facies definition are the bed's thickness, geometry, and the graveljsand ...... (1973), Walker & Mutti (1973), Wright & Wilson (1984),. Pickering et al. (1986, 1989) ...... In: Mar- gina/ Basin Geo/ogy (Ed. by B. P. Kokelaar· & M. F.. Howells), Spec.
c..-'"" . .
l
l
Sedimentology (1992) 39, 423-454
Subaqueous sediment gravity flow deposits: practical criteria for their field description and classification GUIDO GHIBAUDO Dipartimento di Scienze della Terra, Università di Torino, Via Accademia delle Scienze 5, 10123 Torino, ltaly
ABSTRACT A new method for the description and classifìcation of subaqueous sediment gravity flow deposits is proposed. The classifìcation scheme employs a convenient letter code and divides deposits (individuai beds) into descriptive categories oftwo hierarchicallevels: facies and subfacies. Facies, as the higher rank categories, are distinguished chiefly on the basis of sediment type (i.e. bed grain size/texture). A total of 13 facies have been distinguished : G = gravel ; GS = gravel- sand couplet; GyS = gravelly sand ; S =sand ; SM = sand-mud couplet; MS = mud- sand couplet; TM = silt- mud couplet; MT = mud-silt couplet; M= mud; MyS =muddy sand; SyM = sandy mud ; MyG = muddy gravel; GyM = gravelly mud. Subfacies, as the l.ower rank categories, are distinguished within the individuai facies on the basis ofthe bed's internai structures. The number of subfacies is unlimited, and their labelling code includes particular facies symbols (see above) preceded by lower-case letters denoting specifìc sedimentary structures and their vertical arrangement. Subfacies thus refer to the bed's intervals, or divisions, which are labelled as follows: m= massive (unstratifìed and ungraded) ; g = graded (unstratifìed and graded); s = plane-stratified; x= cross-stratitìed; l= para Ilei- and/or cross-laminated ; q= liquetìed. For example, subfacies gsG (graded to plane-stratifìed gravel) are grave( beds that have a lower graded interval and an upper plane-stratitìed interval; subfacies xG (cross-stratifìed gravel) are gravel beds that are cross-stratitìed throughout ; subfacies siS (plane-stratifìed to laminated sand) are sand beds that have a lower plane-stratitìed interval and an upper laminated (parallel- andfor cross-laminated) interval.
INTRODUCTION A new classification scheme for modern and ancient deep-water deposits has recently been proposed in a review by Pickering et al. (1986). This scheme, and its slightly revised later version (Pickering et al., 1989), incorporate (al bei t with substantial modifications) the previous classification schemes of Mutti & Ricci Lucchi (1972, 1975), Walker & Mutti (1973), Walker (1975a, 1978) and Mutti (1979), and represent the subject's most complete treatment currently available. Despite its outstanding character, the classification scheme of Pickering et al. (1986, 1989), as well as its predecessors, appears to be impractical for field-work, especially for the detailed bed-by-bed Jogging of stratigraphic sections. The main difficulties in applying these schemes in the field are: (l) the necessity to memorize and know beforehand the sedimentary characteristics of some
tens of individuai facies, denoted by letter and number codes that are not particularly informative in themselves; and (2) their inadequacy as the means of providing a simple description of individuai beds in terms oftheir component, textural/structural divisions or depositional intervals. A reasonably universal method for describing sediment gravity flow deposits, based on a widely acceptable classification scheme, is stilllacking. The demand for such a practical scheme is dictated not only by the researcher's need fora more uniform approach and easier communication, but also by the increasing need for computer storage, rapid numerica) analysis and comparison of large data sets (for example the cored sections of modern turbidite systems and detailed bed-by-bed sections of similar ancient systems). This paper is intended to meet these practical 423
v
G. Ghibaudo
424
purposes. The description method and classification scheme for sediment gravity flow deposits proposed here are meant to serve as a practical tool for both field studies and laboratory analytical work. The descriptive approach and the corresponding classification scheme ha ve been based chiefly on analysis of a number of ancient turbiditic systems, including the detailed bed-by-bed measuring of numerous sections in the U p per Eocene to Lower Oligocene Grès d'Annot Formation of the western French Alps (Ghibaudo et al., in preparation). The scheme presented here has been used successfully to describe about 30 000 turbiditic beds from this formation, and is currently being applied in a study of resedimented shallow- and deep-marine clastics in the Tertiary Piedmont Basin of northern Italy. This new classification pertains to subaqueous sediment gravity flow deposits in generai, not necessarily those of deep-water environments. Therefore, no account has been made here of the full spectrum of facies commonly associated with deep-water turbidites, such as hemipelagic muds, contourites, volcanic ash beds, or various biogenic oozesjmuds. The broader, more comprehensive review of Pickering et al. (1986, 1989) remains as a valuable tool for the classification of deep-water deposits in generai.
CLASSIFICATION OBJECTIVES The principal objectives of the proposed classification are as follows. (l) To offer a practical tool for the field description and descriptive classification of sediment gravity flow deposits, employing conventional macroscopic observations. The classification therefore focuses on relatively coarse-grained deposits such as gra ve!, sand and siit, rather than on the varieties of mud, whose complete description often requires speciallaboratory analyses. (2) To formulate a fully descriptive framework for the classification of sediment gravity flow deposits that is sufficiently broad and flexible to include both the known common facies and those that may be recognized or distinguished in the future. (3) T o provide a useful, informative, descriptive code for the bed types (facies) and their depositional intervals, or internai 'divisions', employing a letter code derived from conventional descriptive terms, to provide a straightforward visualization of the bed's texture and internai structure. This coding
system allows us to summarize the essential characteristics of a bed and to ascribe the bed readily to a corresponding facies and subfacies.
FACIES DEFINITION AND TERMINOLOGY The term 'facies' is defined here, following Mutti & Ricci Lucchi (1975), as 'a bed or a group of beds showing lithologic, geometrie and sedimentologic characters which are different from those of adjacent beds. A facies is considered to be the product of a specific depositional mechanism or severa! related mechanisms acting a t the same time.' The basic sedimentary characteristics used here to define different facies and subfacies are the sediment type and internai structures of the beds. These characteristics, when coded, are represented by upper- and lower-case letters, respectively, and the letters are usually the initials of some well-known, descriptive sedimentological terms. Secondary sedimentary attributes used in facies definition are the bed's thickness, geometry, and the graveljsand, sand/mud or silt/mud thickness ratio in bipartite beds. For simplicity, the names of sediment grain-size categories (i.e. grave!, sand, silt and mud) have been adopted to de seri be the lithology of a bed, butit should be clear to the reader that the classification pertains equally to unconsolidated and lithified deposits. The grain-size classes correspond to those of Wentworth (1922). Litbology and related code-keys
The lithological categories adopted for the classification scheme are based on the sediment grain size and to some extent also sorting. They are shown in Fig. l ; the corresponding descriptive code-keys are listed in Table l. G =grave!. This category comprises beds that consist of a clast-supported grave! throughout, or have a clastsupported lower part and a matrix-supported upper part. Mud content is less than lO vol.% and mud caps are generally absent. For the purpose of detailed descriptions, the upper size limit of the clasts may be denoted with an appropriate adjective/prefix : PyG=pebbly grave! ; CyG = cobbly grave!; ByG = bouldery grave!. GS=gravel- sand couplet. This bed type consists of a clast-supported, gravelly lower part and a sandy
'
Sediment gravity fio w deposits
G
GRAV E L
GS
GRAVEL- SAND COUPL ET
SM
SAND - MUD COUPL ET
MS
MUD -SAND C OUPLE T
TM ~
SILT - MUD COUPLE T
MT . . ·.
M UD- SILT COUPLET
M
GyS
o
•
•
GRAVELLY SAND
s
425
MUD
MyS
MUDDY SAND
SyM
SAN DY MUD
MUDDY GRAVEL
SAND
GRAVELLY MUD
Fig. l. Schematic graphical presentation and letter-code of the defìned bed facies (lithological categories).
Table l . Summary of the descriptive code for lithologies and depositional intervals. Lithology Gravel Gravel- sand couplet Gravelly sand Sand Sand- mud couplet Mud- sand couplet Silt- mud couplet Mud- silt couplet Mud Muddysand Sandymud Muddy gravel Gravellymud
Depositional interval G GS GyS
s
SM MS TM MT M MyS SyM MyG GyM
massive graded plane-stratifìed cross-stratifìed parallel-laminated cross-laminated muddy interval faintly laminated mud graded mud liquefìed
m 2 ,m 1, m
82> g, , g, 8o s2 , s., s x 2 , x 1, x li ' b, d C,C0
e e, e2 q
426
G. Ghibaudo
upper part. The gravelly part comprises more than 5% of the total bed thickness and the couplet represents a single depositional event. Mud content is less than IO vol. % and mud caps are generally absent. GyS = gravelly sand. This bed type consists of sandy matrix-supported grave!, or has a matrix-supported, gravelly lower part and a sandy upper part. The gravelly part constitutes 5- 100% of the total bed thickness. Mud content is less than IO vol. % and mud caps are usually absent. As with the grave! facies, the upper size limi t of the clasts may be denoted with an appropriate adjective/prefix: PyS = pebbly sand; CyS = cobbly sand ; ByS = bouldery sand. S=sand. Beds of this type are sandy, but up to 5% of their thickness may consist of a basai gra velly sand or grave!. Mud content is less than IO vol. %. Mud caps are usually absent. SM=sand- mud couplet. Beds of this type are bipartite, with a sandy lower part (possibly with some pebbles at the base) and a muddy upper part. The sandy division constitutes more than 50% of the total bed thickness and the sand- mud couplet represents a single depositional event. MS=mud- sand couplet. This is analogous to the previous facies, but here the muddy division constitutes more than 50% of the total bed thickness. For brevity, in both ora! and written communication, this bed type is referred to as a 'mud- sand' couplet to indicate that the bed's muddy division is thicker. TM=si/t- mud couplet. This bed type consists of a silty lower part and a muddy upper part. The silty division constitutes more than 50% of the total bed thickness, an d the silt- mud couplet represents a single depositional event. Silt, according to Folk's (1968) definition, is a clay-poor type of mudrock, with less than 30 vol.% clay. MT=mud- silt couplet. This is analogous to the previous facies, but again the muddy division here constitutes more than 50% of the total bed thickness. As with the MS facies, this bed type is referred to as a 'mud- silt' couplet to indicate that the bed's muddy division is thicker. M = mud. This is a bed consisting largely of a claysilt mixture but with a distinct clay fraction ( > 30 vol.%; see Folk, 1968). Sand ( < 10 vol.%) may be a minor component. MyS=muddy sand. This is a mud-rich sand bed, containing 10- 50 vol.% mud. SyM= sandy mud. This is a sand-rich mud bed, containing 1Q--50 vol.% sand fraction. MyG = muddy grave/. This is a mud-bea ring grave! bed, containing IQ--50 vol.% mud, possibly sandy.
Gy M = gravelly mud. This is a gravel-bearing mud, possi bly sandy, containing less t han 50 vol.% of gravelsized clasts. Depositional features and their code-keys
The following types of bed intervals, or depositional divisions, are distinguished for further, more detailed description of the bed's internai characteristics (Fig. 2 & Table 1). m=massive. This prefix (m) indicates a bed, or a division of a bed, characterized by the absence ofboth stratification and vertical clast-size grading. The following varieties are distinguished with respect to the sediment's texture : m 2 =massi ve interval in clastsupported grave! ; m 1 =massive interval in matrixsupported grave! ; m = massi ve interval in sand. g = graded. This prefix (g) indicates a bed, or a division of a bed, characterized by vertical clast-size grading (of any type), but lacking stratification. The grading may be norma!, inverse, or inverse to norma!, and may be developed as either 'distribution', 'coarse tail', 'content' or 'top-only' varieties. The clast fabric may be chaotic or show some pattern. The following varieties are distinguished with respect to the sediment's texture: g 2 = graded interval in clast-supported grave! ; g 1 =graded interval in matrix-supported grave!; g=graded interval in sand ; g0 =graded interval in silt. s=plane-stratified. This prefix (s) indicates a bed, or a division of a bed, characterized by horizontal or subhorizontal parallel stratification. The piane-parailei strata may range from l to 20 cm in thickness, exceptionally up to 50 cm, depending on grain size. The strata may show gradational or sharp contacts, and either inverse or norma! internai grading. They are defined by variations in grain size or other textural attributes. The following varieties are distinguished : s2 = plane-stratified interval in clast-supported grave! ; s 1 = plane-stratified interval in matrix-supported gra ve! ; s = plane-stratified interval in sand. x= cross-stratified . This prefix (x) indicates a bed, or a division of a bed, characterized by large-scale cross-stratification in simple or multiple sets. The following varieties are distinguished : x2 = cross-stratified interval in clast-supported grave! ; x 1 = crossstratified interval in matrix-supported grave!; x= cross-stratified interval in sand. b, d = paral/e/-/aminated. These prefixes (b, d) indicate a bed, or a division of a bed, characterized by plane-parallellamination developed in sand (b) and in alternating silt and mud (d). They correspond to
,
427
Sediment gravity flow deposits
m
massive
S: plane-stratified
D o
.
.
m,
g
s
m
graded
X: cross-stratified
x1
.Q,Q
x
parallel-laminated
d
c
•
cross -la m i nated
c
q
liquefied
e
Co
peli t ic
B
LJ
interval
o
q
Fig. 2. Schematic graphical presentation and letter-code of the depositional features used for detailed bed description and subfacies distinction.
Bouma's (1962) turbidite divisions Band D . The term 'laminae' is used according to McKee & Weir (1953) for strata Iess than l cm thick, ranging in grain size from very coarse sand to silt. The coarser-grained and thicker (5- 1O mm) planar lamination is thus transitional to planar stratification (s), as defined above, and is often developed at the top of thick, planestratified or graded sand beds. This thicker lamination, defined by the alternating laminae of coarse to very coarse and medium to fine sand, is denoted with the symbol l$ to distinguish it from the thinner and finergrained parallellamination (b) that is more commonly developed in the 'classical', Bouma-type turbidites. Both types of parallellamination may show a parting lineation . The following varieties are distinguished: ~ , b = parallel Iamination in sand; d= parallel lamination (planar or undulatory) in alternating silt and mud. c= cross-laminated. This prefix (c) indicates a be d,
or a division of a bed, characterized by ripple crosslamination an d corresponding to Bouma 's (1962) turbidite division C. The following varieties are distinguished : c= cross-lamination in sand; c0 = cross-lamination in sii t. l= laminated . This is used as a generai prefix to indicate a bed, or a division of a bed, characterized by parallellamination (b, d) and/or cross-lamination (c).
e= peli tic interval. This prefix indicates the turbidite's mud division and corresponds to Bouma's (1962) division E. The following varieties are distinguished for more detailed laboratory descriptions : e 1 = faint parallel-lamination in mud ; e 2 = graded interval in mud . q= liquefied. This prefix indicates a bed, or a division of a bed, characterized by abundant fiuidescape structures (e.g. dishes and pillars; 'fiat consolidation laminae' of Lowe, 1975; 'wavy lamination' of
428
G. Ghibaudo
Corbet, 1972). Primary sedimentary structures in such units are, for the most part, completely destroyed or strongly obliterated. A number of additionalletters are also introduced as a means of coding other specific bed features: t= thinbedded- this key is used specifically for subfacies tgSM and trSM (see Fig. 3 & Table 2); r=form-set rippled- this key is used specifically for subfacies trSM (see Fig. 3 & Table 2); (i)=inversely graded; (in)= inverse to normally graded ; (S), (M)= keys to indicate some very thin, often discontinuous sand and mud caps in grave! and sand beds, respectively. Such beds are coded as G(S) and S(M), and their caps are considered to be of negligible thickness.
CLASSIFICATION SCHEME AND BED DESCRIPTION METHOD The proposed classification scheme is shown in Fig. 3; the corresponding facies nomenclature is given in Table 2. A synopsis of the facies is shown in Table 3. The classification divides the beds of sediment gravity flow deposits into descriptive categories oftwo different, hierarchical levels : facies and subfacies. F acies are of higher rank and are distinguished mainly on the basis ofthe bed's lithology. Thirteen facies are defined here, each labelled according to their lithology (e. g. facies G =grave! ; facies S =sand; etc.). Subfacies are of lower rank, and are distinguished on the basis ofthe particularcombinationsoflithology and internai structures shown by individuai beds. Accordingly, they are labelled according to a combination of their lithology and sedimentary structures (e.g. subfacies gS = graded sand; subfacies g!S = graded to laminated sand; sS = plane-stratified sand). The number of subfacies is unlimited ; the most common subfacies found in subaqueous sediment gravity flow deposits are shown in Fig. 3 and listed in Table 2. The distinction of subfacies in the field may require a more detailed characterization of the individuai beds in terms of their recognizable texturalfstructural depositional intervals. This additional descriptive information is coded with symbols that assign particular sedimentary structures to the bed's particular textural divisions (e.g. s 2 = plane-stratified interval in clast-supported grave! ; s 1 = plane-stratified interval in matrix-supported grave! ; s = plane-stratified interval in sand), and the multiple code thus conveys ali the essential information available for a particular bed. The classification proposed here is directly related to
this simple descriptive scheme adopted for the beds, and can be applied at various selected levels of bed detail, depending on the researcher's requirements or the type of available data. Bed description and facies designation
Individuai beds deposited by sediment gravity flows consist of various combinations of the depositional intervals already listed. These intervals are: (l) massive (unstratified and ungraded) ; (2) graded (unstratified, but variously graded); (3) plane-stratified ; (4) cross-stratified; (5) parallel-laminated ; (6) ripple cross-laminated ; (7) liquefactionally homogenized. Pelitic caps (the E division of Bouma, 1962) may or may not be presentat the bed tops. The descriptive code for denoting bed types, when Jogging sedimentary successions in the field or transferring pre-existing descriptive data into a computer file, is as follows (Fig. 4). (l) The bed's sediment type, or lithology, is specified (using one of the following: G, GS, GyS, S, SM, MS, TM, MT, M, MyS, SyM, MyG, or GyM). (2) The sedimentary structural features of the bed, from its base upwards, are denoted (using the following descriptive prefixes : m, g, s, x, b, c, d, e, q).
The facies designation of a particular bed is thus based on, and guided by, the lithological key (uppercase letters) and the sequence of prefixes denoting the bed's internai organization (lower-case letters). Any additional characteristics, such as the presence of inverse or inverse to norma! grading in the bed's lower part, are denoted using specific keys that precede, in parentheses, the principal keys fora given interval of a bed (see Fig. 4). Subfacies designation
Beds are thus categorized as facies directly in the field, when described and appropriately coded at the outcrop. Similarly, the subfacies designation of a bed can be readily made either directly in the field or at a later stage, when the field data can also be tabulated and stored in a computer file. The criteria adopted to distinguish particular subfacies are as follows. (l) Beds characterized by the absence of primary sedimentary structures are defined as massi ve and grouped into the corresponding subfacies (e.g. mG, mGyS, mS, etc.).
429
Sediment gravity fio w deposits
mSI7J LJ Cl)
cn Cl)
cn
~ ~
Cl)
xs~ ~ gxS
N u
u
gS f ) gtS
[5j..~ - . .-.. .. ·
gsS :'·:.>;'.
sS sxS
~
gsxS
stS
lZGill
gstS
(!) cn
Q. LiJ
gSM
~
l5l
W9tSM ~
Cl)
tSMg
trSM~
xSM~ tgSM ~
9 tMS~ tMS~ tTM i i
9 TM
1MT C!
gMT U
tgM[] cn Cl)
Fig. 3. Facies and subfacies scheme employed in the proposed classification of subaqueous sediment gravity flow deposits.
430
G. Ghibaudo
Table 2. The names and letter-codes used for the facies and subfacies distinguished in the present review. Facies G (grave! beds) mG : massi ve grave! xG : cross-stratifìed grave! sG : plane-stratifìed grave! gG : graded grave! gsG : graded to plane-stratifìed grave! Facies GS (gravel- sand couplets) gGS : graded gravel- sand couplet gxGS : graded to cross-stratifìed gravel- sand couplet gsGS : graded to plane-stratifìed gravel- sand couplet glGS : graded to laminated gravel- sand couplet gsxGS : graded to plane-stratifìed to cross-stratifìed gravel- sand couplet gslGS : graded to plane-stratifìed to laminated gravelsand couplet Facies GyS (gravelly sand beds) mGyS : massi ve gravelly sand xGyS : cross-stratifìed gravelly sand gGyS: graded gravelly sand gxGyS: graded to cross-stratifìed gravelly sand gsGyS: graded to plane-stratifìed gra velly sand glGyS: graded to laminated gravelly sand gsxGyS: graded to plane-stratifìed to cross-stratifìed gravelly sand gslGyS : graded to plane-stratifìed to laminated gravelly sand sGyS: plane-stratifìed gravelly sand sxGyS : plane-stratifìed to cross-stratifìed gravelly sand slGyS : plane-stratifìed to laminated gravelly sand Facies S (sand beds) mS : massive sand xS : cross-stratifìed sand gS : graded sand gxS : graded to cross-stratifìed sand gsS : graded to plane-stratifìed sand glS: graded to laminated sand gsxS : graded to plane-stratifìed to cross-stratifìed sand gslS : graded to plane-stratifìed to laminated sand sS : plane-stratifìed sand sxS : plane-stratifìed to cross-stratifìed sand slS : plane-stratifìed to laminated sand Facies SM (sand- mud couplets) gSM : graded sand- mud couplet glSM : graded to laminated sand- mud couplet ISM : laminated sand- mud couplet xSM : cross-stratifìed sand- mud couplet tgSM : thin-bedded, graded sand- mud couplet trSM: thin-bedded, rippled sand- mud couplet Facies MS (mud- sand couplets) g!MS : graded to laminated mud- sand couplet IMS : laminated mud- sand couplet Facies TM (silt- mud couplets) ITM : laminated silt- mud couplet gTM : graded silt- mud couplet
Facies MT (mud- silt couplets) IMT: laminated mud- silt couplet gMT: graded mud- silt couplet Facies M (mud beds) lgM : laminated to graded mud gM: graded mud Facies MyS (muddy sand beds) mMyS : massi ve muddy sand gMyS: graded muddy sands Facies SyM (sandy mud beds) mSyM : massi ve sandy mud gSyM : graded sandy mud Facies MyG (muddy grave! beds) mMyG : massi ve muddy grave! gMyG : graded muddy grave! Facies GyM (gravelly mud beds) mGyM : massi ve gravelly mud gGyM : graded gravelly mud
(2) Beds characterized solely by plane-stratifìcation or cross-stratifìcation are defìned as plane-stratifìed and cross-stratifìed, respectively, and are grouped into the corresponding subfacies (e.g. sG, sGS, sGyS, sS ; xG, xGyS, xS). (3) Ali 'plane-stratifìed' (s) beds having cross-stratifìcation or parallel/cross-lamination at the top are defìned as plane-stratifìed to cross-stratifìed and plane-stratifìed to laminated, respectively, and grouped into the corresponding subfacies (e.g. sxGS, sxGyS, sxS ; slGS, slGyS, slS). (4) Ali beds that are unstratifìed and characterized solely by grading are defìned as graded and grouped into the corresponding subfacies (e.g. gG, gGS, gGyS, gS, etc.). (5) Ali 'graded' (g) beds having parallel lamination and/or cross-lamination at the top are defìned as graded to laminated and grouped into the corresponding subfacies (e.g. glGS, glGyS, glS, glSM, etc.). (6) Ali 'graded' beds having plane-stratifìcation or cross-stratifìcation a t the top are defìned as graded to plane-stratifìed and graded to cross-stratifìed, respectively, and grouped into the corresponding subfacies (e.g. gsG, gsGS, gsGyS, gsS; gxGS, gxGyS, gxS). (7) Ali 'graded to plane-stratifìed' (gs) beds having cross-stratifìcation a t the top are defìned as graded to plane-stratifìed to cross-stratifìed and grouped
431
Sediment gravity fio w deposits Table 3. Synopsis of the facies and subfacies. Lithology G
GS
mG gG gsG sG xG
GyS
s
mGyS
mS
SM
gGS gS gGyS gxGS gxGyS gxS g!GS g!GyS g!S gsGS gsGyS gsS gsxGS gsxGyS gsxS gs!GS gs!GyS gs!S sGyS sS sxGyS sxS s!GyS s!S xGyS xS
MS
gSM g!SM !SM
g!MS !MS
TM
MT
M
gTM
gMT
gM
!TM
!MT
SM
~
gS
Desc ription
gS
b
Fac ies d epo sition a l int e r va l s
GS
Subfacies : Description ·
g
S
Subfacies
lithol o g y
e c
SM giSM gbceSM
lith o logy
~ u
Facie s dep os iti o n a l i n t e rva l s
Sublac ies Description
GS gsGS g2gqsGS
1i t hol o g y
x
Fac ies
s
depositi o na l int e rv a l s
g,
G
i
Sublacies De scrrpti o n ·
GyS gsxG yS g1sxGyS
li t h o l ogy Fa c ies
g, d e pos i tiona l int e r va ls
MyG
GyM
mMyS
mSyM
mMyG
mGyM
gMyS
gSyM
gMyG
gGyM
xSM t gSM trSM
Faci es
g
SyM
lgM
li t holo gy
depositiona l interval s
MyS
Su bl acies D esc rrpt ion
G (in)gG (in)g 2g 1G
Fig. 4. Examples showirÌg how beds are described and classified according to the present scheme.
into the corresponding subfacies (e.g. gsxGS, gsxGyS, gsxS). (8) All 'graded to plane-stratified' beds with parallel lamination and/or cross-lamination at the top are defined as graded to plane-stratified to laminated and distinguished as the corresponding subfacies (e.g. gs!GS, gslGyS, gslS). (9) All beds characterized solely by parallel lamination and/or cross-lamination are defined as laminated and distinguished as the corresponding subfacies (e.g. lSM, lMS, lTM, IMT). For simplicity, some of the sedimentary features routinely observed in the field, such as the type of grading and clast fabric, are not included in the criteria for subfacies definition. Their combination with the other sedimentary characteristics would result in an excessively large number of subfacies. Moreover, bed attributes such as clast-fabric type or basai inverse grading are often laterally impersistent or variable, changing within the same bed even over relatively short distances (Surlyk, 1984 ; C. H . Eyles, 1987; Yagishita, 1989). Beds that differ merely by the presence or absence of basai inverse grading are considered to be varieties of the same subfacies (cf. Fig. 3). Subfacies varieties characterized by the presence of inverse or inverse to norma! grading in their basai parts are coded with the use of keys (i) an d (in) that precede the corresponding subfacies code (cf. Fig. 4). Similarly, the presence or absence of fluidescape structures is not considered sufficiently criticai to be used in the distinction of subfacies; such features are secondary, early or late post-depositional, typically superimposed on the pre-existing, primary sedimentary structures. A liquified bed division with com-
432
G. Ghibaudo
pletely obliterated primary structures is therefore not distinguished from either 'massi ve' (m) or 'graded' (g) bed divisions in this scheme (Fig. 4). If the primary structures are stili recognizable, only these are considered in the bed description (letter code) and subfacies designation. The presence of fluid-escape structures is, in such case, reported separately in the field. As has been mentioned, the number of possible subfacies is unlimited in the scheme adopted here. Figure 3 and Table 21ist some 55 subfacies representative of the entire spectrum of sediment gravity flow deposits. Other possible subfacies, not depicted in the present scheme (Fig. 3), can readily be distinguished in the fie id whenever necessary. Pickering et al. (1989, p. 23), for example, describe sand- mud couplets that show a basai plane-stratified division overlain by a graded division and capped with parallelfcrossJamination and a peli tic division. Those bipartite beds would be described as sgbceSM in the present scheme, but a separated new subfacies (sgiSM, plane-stratified to graded to Jaminated sand-mud couplets) might also be distinguished for descriptive and classification purposes. The same is true of other, relatively uncommon bed types, such as gsmGS (Aalto, 1989), sgGyS (Slaczka & Thompson, 1981), gxlSM (Hubert, 1966a, b; Thompson & Thomasson, 1969; Spalletti et al., 1989), xlSM (Thompson & Thomasson, 1969 ; Rocheleau & Lajoie, 1974; Mutti, 1979 ; Mutti & Normark, 1987), glMT (Stow & Piper, 1984; Anastasakis & P iper, 1991) an d m M (McCa ve & J ones, 1988 ; Jones & McCave, 1990). Moreover, some additional descriptive code-keys may be introduced to denote specific sedimentary structures and to distinguish new, Jess typica1 subfacies, if necessary (e.g. separate keys for Jow-angle cross-stratification, or antidune crossstratification).
DEPOSITIONAL PROCESSES The existing models for the transport and deposition of sediment from gravitational flows (e.g. Pickering et al., 1986, 1989) permit the depositional processes of individuai facies and subfacies to be interpreted in terms of: (a) laminar versus turbulent flow , (b) high versus low flow concentration, (c) the grain size and sorting of the transported load, (d) the rate of deposition, and (e) the actual mode of deposition (i .e. whether en masse, chiefly tractional, or tractivesuspensional). As pointed out by the recent discussions of the transport mechanisms and depositional processes of
sediment gravity flows (Walker, 1978; Lowe, 1982 ; Pickering et al., 1986; Stow, 1986 ; Pickering et al. , 1989; N emec, 1990; Einsele, 1991 ), the Jong-distance subaqueous transport of both fine and coarse material over a basi n floor may involve two principal processes: turbidity currents of various concentration and debris flows. ìiY\IM'L-flc~ The transport mechanism for coarse,Vgravelly materia! is, however, stili rather poorly understood. Theoretical calculations (Davies & Walker, 1974) suggest that some very fast (up to 10- 20 m s - t), highconcentration turbidity currents may be capable of carrying materia! up to cobble size in dispersion a bo ve the basin floor (see also Komar, 1970 ; Postma et al., 1988). The higher the sediment concentration in a turbidity current, the lower the ability of clasts to move freely relative to one another. The presence or absence of latera! or vertical clast-size segregation (norma! grading) thus depends mainly on the flow concentration and distance travelled (duration) by the flow before the actual deposition (Davies & Walker, 1974 ; Walker, 1975a, 1977). Flow transformation processes related to deceleration (Fisher, 1983, 1984; Postma et al., 1988) may cause the flow to separate into a basai, highly concentrated, non-turbulent gravelly portion (traction carpet, inertia-flow layer) and an upper, less concentrated, sandy or sandygravelly turbulent portion. In the highly concentrated basai layer, clasts are driven by the downslope componentof gravityand by the shearstress associated with the overlying turbulent suspension. The mecha- " nism of clast support in the basallayer is provided by the combined effect of dispersive pressure, enhanced buoyancy, and hindered settling due to the high grain concentration (Lowe, 1982 ; Pickering et al. , 1986; Postma et al., 1988). Because of the high dispersive pressure, inverse grading typically develops in the basai layer. The 'freezing' of such a layer, possibly followed by suspension deposition of finer grave!, produces an inverse or inverse to normally graded gravelly uni t at the base ofthe corresponding turbidite bed. Some relatively thick, mud-free (cohesionless), pebblefcobble gravelly units with inverse or inverse to normal grading are therefore often interpreted as the depositional products of some powerful high-concentration turbidity currents. Analogous beds with welldeveloped norma! grading indicate settling of turbulent suspension and suggest even more powerful, Jess concentrated flows. In such bipartite currents, moreover, the upper, less concentrated, sandy or sandygravelly turbulent portion is more mobile than the lower gravelly one and capable of bypassing the area
Sediment gravity fio w deposits
where the underlying coarse materia) is deposited (Lowe, 1982 ; Postma et al., 1988). This se1f-splitting process, when repeated by successive flows over a longer period of time, may lead to preferential downslope segregation of finer materia! and thus also facies (Lowe, 1982; Porebski, 1984). Accordingly, some downcurrent transitions from gravelly facies (G, GS, GyS) to sandy facies (S) may be expected. The transport mechanism for some thick to very thick beds of essentially structureless, mud-free, coarse polymodal grave) or gravelly sand is more speculative. Highly concentrated, non-turbulent flows, variously referred to as 'density-modified grain flows' (Lowe, 1976, 1982; Rodine & Johnson, 1976), 'sandy debris flows' (Hampton, 1975 ; Lowe, 1976, 1982; Naylor, 1980), 'combined grain flows/debris flows' (Hampton, 1979), or 'cohesionless debris flows' (Nemec & Steel, 1984; Postma, 1986; Nemec, 1990), have beeninvoked as a transport mechanism for such massi ve units (see also Surlyk, 1978, 1984; Nemec et al. , 1980; Cook & Mullins, 1983 ; Lash, 1984; Okada & Tandon, 1984; Porebski, 1984; Morris, 1987; Kleverlaan, 1989). In a laminar gravelly flow, the clasts at the base would move by sliding, rolling and bouncing, and the resulting basa! dispersive pressure might be sufficiently high to be transmitted upward (see discussion of the so called 'granular temperature' by Nemec, 1990) and to provide mobility to the overlying materia!, in combination with the effect of bouyancy and the lubricating effect of interstitial fluida! matrix. As Jittle as 1- 2 vol.% mud might be enough to renderthis Jatter effect important (see Hampton, 1975, 1979 ; Lowe, 1982). These flows would either travel and ' freeze' in a roughly steady/uniform fashion, or be subject to 'surface transformation' phenomena (Fisher, 1983, 1984), generating an overlying turbulent flow capable of carrying pebb1e grave! and sand (Hampton, 1972; Krause & Oldershaw, 1979; Lash, 1984). Asdiscussed ear1ier, the 'entrained' turbulent current might effectively bypass the area where the underlying laminar flow is deposited and continue for some distance as an
433
independent flow. The process of surface transformation of a cohesionless debris flow is, however, highly speculative and invoked mainly by analogy with cohesive flows (cf. Hampton, 1972). The mechanics of a cohesionless debris fio w, especially one dominated by coarse grave!, are stili rather poorly understood. The internai organization of sediment gravity flow beds is thought to be mainly the result of the final stage of transport and the depositional stage itself (see discussion by Postma, 1986). Three processes may be of particular importance during the deposition of a bed : (a) tractional grain-by-grain deposition from suspension, (b) grain-by-grain deposition from suspension with rapid burial of the clasts and no traction transport on the bed, and (c) en masse deposition by frictional or cohesive freezing (Lowe, 1982; Pickering et al., 1986). These processes are related principally to the sediment concentration in the fiow. High sediment concentrations result in high sedimentation rates, with en masse deposition of the bedload or rapid dumping of turbulent suspension-Joad that may effectively suppress near-bed turbulence and traction. Lower sediment concentrations are accompanied by lower sedimentation rates, with more effective tractional grain segregation and Jess intense suspension fall-out. The main transport processes inferred for the principal bed types, or facies, and their specific depositional divisions, are reviewed in Tables 4 & 5, respectively. A detailed interpretation of the depositional process for a particular bed can be established by summing the ' partial' processes that correspond to the bed's consecutive divisions (see Table 5), as the latter represent the successive depositional stages, or near-base dynamic conditions, ofthe waning sediment gravity flow. An example is given in Fig. 5, where the gravel- sand couplet, described as g 2 sbcGS, is attributed to a high-concentration turbidity current (cf. Table 4). Its sequence of depositional processes is interpreted as (cf. Table 5): rapid settling of grave! from turbulent suspension, such that norma) grading
Table 4. Inferred transport processes for particular facies and subfacies. Turbidity currents High concentration G GS GyS
s
Moderately high concentration g!SM glMS
Debris fiows Low concentration ISM IMS TM MT M
Cohesive
Cohesionless
MyS SyM MyG GyM
mG mGyS mS
Table 5. Depositional processes inferred for different bed divisions, or sedimentary features, that typify the deposits of high- and low-concentration turbidity currents.
DEPOSITIONAL LLI
>
Ui
high
v.t
c
::lE
A
Q LLI Q
c
a:
Cf)
t::l
m2
rapid ~ sedimentation by frictional freezing from a highly concentrated grave! clast dispersion.
m1 m
rapid ~ sedimentation by frictional freezing from a highly concentrated pebble-sand dispersion.
g2
rapid grain by grain suspension sedimentation from a highly concentra!ed grave! clast dispersion with or without development of a basai, inversely graded, traction carpe!.
g1 g
rapid grain by grain suspension sedimentation from a highly concentrated pebble-sand dispersion with or without development of a basai, inversely graded, traction carpet.
_J
~
a:
LU
l-
z --
Qo UJ
le
~ a: ~~ cr=c ~e: v.t
z o
--
_J
g (Lowe, 1982); g0 (Pickering et al., 1986, 1989); s 2 , s 1 (Davies & Walker, 1974; Walker, 1975a, b, 1977; Hein, 1982; Pickering et al. , 1986, 1989); s (Hiscott & Middleton, 1979, 1980); X z, x 1 (Winn & Dott, 1977, 1979; Hein, 1982) ; x (Allen, 1970, 1982 ; Mutti & Ricci Lucchi, 1975 ; Mutti, 1979; Pickering et al., 1986, 1989; Mutti & Normark, 1988 ; Stanley, 1988); b, c, c0 (Sanders, 1965 ; Walker, 1965); d, e 1 (Stow & Bowen, 1978, 1980; H esse & Chough, 1980); e 2 (Pickering et al., 1986, 1989).
435
Sediment gravity.flow deposits
GS gsl GS
g 2 s b c GS
Fig. 5. Interpretation of the transport and depositional processes fora particular subfacies (see text).
develops, but no significant tractional transport occurs (g 2); the development and 'freezing' of a series of consecutive, coarse sandy traction carpets (s); and a final stage of greatly reduced sedimentation rate, with traction plus fall-out deposition of fine sandy Ioad (b and c).
FACIES CHARACTERISTICS A large number of the facies and subfacies specified by the present classification (Fig. 3 & Table 2) are widely known and comprehensively described in the existing literature, and in previous reviews and classifications of deep-water sediment gravity flow deposits. Therefore, only the mai n facies are described
h ere, o n the basis ofthe author's own fie ld observations and relevant Jiterature. A useful cross-reference, linking the facies names proposed here with those used in the previous Jiterature, is given in Table 6. Subfacies are not described in detail here, but the most common subfacies are listed in Table 2 and shown in Fig. 3. For further descriptive details of specific subfacies, the reader is referred to Mutti & Ricci Lucchi (1972, 1975), Walker & Mutti (1973), Ricci Lucchi (1975a, b, 1984), Mutti (1977, 1979), W alker (1978), and particularly to the reviews by Pickering et al. (1986, 1989). ' Facies G: gravel beds These gravity flow beds typically consist of grave) that ranges from pebble to boulder grade, is clast supported and poorly to moderately well sorted. Bed thickness can be over lO m, and mud caps are usually absent. Individuai beds may be clast supported throughout, or have a clast-supported Iower part passing into a matrix-supported upper part. The beds are commonly amalgamated and their distinction within thicker sequences of this facies may be difficult. The grave) beds have an irregular geometry, due to either erosion or uneven depositional relief. At outcrop, the beds
Table 6. Comparison of the present classification for subaqueous sediment gravity ftow deposits and related classification schemes from the literature. Previous classifications This paper
Multi & Ricci Lucchi Mutti & Walker (1972) (1973)
Multi & Ricci Lucchi (1978)
Walker (1978)
G
A
A t> A z
A,
GS
A
Az
A,
GyS
A
A3, A4
A,
clast-supported conglomerate clast-supported conglomerate pebbly sandstone
s
A, B C, D, E C, D
B, , c Bz, C, D, E C, D
A 1 , 8 1, C 1 Bz, C z, D, , E Cz , Dz
massi ve sandstone classic turbidites classic turbidites
SM MS TM
MT M MyS SyM MyG
F
F
Az
GyM
F
F
Az
DJ matrix-supported conglomerate matrix-supported conglomerate
Pickering et al. (1986a, 1989) A 1 . 1 , A z., , A z.z, Az·3• Az.4 A z.z, Az·J• Az.4 A, .4, A z.s, A z·6• A z.,, Az.s B, .,, Bz.,, Bz.z, C z., B, .z, Cz.z, Cz.J Cz.z, Cz.J, Cz.4 Dz., , Dz.z, Dz·3• D, .,, D, .z Dz.,, Dz.z, Dz·3• D, .,, D, .z Ez., c, ., c, ., A, .z A, .J
436
G. Ghibaudo
most commonly have an irregular, lenticular geometry due to basai erosion, or, more rarely, a mound-like morphology due to the preservation of large, dunelike bedforms. Intemally, the grave! beds may be massive, 'structureless', but more frequently show a fairly well-developed norma! or inverse to norma! grading and often clast a-axes imbrication (cf. R 2 and R 3 divisions of Lowe, 1982). Less common sedimentary structures in coarse, clast-supported grave! beds are faint or well-developed, horizontal or subhorizontal parallel stratification and high-angle cross-stratification. The latter is rather rare, but the sets of gra velly cross-strata, when present, may be up to 4 m thick. The horizontal or subhorizontal stratification is usually better developed in the finer grave! in the upper or uppermost portions of some thicker, normally graded, clast-supported grave! beds. Individuai horizontal strata may be up to 50 cm thick and are marked mainly by an alternation of coarse and fine, or of clastand matrix-supported, pebbly grave! (see Davies & Walker, 1974). Individuai planar strata may be severa! tens of metres in latera! extent, and where the horizontal stratification is exceptionally thick, the strata may be difficult to distinguish from some thinner,"clast-supportedgravel beds. The latera! extent of facies G beds ranges from a few metres up to a few hundred metres, and they usually fili large erosional features and pinch out laterally by onlapping erosional surfaces. The commonest subfacies recognized in facies G beds are: massi ve (m G), graded (gG), plane-stratified (sG), graded to plane-stratified (gsG), and crossstratified (xG) beds. Selected references mG = massive gravels : Walker & Mutti (1973), Walker (1975a, 1978), Carter et al. (1978), Surlyk (1978, 1984), Johnson & Walker (1979), Hein (1982), Clifton (1984), Neefet al. (1985), Pickering et al. (1986, 1989), C. H . Eyles (1987), Kelling et al. (1987), Barnes (1988), Ineson (1989). gG=graded gravels: Aalto (1972), Walker & Mutti (1973), Davies & Walker (1974), Walker (1975a, 1977, 1978), Surlyk (1978, 1984), Johnson & Walker (1979), Hein (1982), Lowe (1982), Okada & Tamdon (1984), Pickering et al. (1986, 1989), C. H. Eyles (1987), Barnes (1988), Ineson (1989), Mutti & Normark (1991). sG = plane-stratified gravels: Walker & Mutti (1973), Davies & Walker (1974), Walker (1977), Eriksson (1982), Hein (1982), Pickering et al. (1986, 1989), Ito (1987), Ineson (1989). gsG = graded to plane-stratified gravels : Hendry (1972), Walker & Mutti (1973), Davies & Walker (1974), Walker (1977), Johnson & Walker (1979), Hein (1982), Massari (1984), Okada & Tamdon (1984), Surlyk (1984), Pickering et al. (1986, 1989), Ineson (1989).
gxG = graded to cross-stratified gravels: Hendry ( 1972), Rocheleau & Lajoie (1974), Walker (1978), Hein (1982, 1984), Surlyk (1984), Spalletti et al. (1989). xG=cross-stratified gravels: Winn & Dott (1977, 1979), Farquharson et al. (1984), Piper et al. (1985), Hughes Clark & Mayer (1986), Mal inverno & Ryan (1986), Pickering et al. (1986, 1989), Shor et al. (1986), Ineson (1989), Normark & Piper (1991).
Facies GS: gravel-sand couplets
Facies GS beds are couplets of clast-supported grave! overlain by sand. The lower division usually consists of pebbly, cobbly, or less commonly bouldery grave!, whereas the upper division is usually granule-bearing, very coarse to coarse sand. Their upward transition is typically rather sharp (abrupt grading), but more graduai transitions, with an intermediate zone of sand-supported grave!, are common as well. Beds are often amalgamated and mud caps are usually absent. The graveljsand thickness ratio is highly variable ( < l to > l). Beds with low thickness ratios usually have the lower, clast-supported portion confined to basai scours, up to severa! tens of metres wide and severa! decimetresdeep. Bedsoffacies GS are typically 0·5-3 m thick, but occasionally exceed IO m in thickness. The beds of this facies, like those of facies G , have an irregular, lenticular geometry with marked latera! thickness variations even at an outcrop scale. The lower boundaries are irregular and erosional, whereas the top surfaces are planar or slightly convex, except where subsequent erosion has produced an irregular relief. The latera! extent of these beds, however, tends to be somewhat greater than that of facies G beds. Large intraformational clasts, up to severa! metres in length, are common and tend to be concentrated in the lower, gravelly part of the beds or at their tops. The most common internai structure is crude norma! grading developed in both the gravelly and the sandy division of a bed. The gravelly division may locally show inverse grading at the base, passing upwards into crude norma! grading. Clast a-axes alignment and imbrication may also be present. Sedimentary structures that are less frequently developed in facies GS beds are plane-stratification and/or cross-stratification. The former is more common and may be developed in both the gravelly part and the sandy upper part of a bed, whereas crossstratification usually occurs in the sandy division only. Parallellamination and/or ripple cross-lamination is occasionally found at the top of either graded or planestratified GS beds. The beds of this facies, especially
Sediment gravity flow deposits
those with low grave!/ sand thickness ratios, may grade laterally into : (a) facies GyS (gravelly sand), due to a graduai 'ditfusion' of the gravelly division; or (b) to facies S (sand), as a result ofprogressive thinning and pinch-out ofthe gravelly division and a corresponding relative thickening of the sandy division. The commonest subfacies recognized are: graded (gGS), graded to cross-stratified (gxGS), graded to plane-stratified (gsGS), graded to laminated (g!GS), graded to plane-stratified to cross-stratified (gsxGS), and graded to plane-stratified to laminated (gs!GS) beds. Selected references gGS=graded gravel-sand couplets : Mutti (1969), Hubert et al. (1970), Hendry {1973), Walker {1975b), Surlyk {1978, 1984), Nemec et al. (1980), Mutti et al. (1981), Eriksson (1982), C1ifton {1984), Porebski {1984), Arnott & Hein {1986), C . H. Ey1es (1987), Morris & Busby-Spera (1988), Ineson (1989). giGS=graded to laminated gravel- sand couplets: Hendry (1972), Cook (1983), Carter et al. (1982). gxGS = graded to cross-stratified gravel-sand couplets: P i per {1970), Hendry {1978), Clifton {1984), Neef et al. {1985), Maejima ( 1988), Spalletti et al. ( 1989). gsGS = graded to p/ane-stratified gravel- sand coup/ets : Hendry (1978), Rocbe1eau & Lajoie (1974), Wa1ker {197;a, 1978), Jobnson & Walker (1979), Sagri (1980), Clifton (1984), Farqubarsonet al. {1984), Porebski (1984), C . H .Ey1es {1987), Bam es ( 1988). gsxGS = graded to plane-stratified to cross-stratified gravelsand couplets: Davies & Wa1ker {1974), Walker {1975a, b). gsiGS=graded to plane-stratified to /aminated gravel- sand couplets: Mutti (1969), Farqubarson et al. (1984).
Facies GyS: graveUy sand beds
Facies GyS beds consist of matrix-supported, pebbly or occasionally cobbly to bouldery grave!, grading upwards into very coarse/granule or coarse to medium sand. Pebbly sand (PyS) beds are particularly common. The upward transition from the matrix-supported gravelly division into the sandy division is usually gradational, and the bed as a whole shows a coarse-tail norma! grading. Graded gravelly sand beds with clasts dispersed throughout the bed thickness, or with a truly massive overall structure, are much less common. Basai inverse grading as well as clast a-axes imbrication may be present. As with the previous gravelly facies (G, GS), the beds of facies GyS are commonly amalgamated and usually lack mud caps. Bed thickness can be over 5 m and shows rapid
437
variation along depositional strike. The latera! extent of these beds, however, is greater than that of either facies G or GS, and many, especially those of the PyS variety, maintain a tabular geometry and uniform thickness over distances exceeding severa! hundred metres. The basai bed boundaries are always sharp, showing large scours or, more rarely, flute and groove casts. Large intraformational clasts are common, mostly concentrated in the lower parts of the beds. Elongate, lobate concentrations of coarser, clastsupported grave!, some metres in length but only up to a few clasts in thickness, are present at the base of some pebbly sand (PyS) beds. These concentrations of cobble-sized grave! possibly represent subaqueous 'sieve' deposits (Fitzgerald & Gorsline, 1989). As mentioned earlier, the beds ofthis facies tend to be well graded, but generally show no other sedimentary structures except for possi ble fluid-escape features in the upper, sandy portion of a bed. Internai scours, severa! centimetres deep and up to severa! metres wide, commonly !ined with aligned pebbles, occasionally occur in the basai or middle parts of finer grained (PyS) beds. Less common is plane-stratification or cross-stratification. These features tend to be better developed in the finer, PyS beds. The plane-stratification may be developed throughout a bed or in the bed's upper, sandy part only, and is defined by the alternation of pebble-rich and pebble-poor layers, or granule and mediumfcoarse sand layers (cf. s2 division of Lowe, 1982). Cross-stratification is less common, but a set of cross-strata may be severa! decimetres thick and comprise an entire bed, or be developed in the bed's sandy, upper part only. Minor parallel lamination and/or ripple cross-lamination may occur at the top of some beds. Facies GyS commonly passes laterally into facies S, due to a graduai decrease in the grave! content of a bed. The commonest subfacies are: massi ve (mGyS), cross-stratified (xGyS), graded (gGyS), graded to cross-stratified (gxGyS), graded to plane-stratified (gsGyS), graded to laminated (g!GyS), graded to plane-stratified to cross-stratified (gsxGyS), graded to plane-stratified to laminated (gs!GyS), plane-stratified (sGyS), plane-stratified to cross-stratified (sxGyS), and plane-stratified to laminated (s!GyS) beds.
Selected references mGyS=massive gravelly sands : Unrug (1963), Walker & Mutti (1973), Surlyk (1978, 1984), Aiello et al. (1977), Pickering et al. ( 1986, 1989), Ba m es ( 1988), C boe & Cbougb (1988).
438
G. Ghibaudo
gGyS = graded gravelly sands : Unrug (1963), Walker & Mutti (1973), Kelling & Holroyd (1978), Surlyk (1978, 1984), Aiello et al. (1977), N emec et al. (1980), Mutti et al. (1981 ), Strong & Walker (1981 ), H e in (1982), Busby-Spera (1985), Pickering et al. (1986, 1989), Watkins (1986), Postma et al. (1988), Mutti & Normark (1991). giGyS=graded to laminated gravelly sands : Aalto (1976), Forde et al. (1981), Slaczka & Thompson (1981), Hein & Walker (1982), Surlyk (1984), Postma et al. (1988). gxGyS = graded to cross-stratified gravel/y sands : Winn & Dott (1978), Eriksson (1982), Hein (1982), Pickering et al. (1986, 1989), Spalletti et al. (1989). gsGy S = graded to plane-stratified gravelly sands : Walker & Mutti (1973), Aalto (1976), Walker (1978), Eriksson (1982), Hein (1982), Hesse & Ogunyomi (1982), Surlyk (1984), Pickering et al. (1986, 1989), Watkins (1986), Postma et al. (1988), Ineson (1989). gsxGyS = graded to plane-stratified to cross-stratified gravelly sands: Rocheleau & Lajoie (1974). gsiGyS = graded to plane-stratified to laminated gravelly sands: Kelling & Holroyd (1978), Postma et al. (1988). sGyS=plane-stratified gravelly sands : Rocheleau & Lajoie (1974), Massari (1984), Surlyk (1984), Aiello et al. (1979), Surlyk & Hurst (1984), Busby-Spera (1985), Pickering et al. (1986, 1989). . sxGyS = plane-stratified to cross-stratifiedgravelly sands: Low e (1982). siGy S = plane-stratified to laminated gravelly sands: Hubert et al. (1970). xGy S =cross-stratified gravel/y sands : Hubert et al. (1970), Rocheleau & Lajoie (1974), Walker (1978), Hendry (1978), Johnson & Walker (1979), Eriksson (1982), Hein (1982), Hein &. Walker (1982).
Facies S: sand beds
Facies S comprises coarse sand beds 0·5- 10 m thick that usually lack mud caps. The beds typically show a relatively well-developed norma! grading. Coarse-tail grading predominates, but many beds show coarsetail grading in their lower parts and distribution grading in the upper parts. Beds where the grading is limited to the bed top only (top grading) are also common. The bed grain size ranges from medium/ coarse or granule sand a t the base, possibly with small pebbles along the basai surface, to fine/medium sand at the top. Despite the relatively large number of 'structureless' examples reported in the literature, true massive sand beds are probably not very common. Top grading, in particular, is a common feature of many 'massive' sand beds. The beds of this facies, where superimposed, are usually amalgamated due to the absence of muddy interbeds. Minor parallel lamination and/or ripple cross-lamination, the latter often developed as a solitary ripple-train ' form set',
may occur at the top of a bed. Muddy intraclasts, up to a few decimetres long, are common. The sand beds of this facies represent Bouma's T(A), T( AB), T(ABC) and T(AC) turbidites. They are considered here, however, as one separate facies because of the generai absence or scarcity of traction plus fall-out lamination a t the tops ofthe thick, graded basai intervals of the beds, an d because of the generai absence of mud caps. A relatively common feature of facies S beds is the presence of horizontal or subhorizontal stratification (Hiscott & Middleton, 1979, 1980), developed either throughout a bed or in a part of the bed (commonly upper part) only. The strata are from l to 20 cm in thickness and tend to be thinner and finer grained upwards. They often pass gradationally upwards into horizontal lamination, especially of the coarse type 1,J. The horizontal strata tend to be inversely graded, less commonly normally graded, and have a latera! extent of severa! metres. In some beds, the planestratified divisions alternate with massive divisions, the latter commonly with fluid-escape structures. Cross-stratification is less common, but may be developed, as either planar or trough variety, at the tops of both graded and plane-stratified beds, or may comprise an entire bed. Water-escape structures are common in facies S beds and are represented by both dish structures and flat 'consolidation laminae' (Lowe & Lo Piccolo, 1974; Lowe, 1975). Dish and associated pillar structures tend to be developed in the upper parts of the beds and may completely obliterate the sand's primary structure. Flat consolidation laminae are often superimposed upon the primary horizontal stratification and tend to be developed along the permeability barriers corresponding to grain-size changes at the stratification surfaces. The horizontal stratification in facies S beds, if crudely developed, may be indistinct and difficult to recognize, especially in weathered outcrops ; in such cases it is often indicated by the presence of regularly spaced (5- 20 cm apart), flat consolidation laminae. Facies S beds ha ve a relatively large latera! extent, ranging from hundreds of metres to severa! kilometres, except for the cross-stratified beds (subfacies xS), whose latera! continuity rarely exceeds tens or hundreds of metres. The beds typically pinch out over distances of only a few metres. The lower bed boundaries are erosional and may be irregular, flat with erosional steps, or slightly concave upwards. The upper surfaces are flat, slightly convex upwards, undulatory over longer distances, or may be irregular
Sediment gravity fio w deposits
due to subsequent erosion. Mud clasts, both isolated and concentrated, are common. Some beds contain intraformational slabs, up to severa! metres in length, of deformed, thin-bedded turbidites. Sole marks are common, although often poorly developed due to bed amalgamation; they are largely tool marks, frondescent marks and large flute casts. The commonest subfacies are: massi ve (mS), crossstratified (xS), graded (gS), graded to cross-stratified (gxS), graded to plane-stratified (gsS), graded to laminated (g!S), graded to plane-stratified to crossstratified (gsxS), graded to plane-stratified to laminated (gs!S), plane-stratified (sS), plane-stratified to cross-stratified (sxS), and plane-stratified to laminated (s!S) beds. Selected references mS= massive sands: Stauffer (1967), Stan1ey (1969, 1980), Stan1ey & Unrug (1972), Sur1yk (1978, 1984, 1987), Wa1ker ( 1978), Jordan (1981), Hurst & Surlyk (l 982, 1983), Pickering et al. (1986, 1989), Barnes (1988), Wuellner & James (1989). gS=graded sands : Unrug (1963), Ghibaudo & Mutti (1973), Walker & Mutti (1973), Carter & Lindqvist (1975), Wa1ker (1978), Sta nley et al. (1978), Hiscott (1980), Hiscott & Middleton (l 980), Stan1ey (l 980), Hein (1982), Spalletti et al. (1989). giS=graded to lamina/ed sands: Carter & Lindqvist (1977), Link (l 975), Mutti & Ricci Lucchi (l 975), Ricci Lucchi (1975a), Hirayama & Nakajima (1977), Piper et al. (1978), Mutti (1979), Ghibaudo (1980), Stanley (1980), Advocate et al. (1988). gxS = graded to cross-stratified sands: Unrug (l 963), Normark & P iper (1969), Piper (l 970), Piper et al. (l 978), Hiscott (l 980), Hiscott & Middleton (l 979, 1980), Slaczka & Thompson (1981 ), Smith (l 987), Morris & Bus by-Spera ( 1988), Spa lletti et al. ( 1989). gsS=graded to plane-stratified sands: Hiscott & Middelton ( 1979), H e in ( 1982), Maejima ( 1988), Kleverlaan ( 1989). gsxS = graded to plane-stratified to cross-stratified sands : Slaczka & Thompson ( 1981 ), Morris & Bus by-Spera ( 1988). gsiS = graded to plane-stratified to lamina ted sands : Stanley et al. (l 978), Slaczka & Thompson (1981 ). sS = plane-stratified sands: Mutti & Ricci Lucchi ( 1972, 1975), Ricci Lucchi (1975a, b), Aiello et al. (1977), Hiscott & Middleton (1979, 1980), Mutti (1979), Mutti et al. (1981), Pickering et al. (l 986, 1989). sxS = plane-stratified to cross-stratified sands : Van V1iet (l 978), Co1ella (1979), Mutti et al. (l 985), Mutti & Normark (1991). siS = plane-stratified to laminated sands: Ricci Lucchi ( 1975a, 1981 ), Mutti & Ricci Lucchi (1972, 1975), Mutti (1979), Pickering et al. ( 1986, 1989). xS=cross-stratified sands: Hubert et al. (1970), Hess & Normark (1976), Pickering (1982a), Hein (1982), Link et al.
439
(1984), Va1entine et al. (1984), Pickering et al. (1986, 1989), Kemp (l 987), Stan1ey (l 988).
Facies SM: sand-mud couplets Facies SM and related facies MS (see next section) are bipartite beds that compri se a lower sandy division and an upper muddy division. The two facies are distinguished by the difference in their sand/mud thickness ratios. Facies SM beds have a sandfmud thickness ratio > l. 8oth SM and MS couplets can be considered in terms of complete and either top- or base-cut'-out turbidites of Bouma (1962), although there are beds characterized by internai structures (e.g. cross-stratification) that differ from the classica! mode!. Facies SM beds with complete Bouma sequences are 0·5-3 m thick and characterized by sharp, erosional, planar or slightly irregular lower boundaries, with possible !oca! amalgamation where directly superimposed upon one another. Various sole marks are present. The upper surfaces are commonly planar. The beds typically ha ve a tabular, sheet-like geometry and a large latera! continuity of up to severa! kilometres. Bed grain size ranges from medium or very coarsefgranule sand, locally with small pebbles at the base, to fine sand at the top. Beds have a welldeveloped norma! grading and typically consist of a lower graded interval (Bouma's division A) overlain by plane-parallellamination and ripple cross-lamination (Bouma's divisions 8 and C), followed by an upper silt/mud interval (Bouma's divisions D and E). Di vision D of silty parallellaminae is often difficult to recognize in the field and, in most cases, cannot be differentiated from the overlying muddy division. The graded basai division in facies SM beds is usually thicker than, or of about the same thickness as, the overlying laminated divisions and commonly contains scattered or clustered, flow-aligned intraformational mudclasts, up to a few decimetres in length. Scattered mudclasts occur mainly in the lower, graded interval, with aligned clasts more common at its top. Where the aligned mudclasts are abundant, a distinct, continuous layer rich in intraclasts, or 'slurried' division, is sandwiched between the graded lower division and the laminated upper division (Ricci Lucchi, 1965, 197Sb, 1981, 1984; Kruit et al., 1975 ; Van Vliet, 1978; Mutti et al., 1978; Mutti & Nilsen, 1981 ; Postma et al., 1988). Bipartite SM beds analogous to Bouma's (1962) topcut-out turbidites often consist of a thick to very thick, graded sand unit overlain sharply by a considerably
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G. Ghibaudo
much thinner, turbiditic mud cap. These couplets can be regarded as Bouma's T(AE) turbidites. Facies SM beds corresponding to Bouma's basecut-out turbidites are 3- 50 cm thick and consist of a lower, fully laminated, normally graded, fine to medium sand division gradationally or sharply overlain by a thinner mud cap. In terms of the Boumatype sequence, these beds can be described as T(BCDE), T(CDE), T(BCE), T(CE), T(BDE) or T(BE) turbidites. The beds typically have a sheet-like geometry and are bounded by even, parallel surfaces over distances of hundreds of metres to severa! kilometres. The lower surface may be smooth or have small scour and tool marks and various trace fossils, whereas the upper surface is often slightly or densely bioturbated. Parting lineation in the parallel-laminated division and !oca! convolute structures are characteristic of many of these beds. Facies SM also includes sandy units 5- 100 cm thick with high-angle cross-stratifi.cation, overlain by thinner mud caps. The sandy di visions are typically coarse to very coarse grained and commonly have sharp bases and tops. The latera! extent of these beds ranges from a few metres to a few hundred metres, and the beds generally have a broadly lenticular geometry with flat bases and undulatory or convex-upward tops. Discontinuous beds, with wavy or pinch-and-swell geometry, are also common. Internally, the beds show crude or well-developed, high-angle cross,stratification usually developed and/or preserved as a single dune-type cross-set. The cross-stratifi.ed sand- mud couplets occur as discrete beds, but can often be traced laterally into thinner sand- mud couplets showing either ripple form-sets or simple grading (see later). These beds seem to have formed on the sea floor as 'patches' of coarse sandy tractive lag, either originally deposited as such or reworked into these patches by subsequent tractional processes (Mutti, 1979; Mutti & Normark, 1987). This interpretation is supported by the presence of irregularly spaced, 1- 5 cm thick, mud interlaminae in some of the beds, suggesting a compound origin, with multiple tractional reactivation of the bedforms and intervening stages of muddy suspension settling (Obrador et al., 1978 ; unpublished data from the Gres d'Annot Formation). Facies SM also includes: (a) thin- to mediumbedded (3-20 cm), coarse to very coarse/granule sandmud couplets characterized by poor grading, sharp bases and tops, and discontinuous bedding (facies E pro parte of Mutti & Ricci Lucchi, 1975); and (b) thin ( < 5 cm), laterally discontinuous, relatively coarsegrained sand-mud couplets with ripple form-set
features (facies E pro parte of Mutti & Ricci Lucchi, 1975 ; facies E of Mutti, 1979). The former are regarded as a variety of subfacies gSM and classifi.ed (coded) as thin-bedded, graded SM couplets (tgSM). The latter are considered to be a special case of subfacies ISM (laminated SM couplets ; see below) and are classifi.ed as thin-bedded, rippled SM couplets (trSM). The most common subfacies recognized in facies SM are : graded (gSM), graded to laminated (g!SM), laminated (ISM), cross-stratified (xSM), thin-bedded, graded (tgSM), and thin-bedded, rippled (trSM) beds. Selected references gSM=graded sand- mud couplets : Walker & Mutti (1973), Cleary & Connolly (1974), Walker (1979), Mutti et al. (1981 ). giSM = graded to laminated sand- mud couplets: Bourna (1962), Walker (1965, 1967, 1978, 1979), Mutti & Ricchi Lucchi (1975), Walker & Mutti (1973), Ricci Lucchi (1975a, b, 1984), Mutti (1979), Pickering et al. (1986, 1989). ISM=Iaminated sand- mud couplets : Mutti & Ricci Lucchi (1975), Walker & Mutti (1973), Ricci Lucchi (1975a, b, 1981 , 1984), Mutti (1977, 1979), Pickering et al. (1986, 1989). xSM = cross-stratifìed sand- mud couplets : Mutti & Ricci Lucchi (1975), Mutti (1979), Colella (1979), Gokçen & Kelling (1983), Mutti & Normark (1987), Spalletti et al. (1989). tgSM = thin-bedded, graded sand- mud couplets: Walker (1967), Mutti & Ricchi Lucchi (1972), Ghibaudo & Mutti (1973), Walker & Mutti (1973), Wright & Wilson (1984), Pickering et al. (1986, 1989), Thomburg & Kulm (1987a, b), Mutti & Normark (1991). trSM = thin-bedded, rippled sand- mud couplets: Mutti ( 1979), Mutti & Normark (1991).
Facies MS: mud-sand couplets
Facies MS comprises Bouma's (1962) classic turbidites characterized by sand/mud thickness ratios < l. The geometry and internai structures of facies MS beds are similar to those in facies SM beds, with complete or base-cut-out Bouma-type sequences. The two facies differ principally in their sand/mud thickness ratio, butto some extent also in other characteristics. Facies MS beds tend to show greater variability in thickness, somewhat greater internai complexity, and greater latera! continuity. MS couplets with complete Bouma-type sequences are 1- 20m in thickness, have a tabular geometry and are laterally extensi ve (up to severa! tens of kilometres, downcurrent, before changing into a base-cut-out MS couplet). The basai grain size ranges from medium to very coarse/granule sand and most of the beds show well-developed, distribution norma! grading. The
Sediment gravity fio w deposits
mean sand/mud thickness ratio in these beds is from l : l to l :2. Siliciclastic MS couplets characterized by complete Bouma-type sequences tend to be thinner and have higher sand/mud ratios than calcareous or hybrid couplets. Internally, the beds usually have a graded lowerdivision that is thinnerthan the overlying parallel- and/or cross-Iaminated division, and some of the very thick MS beds show climbing dune-type cross-stratification developed between the Iaminated turbidite divisions B and C. Wavy or convolute Iamination and vertical fluid-escape structures are common. MS couplets with base-cut-out Bouma-type sequences have even, parallel boundaries and are also Iaterally extensive (from severa) kilometres for the thinner beds to severa! tens or hundreds of kilometres for the thicker ones). The beds consist of medium/fine to very fine sand at the base; their lower, sandy division is typically laminated throughout and considerably thinner than the overlying mud division. The sandy division comprises both parallellamination and ripple-drift cross-lamination, as in the classic turbi dite sequence, and most of the beds can be reguded as Bouma-type T(B- E), T(C- E), T(BCE), or T(CE) turbidites. The sand/mud thickness ratio in these beds is more variable than in the corresponding, laminated SM couplets, usually between l : l and l :4, but ratios of l : lO or more are not uncommon. Bed thicknesses themselves are also highly variable, ranging from lO cm to 16m. Siliciclastic base-cut-out MS couplets tend to be thinner than calcareous or hybrid couplets. Facies MS also includes some very thick, fully laminated beds characterized by great latera) extent (tens of kilometres) and internai evidence ofrepetitive flow reversals during deposition (cf. facies C2.4 of Pickering et al., 1986). Internai structures include dune-form cross-strata sets, ripple and ripple-drift cross-lamination with opposite flow directions, and wavy or flat parallellamination. Mud films between divisions with opposi te flow directions are also typical. These beds are interpreted to represent deposition from large-volume turbidity currents with multiple reftections or deflections of the flow by the marginai slopes in relatively small basins (cf. Pickering & Hiscott, 1985 ; Pantin & Leeder, 1987; Marjanac, 1990; Porebski et al., 1991). The commonest subfacies are: graded to laminated (g!MS) and Iaminated (IMS) beds. Selected references giMS=graded to laminated mud-sand couplets: Sagri (1971), Ricci Lucchi & Pialli (1973), Debroas et al. (1983).
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IMS=Iaminated mud-sand couplets : Mutti & Ricci Lucchi (1972, 1975), Ne1son & Ku1m (1973), Mutti (1977, 1979), Ricci Lucchi (1981), Ricci Lucchi & Va1mori (1980), Pickering & Hiscott (1985), Pickering et al. (1986, 1989).
Facies TM and MT: silt-mud and mud-silt couplets Facies TM and MT are bipartite beds comprised of a silty lower division and a muddy upper division. The two facies are thus lithologically similar couplets and are distinguished mainly by their different silt/mud thickness ratios : facies TM has a thicker siit division (thickness ratio >l), whereas facies MT has a thicker mud division (ratio
