thinning- and fining-upward
sequences associated with channel migration
and abandonment, as well as thickening- and
coarsening-upward cycles associated with progradation of outer-fan depositional lobes (e.g.
Ricci-Lucchi 1975; Mutti et al. 1978; Walker
1978).
TE.., NOOS
, J/Y
TRENCH " , , : : ? / ¢ /
.
so
~"%~,i,~,F A N ' , '>Y, ~:: . . . . . .
,'
" - ~ " - - "~
-'~/<x..\~\
FACIES'- ('A),B,
C, D, E, (F), 6
Associo[ions :
INNER TO OUTER
FAN
Paleocorrents:
RADIAL ; MAY
BE DOMINANTLY
L OMG/TUDINAL
,
"
C H A N N E L"' :, "'(".,"~
~
:
~
-
-
Associations :
INNER TO MID
FAN
Poleocurrents :
LONG/TUDINAL
J
~
C, D, (F), G
Associations:
-
\ .....
: ~
F A C I E S:
~
(D],(F), G
OUTER FAN TO
BASIN
PLAIN
Poleocurrents:
L ONGI TUD/NA L
Associations =
SLOPE
PLAIN
OR B A S I N
FIG. 4. Conceptual diagram showing the turbidite facies and palaeocurrent patterns predicted for
sediment bodies on the trench floor. See Table 1 for definition of facies terminology. Minor turbidite
facies are shown in parentheses, and equivalent and/or similar associations, including submarine-fan
facies associations, are also indicated. Adapted from Underwood et al. (1980).
Downloaded from http://sp.lyellcollection.org/ at Oregon State University on June 8, 2012
Sedimentary facies associations within subduction
Trenches containing large axial channels do
not fit well into a model of submarine-fan
sedimentation. Thick sequences of channellized
sandstone (facies B & C) are likely in such
cases; these deposits would pass sharply into
associated fine-grained levee and overbank deposits (facies E, with D & G). Palaeocurrents
reflect longitudinal flow (Fig. 4). Facies associations are similar to those of inner- to mid-fan
facies, but more distal fan facies associations
are notably lacking. Basin-floor channels of this
type have been described in the Alps and
Apennines by Sagri (1979).
Evidence from seismic reflection data suggests that non-channellized sheet flow is common in modern trenches. Facies associations
related to this style of sedimentation are probably similar to non-channellized outer-fan and
basin-plain facies associations,
although
palaeocurrent patterns would be dominantly
longitudinal (Fig. 4). Facies C and D turbidites
and interlayered hemipelagic mudstones (facies
G) are expected, but well-developed vertical
cycles are unlikely. Analogies with modern
abyssal-floor basins suggest a decrease in both
sandstone/shale ratios and sand-layer thickness
with increasing distance from the sediment
source (Pilkey et al. 1980).
Trenches may be starved of coarse clastic
material either because no terrigenous sands
reach the trench or axial transport on the trench
floor is blocked. The sedimentary facies
associations expected with a 'starved' trench are
similar to those of the slope or basin plain (Fig.
4); deposits are dominated by facies G
hemipelagic muds and fine-grained facies D
turbidites, along with possible facies F slump
bodies.
Trench-slope deposits
The structure and morphology of forearc regions are diverse and variable (Seely 1979;
Dickinson & Seely 1979). Large sedimentary
basins such as the Aleutian Terrace (Marlow et
al. 1973; Scholl 1974) and the Mentawai
Trough, off Sumatra (Karig et al. 1980) are
commonly located between the active magmatic
arc and the trench-slope break. Several smaller
basins or benches may develop on the upper
trench slope, such as off Oregon-Washington
(Barnard 1978; Kulm & Scheidegger 1979) and
Peru-Chile (Coulbourn & Moberly 1977;
Moberly et al. 1981; Kulm et al. 1977; Kulm et
al. 1981). Models of forearc-basin sedimentation
are
well-established.
Sedimentary
sequences range from non-marine, deltaic, and
complexes
543
shelf deposits to large complexes of coalescing
submarine fans (Dickinson 1971, 1974; Ingersoll 1978b; Dickinson & Seely 1979). In this
paper, forearc basins are considered only in so
far as they affect sedimentation along the lower
trench slope.
Sedimentary basins along the lower slope are
generally elongate and bounded by tectonically
active ridges (e.g. von Huene 1972, 1979; Kulm
& Fowler 1974; Carson et al. 1974; G. F. Moore
& Karig 1976; White & Klitgord 1976; Karig et
al. 1979; Arthur, von Huene et al. 1981). Locally, slope basins may contain: (1) hemipelagic
deposits or remobilized material slumped off
adjacent bathymetric highs; (2) turbidites of
shallow-water origin transported beyond the
trench-slope break via submarine canyons; or
(3) debris-flow deposits containing blocks of the
underlying accretionary complex (G. F. Moore
& Karig 1976). Any small basin or terrace is a
potential site for the accumulation of sand-sized
detritus, and sand layers have been cored in
lower-slope basins off Kodiak Island (Kulm,
yon Huene et al. 1973; Howell & v o n Huene
1978; M. Hampton, pers. comm. 1979), New
Zealand (Lewis 1981), and Oregon-Washington
(Kulm, von Huene et al. 1973; Kulm & Fowler
1974; Barnard 1978; Kulm & Scheidegger
1979). Because of upslope blockage of sediment
transport by tectonic ridges and deposition
within basins located higher on the slope, the
chances of coarse sediment reaching a lowerslope basin decreases with increasing distance
down the trench slope (Underwood et al. 1980).
Where basins are not present, the apron of
sediments covering the lower trench slope
generally consists of hemipelagic muds and
occasional thin beds of fine sand and silt (Ross
1971; Kulm, von Huene et al. 1973; Barnard
1978; Langseth, Okada et al. 1978; Moore,
Watkins et al. 1979; Kulm & Scheidegger 1979;
Krissek et al. 1980; von Huene, Aubouin et al.
1980; McMillen & Haines 1981 ; McMillen et al.
1981; Arthur, von Huene et al. 1981). In many
cases, the slope muds overlie much coarser
trench sediments that were accreted at the base
of the lower slope ( e . g . J . C . Moore & Karig
1976; Moore, Watkins et al. 1979). Tectonic
loading and oversteepened slopes may cause
sediment failure (slumps and slides) within the
apron of slope muds (e.g. Hampton et al. 1978).
Upslopetrapping
Small submarine canyons play an important
role in the distribution of sediment along the
trench slope by providing an effective transport
route for coarse detritus. Most of the canyons
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544
M. B. Underwood
on the landward slope of the northern Middle
America Trench head downslope of the continental shelf (Figs 1 & 2; Fisher 1961), and, as
a result, they probably do not receive a direct
supply of terrigenous detritus with the present
position of sea-level. It is likely, however, that
sand-sized material entered the canyons during
global low stands associated with Pleistocene
glacial epochs. Moreover, unconfined mass
movements (sediment creep, slumps, slides,
debris flows) may carry coarse debris beyond
the shelf break (Field & Clarke 1979; Nardin et
al. 1979). Small submarine canyons can then
intercept and funnel downslope the turbidity
currents generated by mass movements occurring upslope or on the outer shelf.
Tectonic ridges and other bathymetric highs
on the trench slope commonly cut off the paths
of submarine canyons and block the transport
of turbidites and other mass flows. The mechanics of upslope trapping are well-documented in
the northern Middle America Trench, where
detailed local bathymetry shows that the small
canyons typically coalesce downslope into
fewer and larger canyons (Karig et al. 1978;
Underwood et al. 1980). As an individual
canyon approaches a ridge, such as the trenchslope break, the canyon channel is blocked, the
canyon gradient decreases, and sediment becomes ponded behind the ridge in a slope or
forearc basin. Reflection profiles display far
fewer submarine canyons below the trenchslope break (Underwood et al. 1980). In general, only the largest canyons continue across the
active ridges of the trench slope (Underwood &
Karig 1980). Approximately 80% of the submarine canyons off Mexico end without
reaching the trench floor; most terminate above
the trench-slope break (Figs 1 & 2). Where a
greater number of active ridges are present,
such as the Sunda Trench (G. F. Moore &
Karig 1976; Karig et al. 1979), the effects of
structural blockage of sediment transport are
more pronounced, and the chances of submarine canyons maintaining their channels
greatly diminishes downslope.
Ancient analogues
In general, differentiation of trench-floor and
trench-slope deposits within ancient subduction
complexes is difficult on the basis of sedimentary facies analysis alone, except in cases
such as Barbados, where a clear lithological
distinction exists between thin-bedded biogenic
marls (slope deposits) and accreted trench
turbidites (e.g. Speed 1981). Structural style is
perhaps the most commonly used criterion for
& S. B . B a c h m a n
distinguishing between these two types of units,
as trench deposits are generally more highly
deformed than associated slope sediments (e.g.
Bachman 1978; Moore & Karig 1980).
Perhaps the most complete record of lowerslope sedimentation is exposed on Nias Island,
off Sumatra (Moore 1979; Moore & Karig 1976,
1980; Moore et al. 1980). Stratigraphic sequences exposed on Nias define an overall
coarsening- and thickening-upward trend
associated with basin uplift and the progradation of a submarine fan complex (Moore et al.
1980). Basal slope strata overlie m61ange
(accreted trench deposits) and consist of
hemipelagic marls and lutite turbidites (facies D
& G). There is a pronounced increase upsection
in coarser facies A, B, and C deposits, and
small-scale vertical cycles suggest that deposition of coarser facies occurred within outer- to
inner-fan environments (Moore et al. 1980).
The stratigraphic record on Nias demonstrates
the influence of tectonic processes on lowerslope sedimentation. Apparently, individual
basins, initially isolated from sources of coarse
detritus, were uplifted and eventually linked
with downcutting submarine canyons which
headed off the coast of Sumatra (Moore 1979;
Moore et al. 1980). Continued uplift and deposition allowed progradation of submarine
fans and the infilling of the slope basins.
Several examples of possible slope-basin deposits occur within the Franciscan Complex of
California (Howell et al. 1977; Underwood
1977; Bachman 1978, 1981; Smith et al. 1979).
These strata are relatively undeformed and
generally dominated by thick sequences of
channellized facies B sandstone, with associated deposits of levee/overbank origin (facies
E, with minor D & G). Some sections in the
Coastal Belt contain abundant facies C & D
turbidites, but distal fan-facies associations are
generally lacking (Bachman 1978, 1981). Facies
associations suggest deposition within mid- to
outer-fan channels and depositional lobes, or
their facies equivalents (Howell et al. 1977;
Bachman 1978; Smith et al. 1979). The predominance of thick-bedded sandstone can be
attributed to selective trapping of the coarsegrained parts of gravity flows within restricted
slope basins, while more turbulent fine-grained
fractions were able to bypass the basins and
settle farther down the trench slope (Bachman
1978; Smith et al. 1979).
Ancient trench-slope deposits on Kodiak Island consist primarily of thick sequences of
facies G mudstone (Nilsen & Moore 1979).
Chaotic deposits (facies F) and thick beds of
channellized sandstone and conglomerate
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545
Sedimentary facies associations within subduction complexes
(facies A & B) crop out locally and probably
represent slump deposits and canyon/channel
fill, respectively (Nilsen & Moore 1979).
Trench-slope deposits on New Zealand contain
deposits of low energy bottom currents (contourites) in addition to the dominant hemipelagic mudstones and lutite turbidites (Carter et al.
1978).
Sedimentary facies model
In our facies model for trench-slope deposits
(Fig. 5), submarine-canyon and slope facies
associations are generally the same as for tectonically inactive environments (e.g. Whitaker
1974; Kelling & Stanley 1976). Slope deposits
are dominated by facies G hemipelagic mudstones and thin-bedded, fine-grained facies D
turbidites (Fig. 5). The occurrence of chaotic
deposits associated with slope failure (facies F)
may be more common within subduction complexes, however, because of the related tectonic
activity. Channel lag deposits from submarine
canyons consist largely of coarse facies A, B, &
C deposits; in large canyons, levee and overbank deposits (facies D, E, & G) may represent
local deposition outside the channel thalweg.
Slumps off canyon walls (facies F) may also
become interstratified with canyon fill. Inactive
canyons are filled largely with hemipelagic
slope deposits and fine-grained turbidites
(facies D & G). Palaeocurrents from canyon fill
should be dominantly at a high angle to the
margin (Fig. 5).
Because of upslope trapping, trench-slope
basins near the base of the landward slope
generally receive only fine-grained sediments
derived from hemipelagic settling and dilute
density flows (facies D & G). Slumps and slides
derived from tectonically active bounding
ridges are also likely, but such chaotic deposits
would consist primarily of remobilized finegrained slope sediments. In our facies model,
basins that receive only fine-grained material
are classified as 'immature' slope basins
(Fig. 5).
Progressive growth of the accretionary prism
generally causes uplift of trench-slope basins
SUBMARINE cANYoN ~/. ' : / / ~ TERRG
i ENOUS,L
FACIES~XI, B,(C),(DJ, E, ';,)~// . SOURCE~,.
~ ~I..~..<<"~/>. : / /
~Po,.ocu.,n,,,
/ .
,/,,
/
/,,,, ~~ ; ~
/
," / ,
,
..j / . \~,:~/./
~
/'~//~'~
~
:-\
_
-/_-.--
.........
~- ...... i-
~.
i
/
~ - - - - - --~-..-__
..-U~ ;
~
i--
/
i
. . . .
----__J_.___-<_._:::~ . . . .
,
"/'//
.
"
'
//
,~
J,
'.
,
---~~~
I //
!
,
I
- ~ ~
£'/I
¢
//
/I
Poleocurrents:
~
,
BASIN
/
FACIES= / /
I III/
S. . .L. . O P E
,
J'/-
j ~ r f i ~,i,} i
;.
~/~//, /
_~,////.
i,J/'/1,
~ -:_~.~
c "
.' .,
~
.
"~..~.~'.-~-----~"
/./,/~7"
~- \ < < ~ ~ ,
.~\',~\'i~/-~
/ i/I~
- -L:~ -_ i
,.
.FLOOR ?_L./t.',//
>__--J~_ -..
.
TO
V
//
/ .~. . .'. . .\. . , , ,1 /J,//
~ / // ;
.....
A N G L E
i V/ MARGIN
.'---(D),(F),6
/,/,/
HIGH
I,l,17 s o u R c E
,I</Is ~ ~ ~ j , , .
Associations, / /
~
~'~
__.
~==---
SLOPE
- ~
OR
"~-~--'-~-
11
:~:-
/1
BASIN
__:
~ r ~
,
PLAIN/
.~.
'
'~;
7- -?~
~
"
I
. /
h i
~-
J
--
___~---~-.._~_..\7'"~.~ ~ ~
FIG. 5. Conceptual diagram showing the turbidite facies and palaeocurrent patterns predicted for
trench-slope settings. The terminology follows that of Fig. 4 (see Table 1). The terms 'mature' and
'immature' slope basin refer to the presence or absence, respectively, of a source of coarse
terrigenous sediment, and are not necessarily indicative of any particular bathymetric position on the
trench slope. Adapted from Underwood et al. (1980).
/
"
"-
J
"
Downloaded from http://sp.lyellcollection.org/ at Oregon State University on June 8, 2012
546
M. B. U n d e r w o o d & S. B. B a c h m a n
through time; this is associated with a decrease
in tectonic activity with increasing distance
from the active basal slope (Seely et al. 1974;
Karig & Sharman 1975). As a result, bounding
thrust ridges may become inactive during the
uplift history of individual slope basins, thereby
increasing the width of the basins through time
(G. F. Moore & Karig 1976). The combined
effect of these processes may be the sudden
influx of coarse terrigenous detritus into previously isolated slope basins, as basin uplift is
accompanied by progressive downcutting of
submarine canyons (e.g. Moore et al. 1980).
In our facies model, we define any such basin
that receives a direct supply of coarse clastic
material as 'mature', regardless of its actual
position on the trench slope (Fig. 5).
The facies associations for mature slope
basins are varied and depend largely upon the
geometry of the basin. All of the submarine-fan
facies associations may be present in large slope
basins. In contrast, small narrow basins may
selectively trap coarse material and allow finegrained sediment to bypass the basin.
Palaeocurrent patterns may also vary from radial to longitudinal (Fig. 5).
Discussion
The sedimentary facies models presented in this
paper are somewhat speculative, due largely to
the general lack of detailed sampling within
modern subduction zones. The available data
clearly indicate, however, that over-simplified
models cannot account for the observed complexity and diversity of sediment bodies and
patterns of sedimentation. We have attempted
to stress the variety of possible facies associations and identify some of the variables that
affect sedimentation within trench-floor and
trench-slope environments.
Turbidite facies terminology is an important
tool in the analysis of sedimentary strata,
including strata exposed within ancient subduction complexes. However, turbidite facies
associations within subduction complexes may
be other than those of a submarine fan. The
facies associations that do not fit well into the
slope/fan/basin-plain model of deep-basin sedimentation
include
axial-channel,
nonchannellized (sheet-flow) trench-floor, starvedtrench, immature-slope-basin, and certain associations within mature slope basins, such as
small basins that selectively trap coarse detritus. Submarine fans also occur within subduction zones, but we believe that the use of
terminology linked to submarine-fan morpholo-
gies is clearly not appropriate in all cases. The
identification of submarine-fan deposits within
subduction complexes should depend upon the
documentation of appropriate palaeocurrent
patterns, vertical cycles, and fan facies associations that are consistent with submarine-fan
progradation or regression. In the absence of
such documentation, facies association terminology should be linked with the geometries of
the other types of sediment bodies observed
within modern trenches, such as axial channels.
Our facies models have other implications for
environmental reconstructions within accretionary margins, including the identification
and differentiation of separate tectonostratigraphic units. Because of the complexity
and variability of sedimentation that is possible
within a single trench, general lithologies and
turbidite facies associations can change dramatically within a relatively short distance. For
example, structural blockage of axial transport
paths by a seamount on the downgoing oceanic
plate could allow thick sequences of sandy
turbidites to accumulate on one side of the
blockage, with hemipelagic muds dominating
the other side. Subsequent accretion and uplift
of the seamount and trench fill could expose
sections of strata that might appear to represent
completely unrelated geological histories. The
resulting juxtaposition of sandy turbidites,
basalts, and mudstones could be erroneously
attributed to large-scale tectonic events associated with accretion, rather than the original
depositional geometry that existed on the
trench floor. In other cases, it may be nearly
impossible to differentiate between trench fill
and trench-slope deposits. For example, abyssal
sediments, trench sediments, and slope sediments could all be very similar if the trench is
starved of coarse clastic material. If tectonostratigraphic designations are based primarily
upon sedimentary facies relations, then all of
the resulting strata would be included in the
same unit, and the designations would have
little genetic meaning.
In addition to the predicted lithological variations and complexities, significant differences in
sandstone petrology could result for trenchfloor and slope-basin deposits that accumulate
in different sites along the length of an accretionary margin. For example, both regional and
local changes in source rocks and onland drainage patterns could introduce sands of differing
composition into the submarine canyons that
funnel detritus into different sedimentary
basins. Moreover, long-distance axial transport
in the trench could juxtapose detritus of one
composition against sediments of a different
Downloaded from http://sp.lyellcollection.org/ at Oregon State University on June 8, 2012
Sedimentary facies associations within subduction complexes
provenance that were transported via local submarine canyons. It is therefore important to
integrate palaeocurrent data with data on sandstone petrology before reliable provenance determinations can be made.
We suggest that normal sedimentary processes, rather than large-scale tectonic events,
may be responsible in some cases for lithological and compositional changes recorded within
many subduction complexes and accretionary
margins. Clearly, the possible effects of penec o n t e m p o r a n e o u s or subsequent tectonism
must be considered, but the possibility of relating observed changes or differences to a logical
system of sediment transport and deposition
547
cannot be ignored. The separation of strata into
meaningful tectonostratigraphic units should
d e p e n d upon several criteria, such as contrasts
in sedimentary facies associations, sandstone
petrology, structural style, palaeontology, and
general physical properties.
We would like to thank the
following people for valuable contributions to this
paper: M. Hampton, D. Howell, D. Karig, G. F.
Moore, J. C. Moore, W. Normark, and W. Schweller. The manuscript was greatly improved by critical
reviews from C. Ando, J. Leggett, G. F. Moore, W.
Schweller, and the Italian and English sedimentologists who served as anonymous referees.
ACKNOWLEDGMENTS:
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