SOUTHERN APPALACHIANS l. GANAPATHY SHANMUGAM 2 AND GEORGE L. BENEDICT III. Department of Geological Sciences,. University of Tennessee,.
JOURNAL OF SEDIMENTARYPETI~OLOGY,VOL. 48, No. 4, r. 1233-1240 FIGS. 1-7, DECEMBER,1978 Copyright © 1978, The Society of Economic Paleontologists and Mineralogists
FINE-GRAINED CARBONATE DEBRIS FLOW, ORDOVICIAN BASIN MARGIN, SOUTHERN APPALACHIANS l GANAPATHY SHANMUGAM2 ANDGEORGE L. BENEDICT III Department of GeologicalSciences, University of Tennessee, Knoxville, Tennessee 37916 AnsrR.~cr: This paper documents for the first time, an occurrence of a specific type of sediment gravity flow caUedfine-grained debris flow (Hampton, 1975). Fine-graineddebris flows are those in which maximum grain size does not exceed that of pebbles (64 mm). The pebbly-lime mudstones of the Whitesburg Formation (Middle Ordovician) at Nina in east Tennessee have been interpreted as fine-grained carbonate debris-flow deposits based on evidence for (1) laminar and plug flow transport, (2) deposition by 'freezing', and (3) flow strength provided by muddy matrix. Planar-clast fabric with inverse grading, fragile fossils and shale clasts, projected clasts on bedding surfaces, and lack of scour marks imply laminar-flow transport. Massive internal structures, random orientation of clasts, and poor sorting are suggestive of plug-flow transport. Floating of large clasts in a muddy matrix (unsupported framework) implies flow strength as well as deposition by 'freezing'. Multiple episodes of debris flows occurred locally in a basin-margin sedimentary setting. Speculations are that these episodic debris flows correspond with tectonic pulses associated with basin subsidence.
INTRODUCTION
Debris flow is one of the major sediment transport mechanisms in submarine environments. Recognition of debris-flow deposits in the sedimentary record requires evidence for (1) laminar a n d / o r plug flow transport, and (2) deposition by 'freezing' (Johnson, 1970; Fisher, 1971; Hampton, 1972, 1975; Middleton and Hampton, 1976; Enos, 1977). Hampton (1972, 1975) speculated the possibility of fine-grained debris flows in submarine environments. In fine-grained debris flows, maximum grain size does not exceed that of pebbles (64 mm), as opposed to blocks of several meters in diameter found in normal debris flows. Hampton (1975) pointed out that there are no reported occurrences of fine-grained debris flow in modern or ancient marine environments. In this paper, we document one such fine-grained debris flow in a Middle Ordovician basin margin in the Southern Appalachians. GEOLOG1C SETTING
A complex mosaic of carbonate and terfigenous clastic sequences constitute the Midt Manuscript received March 1, 1978; revised May 2, 1978. 2Mobil Field Research Laboratory, P.O. Box 900, Dallas, TX 75221.
die Ordovician of Tennessee in the Valley and Ridge Province of the Southern Appalachians. These sequences have been studied cartier by Rodgers (1953), and Neuman (1955). The ecostratigraphy of the Middle Ordovician of Tennessee has been studied by Walker (1977). He has suggested a facies pattern composed of three pans: (1) a western shelf with a shelf edge skeletal sand b a n k / r e e f tract; (2) a deep-water basin southeast of the shelf; (3) a shallow-water, near shore environment southeast of the basin (Walker, 1977, p. 16). In this study, attention is focused on the western shelf to basinal sequences exposed at Nina (Fig. 1). A measured stratigraphic section at Nina is shown in Figure 2. These sequences are bounded by the Dumphn Valley Fault to the northwest and the Great Smoky Fault to the southeast. Major folding and minor faulting of these sequences are common in this area. The Whitesburg Formation, which is the subject of this paper, has been interpreted as slope deposits at other locahties (Benedict, 1977; Shanmugam, 1977; Walker 1977; Shanmugam and Walker 1978; Benedict and Shanmugam, in preparation). Graptofites collected from the Whitesburg Formation at Nina suggest that these deposits belong to the lower half of the Nemagraptus gracilis
1234
GA NA P A T H Y SHA N M UGA M A ND GE01~ GE L. B E N E D I C T III
_ ~ z ~ '*v
:::::::::.:.~: _ .,._~.~.3.:i:.i!i!~i!i!.~:!i~!i!~% "!i!i!~i!~!:i!:'r:':::.:.:,.. ~ ~ , ~
~I ~ ~' ~
""
OAIVORIDSE •
.......
:~v~
:". Y AREA"::
,
/'-~("
Ft~. l . - - L o c a t i o n map of study area at Nina. G.S.F. = Great Smoky Fault.
Zone. The age of these deposits may range from Upper Llandeilo to Lower Caradoc (S. Finney, pers. com., January 10, 1978). Pebbly-lime mudstone and clay shale are the two characteristic lithologies in the Whitesburg at Nina. Clast size in the pebblylime mudstones ranges from I mm to 64 ram. Most clasts are angular, but some are rounded. Ninety percent of the clasts are inferred to be of deep-water origin based on the presence of pelagic microfossils, and ten percent of unknown origin (some of which were probably derived from the western shelf). Phosphatic peloids are common in the clasts, however, terrigenous grains are absent. Graptolites are abundant both in the pebbly-lime mudstones and in the intervening shales. Shallow-water benthic megafossils and algae are completely absent. A definite mechanism by which the deepwater mudstone clasts were incorporated in the pebbly-lime mudstones is not clear. However, muds deposited and being lithified near the base of the slope (Toe of slope of FonuAr~ol~
saw, "~0
--
.....
$IVItR
s.~t
~
._
7MaN 3 0 %
~RAPrOLIrtS.
CLAIr~
Wilson, 1974), could be transported downslope by local mass movement and redeposited in a nearby basin-margin setting. This interpretation would explain the interbedding of pebbly-lime mudstones with pelagic basinal shales, and the presence of phosphatic peloids in the clasts. Stratigraphically, the Whitesburg Formation is "sandwiched" between underlying shelf deposits (Lenoir Limestone) and overlying basinal deposits (Blockhouse and Sevier Shales). By applying Walther's "law", the sequence at Nina (Fig. 2) can be visualized in a lateral framework, where the Whitesburg Formation would occupy the basin margin/slope environment bounded by shelf on one side (west) and basin on the other. The basinal shales at Nina are composed of pelagites (Blockhouse Formation), and distal turbidites (Sevier Formation) similar to equivalent basinal shales exposed at Boyds Creek (Shanmugam, 1977; Shanmugam and Walker, 1977a, 1977b, 1978; Stow and Shanmugam, in preparation). 40 km west of Nina, in the same strike belt. SEDIMENTARY FEATURES
Bedding
In outcrops, the Whitesburg Formation is composed of a characteristic sequence of alternating pebbly-lime mudstone and shale beds (Fig. 3A). The pebbly-lime mudstones are light gray in weathered surface and very dark gray in fresh broken surfaces. Bed thickness of these pebbly-lime mudstones ranges from 3-4 cm. The beds are laterally continuous and persistent in thickness through the entire outcrop width of nearly 30 meters. The intervening yellowish gray shale beds vary in thickness from 1 to 3 meters.
~LA0~¢-
S*Lr~ S.A~E
Internal L a m i n a t i o n s ~
ut¢popol~s
.
~
,
so
. . . . . . . . . . .
SI~ILF
FIc. 2,--Stratigraphic section at Nina showing characteristic features of individual formation and lithofacies interpretation.
Commonly the pebbly-lime mudstones are internally structureless (massive), but occasionally they show internal laminations (Fig. 3B). Internal laminations are characteristically present in the lower, finer grained zones (Fig. 5B). These laminations are generally accompanied by thin horizons of pyrite up to 1 cm long and 2-3 mm thick. In polished slabs, these internal laminations are discontinuous and end abruptly. A few of the
FINE- G R A I N E D D E B R I S FLO W
1235
served on the bedding surface (Fig. 5A) are also quite friable in nature.
Projected Clasts One of the most outstanding primary sedimentary features in the pebbly-lime mudstones is the projected clasts on the bedding surface (Fig. 5B). This feature is readily seen even on the outcrops, and is accentuated by weathering of the overlying shales. Almost all beds studied show this phenomenon. Some clasts project 1 cm above the bedding surface, but most project less than 1 cm. Commonly the projected clasts are "floating" on the bedding surface (Fig. 5B 1). Some projected clasts are also buried up to 5 mm below the bedding surface (Fig. 5B2). Shallow (1-2 mm) burial of clasts is quite common.
FIG. 3.--(A) Field photograph showing a thin pebblylime mudstone bed alternating with shales. (B) A polished slab showing a vague internal lamination near the center of bed. Notice the parallel alignment of elongate clasts to bedding surface (also Fig. 4A).
laminations resulted mainly from the accumulation of a thin organic layer. Occasional internal micro-graded laminations have been observed. Hence the origin of the laminations can be attributed to change in composition as well as grain size. Internal laminations of the shales are not uncommon.
Fabric In all beds studied, most elongate clasts are aligned parallel to bedding surface i.e. planar fabric (Fig. 4A). A few inclined clasts of random orientation are present.
Inverse Grading Inverse grading is one of the characteristic features of the pebbly-lime mudstones (Fig. 5B). Invariably, the maximum grain size at the bottom of the bed is 0.04 mm (4.68 0). Maximum grain size near the upper surface of the bed ranges from 1 cm ( - 3 . 3 •) to 4 cm ( - 5 . 3 ~1). A plot of vertical variation in maximum grain size in fourteen samples (Fig. 6) shows a drastic increase in maximum grain size near the upper surface of the beds. Gradual upward increase in grain size within a bed is also common. Poor Sorting Poor sorting of the pebbly-lime mudstones is obvious as they contain clasts of varying size (up to 4 cm in diameter) in a muddy matrix (Fig. 5A). No point count was made for size analysis as some areas in the pebblylime mudstones underwent a phase of recrystallization of carbonate mud which resuited in a mosaic of interlocking microspar and relict mud.
Fragile Fossils and Shale Clasts
EVIDENCE FOR DEBRIS FLOW
Graptolites have been well preserved in the pebbly-lime mudstones. Microscopic investigation reveals the three dimensional nature of graptolites as well as their delicate ornamentation (Fig. 4B). Three dimensional preservation of fragile graptolltes is rare in the sedimentary record. Shale clasts pre-
Transport in debris flows occurs under laminar and plug flow conditions. Deposition from debris flows occurs by 'freezing' when internal shear stress no longer exceeds the total yield strength of debris. In Johnson's (1965, 1970) Coloumb-viscous model it can be expressed as:
1236
A
GA N A P A T H Y SHA N M U G A M A N D GEOR GE L. B E N E D I C T I I I
3
4
Ft~. 4.--(A) A close-upview of polished slab in Figure 3B, showingplanar fabric. Scale is in cm. (B) Preservation of graptolite, Glyptograptus sp. on bedding surface with delicate ornamentation in three dimensions.
T < S,
(1)
where T is internal shear stress and S is total yield strength. Total yield strength S is obtained by adding cohesion of material c, and intergranular friction o-n tan 4, Hence, S = c + ~o tan dp,
(2)
where c~n is internal normal stress, ¢b is the angle of internal friction. Thus, evidence for debris flow origin of ancient deposits must come from criteria for (1) laminar a n d / o r plug flow transport, (2) deposition by 'freezing', and (3) flow strength provided by muddy matrix. Parallel alignment of elongate fragments to bedding surfaces (planar fabric) has been observed in debris flows (Fisher, 1971). Preservation of planar fabric in a muddy matrix probably suggests laminar flow conditions. However, such fabric could also be produced by migration of a rigid plug within the debris
flow (Hampton, 1975, p. 843). As mentioned earlier, planar fabric of clasts (Fig. 4A) is well preserved in the pebbly-time mudstones of the Whitesburg Formation indicating either laminar or plug flow transport. Fisher (1971) attributes inverse grading associated with planar fabric debris flow deposits to laminar flow conditions. It has been shown that inverse grading is a characteristic feature of the pebbly-lime mudstones (Fig. 5B). Inverse grading commonly occurs at the base of turbidite beds. However, absence of features characteristic of turbidite beds (Bouma, 1962) in these pebbly-time mudstones precludes deposition by turbidity currents. Preservation of fragile fossils such as graptofites with deficate ornamentation in three dimension and friable shale clasts implies non-turbulent transport. Complete absence of scour marks in these deposits also implies lack of turbulence in the flow. Such
1237
FINE- GRA I N E D D E B R I S FLO W
negative evidence for turbtilent flow m a y be considered as an indication of laminar flow. Presence o f fragile graptofites and shale clasts might also be due to plug flow transport, where shearing is absent. H a m p t o n (1972) notes in debris flows 'rock fragments projecting from the top of natural flows were not tossed about but were gently rafted both in the area o f the rigid plug and in the zone o f flow.' The presence o f proj e c t e d clasts on the bedding surfaces (Fig. 5B) is one o f the most outstanding sedimentary features o b s e r v e d in the pebbly-time mudstones. These p r o j e c t e d clasts suggest either laminar or plug flow transport. Massive internal structure, random orientation of clasts, and p o o r sorting of these pebbly-lime mudstones are suggestive of plug flow transport. Concentration o f coarser grains near the upper bedding surface (Fig. 5B, upper part) also indicates plug flow transport, whereas presence o f internal laminations associated with fine grains (Fig. 5B, lower part) reflects shearing in the lower zones o f flow. Thus, some o f the pebbly-lime mudstones have p r e s e r v e d zones of rigid plug as well as zones of shearing. W h e n the debris flow ' f r e e z e s ' the fluid phase may be partially p r e s e r v e d as matrix mud (Enos, 1977, p. 140). Hence the presence o f mud is a critical factor in documenting ancient debris flows. The m u d d y nature of the pebbly-time mudstones in the Whitesburg
)
Fro. 5.--(A) Preservation of shale clasts on a bedding surface. Poor sorting is seen by the presence of large clasts in a muddy matrix. (B) Photograph of a thin section showing projected clasts and inverse grading. 1. A "floating" clast on the bedding surface. 2. A projected clast buried up to 5 mm below the bedding surface. Notice internal laminations in the lower part.
0
I
I
40
I
i
I
ZO
0 40
20
0 40
ZO
i
I
0 40
20
I
0
I
4 0 20
0
I
40
I
I
20
0 40
20
0 40
I
I
I
20
0 40
20
0
20
0
20
0
¢O
0
I
40
I
I
I
20
|
0
40
I
I
20
0 40
I
20
0 40
I
I
l
40
MAXIMUM GRAIN SIZE iN MM
Flc. 6.--Plots showing vertical increase of maximum grain size in 14 samples. Maximum grain size was measured on faces of slab samples cut perpendicular to bedding,
1238
GA NAPA THY SHA NMUGAM A N D GEORGE L. BENEDICT II1
Formation imphes deposition by 'freezing' of debris flow. Features such as inverse grading, and floating of large clasts in a muddy matrix observed in the pebbly-lime mudstones indicate an unsupported framework. Such a framework develops when a debris flow with high density, high viscosity, and strength 'freezes' in place. An unsupported framework also rules out suspension settling of particles, and hence a Newtonian flow. In summary, the criteria discussed here support (1) laminar as well as plug flow transport, (2) deposition by 'freezing', and (3) strength of flow provided by muddy matrix, all implying debris-flow emplacement of the pebbly-time mudstones in the Whitesburg Formation. Sedimentary features indicative of debris flow are shown in Figure 7. GEOLOGIC IMPLICATIONS
Our extensive search for similar debrisflow deposits of the Whitesburg Formation in east Tennessee has been in vain. This could mean that these deposits are of local occurrence. This is also supported by small bed thickness (3-4 cm) which impfies short flow distances. Small clast sizes (less than 64 ram) in these deposits suggest that the strength of the flow was not sufficient enough to support larger clasts. Because the pebblylime mudstone beds overlie shales without any indications of erosion, low velocity flow is implied. The tectonic significance of these deposits is of special interest as they apparently were formed at a time when the basin was
PROJECTED CLAST SHALE CLAST WITH PELAQIC MICROFOBII,_S SRAPTOLITE WITH DELICATE ONNANtNTATIOM BURIED & PROJECTED CLAST PLANAR FABRIC &
FLOAT NG CLAST
]/ INVERSE
ORAOIN¢ II
POOR SORTING
--
o
MUDDY MATRIX / NON-OR OSIONAL CONTACT URDI[RLYIR@ SHALl[
FIG. 7,--An idealized pebbly-lime mudstone bed of the Whitesburg Formation at Nina with sedimentary features indicative of debris flow origin.
experiencing a high rate of tectonic subsidence (Shanmugam, 1978). The deep-water basin margin/slope sedimentary setting, e.g., the Whitesburg Formation, provides an ideal condition for debris flows (Stanley, 1973). The repeated occurrence of debris flow deposits at Nina implies multiple episodes. We speculate that these episodic debris flows probably correspond with tectonic pulses associated with basin subsidence. CONCLUSIONS
1. In the pebbly-lime mudstones of the Whitesburg Formation, planar fabric with inverse grading, fragile fossils and shale clasts, projected clasts, and lack of scour marks imply laminar-flow transport. 2. Massive internal structures, random orientation of clasts, and poor sorting imply plug-flow transport. 3. Floating of large clasts in a muddy matrix (unsupported framework) implies flow strength as well as deposition by 'freezing'. 4. The above criteria indicate a debris-flow origin of the pebbly-lime mudstones. 5. As the clasts in the pebbly-time mudstones do not exceed the pebble size range of 64 mm ( - 6 I~), they are qualified to be fine-grained debris flow. To our knowledge, this is the first reported occurrence o f finegrained carbonate debris flow to date. ACKNOWLEDGEMENTS
We wish to thank Kenneth R. Walker for his supervision of this work. We thank Jean Lajoie for critically reviewing the manuscript. This research was supported by two National Science Foundation Grants (# DES 72-01611 and EAR 76-11808) awarded to Walker; by the following grants awarded to Shanmugam: a grant from Chvrou Oil Field Research Company through the Geological Society of America (76-77); a Penrose Grant from the Geological Society of America (7778); two Sigma Xi Grant-in-aid of Research (76-78); and by support from the Don Jones Research Fund (Univ. of Tennessee) awarded to both of us. Stan Finney of Memorial University of Newfoundland identified the graptolites. Field assistance provided by the senior author's wife, Jean, is greatly appreciated. This paper is an outgrowth of an ongo-
FINE- GRAINED
ing project on the shelf to basinal sequences of the Ordovician in Tennessee under the direction of K. R. Walker. REFERENCES BENEDICT, G. L. 11I, 1977, Outer Shelf margin-upper slope transition zone in the Lenoir and Whitesburg Formations: in Ruppel, S. C., and K. R. Walker, (eds), The ecostratigraphy of the Middle Ordovician of the Southern Appalachians (Kentucky, Tennessee, and Virginia), U.S.A.: A Field Excursion: Studies in Geology, Number 77-1, University of Tennessee, Dept. of Geol. Sciences, p. 96-101. BOUMA, A. H., 1962, Sedimentology of some flysch deposits, Elsevier Pub. Co., Amsterdam, 168 p. ENos, P., 1977, Flow regimes in debris flow: Sedimentology, v. 24, p. 133-142. FisnEtt, R. V., 1971, Features of coarse-grained, highconcentration fluids and their deposits: Jour. Sed. Petrology, v. 41, p. 916-927. H~uvroN, M. A., 1972, The role of subaqueous debris flow in generating turbidity currents: Jour. Sed. Petrology, v. 42, p. 775-793. , 1975, Competence of fine-grained debris flows: Jour. Sed. Petrology, v. 45, p. 834-844. JOHNSON, A. M., 1965, A model for debris flow: unpub. Ph.D. Thesis. Pennsylvania State University, University Park, 205 p. , 1970, Physical Processes in Geology, Freeman, Cooper and Co., San Francisco, 577 p. MtDDLErON, G. V., AND M. A. HAMPTON, 1976, Subaqueous sediment transport and deposition by sediment gravity flows: in Stanley, D. J., and D. J. P. Swift, (eds), Marine Sediment Transport and Environmental Management: John Wiley & Sons, Inc., New York, p. 197-218. NEUMAN, R. B., 1955, Middle Ordovician rocks of the Tellico-Sevier belt eastern Tennessee: U.S. Geol. Survey Prof. Paper 274-F, p. 141-177. RODCERS, J., 1953, Geologic map of east Tennessee with
DEBRIS FLO W
1239
explanatory text: Tenn. Div. Geology Bull., v. 58, Part II, 168 p. S~Ar~MUGAM, G., 1977, Shelf to basinal sequences at Boyds Creek, Tennessee: in Ruppel, S. C. and K. R. Walker, (eds), The ecostratigraphy of the Middle Ordovician of the Southern Appalachians (Kentucky, Tennessee, and Virginia), U.S.A. : A Field Excursion: Studies in Geology, Number 77-1, University of Tennessee, Dept. of Geol. Sciences, p. 106 115. , ANDK. R. WALKER, 1977a, Small scale sedimentary structures in the Middle Ordovician Sevier Shale, Eastern Tennessee (abs.): Geol. Soc. America Abstracts with Programs, v. 9, p. 183-184. - - , AND--, 1977b, Deep-water sedimentation and tectonics in the Middle Ordovieian Sevier Shale, Tennessee (abs.): Am. Assoc. Petroleum Geologists Bull., v. 61, p. 828-829. - - , 1978, Subsidence analysis of shelf and basinal sequences of the Middle Ordovician in east Tennessee (abs.): Geol. Soc. America Abstracts with Programs, v. 10, p. 197. - - , ANOK. R. WALKER, 1978, Tectonic significance of distal turbidites in the Middle Ordovician Blockhouse and lower Sevier Formations in East Tennessee: Am. Jour. Science, v. 278, p. 551-578. STANLEY, D. J., 1973, Sedimentation in slope and baseof-slope environments: in Stanley, D. J. (ed), The New Concepts of Continental Margin Sedimentation, Short Course Lecture Notes, Second Printing: American Geological Institute, Washington, D.C., P. DJS 8.1-DJS 8.25. WALKER, K. R., 1977, A brief introduction to the ecostratigraphy of the Middle Ordovician of Tennessee (Southern Appalachians, U.S.A.): in Ruppel, S. C., and K. R. Walker, (edx), The ecostratigraphy of the Middle Ordovician of the Southern Appalachians (Kentucky, Tennessee, and Virginia), U S . A . : A Field Excursion: Studies in Geology, Number 77-1, University of Tennessee, Dept. of Geol. Sciences, p. 12-17. WILSON, J. L., 1974, Characteristics of carbonate-platform margins: Am. Assoc. Petroleum Geologists Bul., v. 58, p. 810-824.
