Possible earthquake-induced soft-sediment faulting ...

Possible earthquake-induced soft-sediment faulting and remobilization in Pennsylvanian alluvial strata, southern New Brunswick, Canada A. G. PLINT' Department of Geology, University of New Brunswick, Box 4400, Fredericton, N.B., Canada E3B 5A3

Received October 16, 1984

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by 199.201.121.12 on 06/05/13 For personal use only.

Revision accepted January 17, 1985 Braided river, sheetflood, and playa lake sediments of the Tynemouth Creek Formation (Lower Pennsylvanian) show superficial synsedimentary faulting and subsurface post-depositional sediment remobilization. A prominent palaeosol has been offset 90 cm on two faults, movement on which clearly preceded deposition of the overlying beds. Seventeen to 25 m below this horizon, large load structures, upward-branching siltstone intrusions, and three types of sandstone dike are recognised. Intrusion occurred on several occasions at different depths of burial and was the result of rapid, probably earthquake-induced dewatering. This interpretation is supported by the location of the study area, only a few kilometres to the north of the Cobequid-Chedabucto Fault, on which major strike-slip movement occurred during the Pennsylvanian. Les sCdiments de rivikre anastomosCe, de ruissellement en nappe et de sebkha de la formation de Tynemouth Creek (Pennsylvanien infkrieur) sont coupCs en surface par des failles synskdimentaires et montrent en sous-surface un remaniement postskdimentaire. Un palCosol bien caractCrisC a CtC dCcalC sur deux failles de 90 cm, le mouvement a nettement prCcCdC la sedimentation des couches sous-jacentes. On observe entre 17 et 25 rn sous cet horizon d'importantes structures de charge, des filons de siltstone se ramifiant vers le haut et Cgalement la prksence de trois types de filons de grks. La mise en place de ces filons s'est produite en plusieurs Ctapes et a des profondeurs diffkrentes durant I'enfouissement des stdiments, et elle rksulte d'un asskchement rapide provoquC possiblement par un tremblement de terre. Le fait que la rkgion CtudiCe est situCe a seulement quelques kilomktres au nord de la faille de Cobequid-Chedabucto, laquelle fut responsable d'un dkcrochement majeur durant le Pennsylvanien, rend cette interprktation d'autant plus valable. [Traduit par le journal] Can. I. Earth Sci. 22, 907-912 (1985)

Introduction The purpose of this paper is to describe some possible earthquake-induced sedimentary structures including a prominent synsedimentary fault and various siltstone and sandstone intrusions that occur in the Tynemouth Creek Formation (Lower Pennsylvanian, Westphalian A-B), which is exposed

BAY

of

on the south coast of New Brunswick, about 30 km east of Saint John (Fig. 1). The formation is an upward-coarsening clastic sequence, at least 700 rn thick, deposited on an alluvial fan and associated piedmont. The sediments were derived from a southeasterly source area that consisted of Upper Precambrian (Coldbrook Group) igneous rocks and Lower Carboniferous

FUNDY

Pennsylvanian/? Mississippian

FIG.1 . Location of study area. 'Present address: Department of Geology, McMaster University, Hamilton, Ont., Canada L8S 4M1

CAN. 1. EARTH SCI. VOL. 22. 1985 Depositional Environment

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by 199.201.121.12 on 06/05/13 For personal use only.

,,

M

bg

Post - Depositional Sedimentary Structures

,---------I Playa lake & ? Sheetflood

Large-scale loading

Type 3 sandstone dikes

? Sheetflood

Locally highly disrupted by silt intrusion Local, larae-scale loading

- D

Tvoe 3 sandstone dikes

L

C

? Sheetflood

'R

Local silt intrusion

\ \ \ \ -

Crossbedding

Silt

intrus~on

FIG.2. Detailed stratigraphy of part of the coastal section exposed 300 m east of Tynemouth Creek, showing the distribution of postdepositional sedimentary structures.

(Mississippian) limestones and sandstones (Plint and van de Poll 1982; Plint et al. 1983). The upward coarsening of the Tynemouth Creek Formation suggests increasing proximality of the source terrain, probably as a result of synsedimentary, northwest-directed thrusting. Continued thrusting in postWestphalian ?B -C time emplaced this source terrain upon the Tynemouth Creek Formation itself (Plint and van de Poll 1984). Thrusting was contemporaneous with major dextral

strike-slip movement on the Cobequid-Chedabucto Fault (Wilson 1962; Webb 1969; Keppie 1982), which lies only a few kilometres to the south of the study area. In this tectonic setting, it is likely that major earthquakes would be common. The strata described in this paper are exposed on the Bay of Fundy coast, 300 m east of Tynemouth Creek (Fig. 1). The general stratigraphy of this coastal section is illustrated in Fig. 18 of Plint and van de Poll (1982), 65-95 m above the base of their section 2. Part of this interval is shown in more detail in Fig. 2. The rocks in this interval consist mainly of metre-thick fine- to medium-grained red silty sandstones, which may be structureless or crossbedded. Thin, centimetre-scale, laterally discontinuous layers of structureless red siltstone are interbedded with the sandstones, at the base of which complex burrow systems (Paleophycus) are locally seen. These sandstones are interbedded with four units of laminated, very finegrained red sandstone and siltstone, 15- 100 cm thick, which locally show small-scale ripple cross-lamination and convolute lamination. The lower three units also contain thin (1 -2 cm) beds of pale grey-green sandy limestone containing abundant ostracods and gastropods. This fine-grained facies is uncommon in the Tynemouth Creek Formation and is interpreted as a playa lake deposit (cf. Picard and High 1972; Steel and Aasheim 1978). The sandstone units with which the lacustrine sediments are interbedded are interpreted as braided river and sheetflood deposits (Plint and van de Poll 1982). Dewatering and sediment remobilization structures are best developed at the base of the thickest lacustrine unit (Fig. 2, unit F) and in the sandstone (unit E) immediately below. The structures are present throughout the exposure of this horizon, for about 250 m laterally. Twenty-one metres above this horizon occurs a distinctive palaeosol (laterally exposed for about 70 m), which has been offset by up to 90 cm on two faults, movement on which obviously occurred prior to the deposition of the overlying beds. Faulting took place in a subaerial environment and may provide supportive evidence for seismic activity contemporaneous with sedimentation.

Synsedimentary faulting A distinctive bioturbated and rooted sandstone, which weathers out to form a prominent "pavement," lies 21 m above the lacustrine sediments of unit F. A detailed field sketch of this horizon is given in Fig. 3. The bioturbated sandstone is 10 cm thick and gradationally overlies a unit of crossbedded, coarsegrained pebbly sandstone and conglomerate that is interpreted as the fill of a braided river channel; the bioturbated sandstone cap is interpreted as a palaeosol. In the eastem portion of the exposure, the palaeosol is strongly bleached, intensely bioturbated, and contains stigmarian roots. Towards the west, the intensity of bioturbation diminishes and the surface locally shows straight-crested, symmetrical wave ripple marks. Near the western limit of the exposure, the palaeosol bears a poorly preserved trail (Diplichnites cuithiensis) produced by the giant myriapod Arthropleura (cf. Briggs et al. 1984). The palaeosol and underlying beds are cut by a prominent fault that offsets the surface by up to 90 cm. Minor displacement has occurred on an adjacent smaller fault, 1.5 m to the west, and the intervening block has been heavily fissured (Fig. 4). The palaeosol forms a prominent lip at the top of the fault scarp (Fig. 4) that is directly analogous to that seen on modem fault scarps generated during earthquakes (see Fuller 1912; McCulloch 1968, Figs. 30,31; Eiby 1980, PI. 28). The bleaching and intense bioturbation of the palaeosol on

1

1 I

~

1

?

0 rn WEST

10

metrrrs

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by 199.201.121.12 on 06/05/13 For personal use only.

bedded fins sand

I Tree stump

I Synsedimentary faults

Intensely bioturboted palaeosol

I Coarse pebbly sand

I Stigrnarian roots

I

-

Erosive based channel

n ~3. .Field sketch section showing the relative position of the synsedimentary faults shown in Fig. 4 and associated sedimentary features. The section is located about 250 m east of Tynemouth Creek.

RG.4. Tabular-weathering palaeosol (P) broken by faults (arrowed) that clearly pre-date deposition of the overlying, thin-bedded sandstones. The main fault trends N60° and dips west at 27'. Note prominent lip (L) at top of fault scarp. Rucksack is 1 m high. the eastern, upthrown side of the fault suggest prolonged subaerial exposure, weathering, and biological modification. In contrast, the palaeosol is relatively poorly developed on the downthrown side, where ripple marks suggest that only the downthrown side was flooded immediately after the disturbance.

The palaeosol is everywhere mantled by 1-2 cm of red siltstone, which is overlain by a 2.8 m thick sequence of interbedded red and grey siltstones and current-rippled, finegrained sandstones, the undersurfaces of which show small load, flute, and groove casts, together with rare reptilian footprints. Several beds on the downthrown side either thin or die

910

CAN. J. EARTH

SCI.VOL. 22,

19x5

TABLEI. Sediment remobilization structures

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by 199.201.121.12 on 06/05/13 For personal use only.

Sedimentary structure

Description

Interpretation

Relative age

Siltstone intrusions

Upward-branching irregular layers (Figs. 5, 6) a few millimetres to few centimetres thick that cross-cut up to 1 m of strata. Common in sandstones overlying siltstone beds

Do not resemble flame structures or root casts. Probably formed during upward expulsion of silt-rich pore water (see Mayall 1979)

Early post-depositional to syndepositional

Sandstone dikes ( ~ Y 1) F

Dikes extend upward from unit E to unit F and locally cross-cut silt intrusions. Dikes are up to 20 cm high, 15 cm wide, and have a complex branched pattern in plan (Fig. 7). Dikes consist of medium-grained sandstone containing angular siltstone fragments. "Pipes" of structureless sandstone may occur beneath dikes

Intruded in very shallow subsurface, as there is no evidence for sand extrusion at a sediment surface (see Johnson 1977; Hesse and Reading 1978). Most clastic dikes are straight (e.g., Diller 1889; Newsome 1903; Hayashi 1966; Winslow 1983) and branched pattern is unusual (see Case 1895; Marschalko 1965; Dott 1966; Truswell 1972) and may reflect intrusion into very soft sediment

Early post- or syndepositional

Sandstone dikes ( ~ Y 2) F

Confined to upper part of unit G;2- 10 mm thick, irregular branching sheets of fine-mediumgrained silt-free sandstone in a silty sandstone matrix (Fig. 8)

Result from pore-water expulsion, localized fluidization. and elutriation of the host sandstone

Early post-depositional

Sandstone dikes (type 3)

1-15 mm thick, subvertical planar sheets of fine-medium-grained sandstone that cross-cut beds for over 1 m. Dikes are subparallel and show little compactional deformation

Intruded later than types 1 and 2 and after host siltstones had been largely compacted. Consistent orientation suggests intrusion was influenced by an early (tectonic?) stress field (see Harms 1965; Marschalko 1965; Peterson 1968; Winslow 1983)

Relatively late postdepositional

Load structures

Pervasively deformed sandstone beds up to 70 cm thick that laterally form isolated "pillows"

Result from localized liquefaction and overturn of sediments with reversed density stratification

Early post- or syndepositional

FIG. 6. Complex anastomosing and bed-transgressive pattern of silt intrusion in which silt laminae enclose thin slivers of the host sandstone.

nc. 5 , silt intrusion (arrowed) in unit E showing well developed upward-branching pattern. Very similar structures have been described by Mayall (1979), who tentatively attributed them to earthquake-associated fluidization.

out towards or against the fault scarp. These rocks were deposited under both shallow standing water and, as indicated by the vertebrate footprints, periodic subaerial conditions in a low-lying channel-margin setting that was subject to intermittent influxes of flood-borne sediment. Post-faulting deposition initially occurred only on the downthrown side but, with time, the depression was filled and deposition occurred evenly on both sides. The fault underwent no further slippage after the initial disturbance, possibly as a result of strain hardening (see Pickering 1983).

1

I

1

I I

I I I

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by 199.201.121.12 on 06/05/13 For personal use only.

I

+8dding

plam, lop surface+

FIG.7. Plan view of a part of the top surface of unit E, showing the sinuous paths and tri-radiate branching pattern of type 1 sandstone dikes. Note the presence of two cylindrical sandstone "pipes," which may represent dewatering conduits.

Sediment remobilization A variety of bed-transgressive sediment "intrusions" of both sand and silt are present 20-25 m below the faulted bed. These structures are briefly described and interpreted in Table 1 and Figs. 5-8. Discussion Preservation potential of fault scarps The preservation potential of a subaerial, synsedimentary fault scarp is very low and depends upon a fortuitous combination of environmental conditions. Such features will therefore be rare in the geological record and, to the author's knowledge, only a few such examples have been described (e.g., Sieh 1978) although examples in subaqueous sediments have been well documented (e.g., Nagtegaal 1963; Seilacher 1969; Van Loon and Wiggers 1976; Mayall 1983; Pickering 1983). Generation of the sharp, well defined fault scarp depended upon an appropriate sediment-water content (Goodman and Appuhn 1966) and an earthquake of sufficient magnitude and proximity. Given these conditions, the preservation of the example described here may be attributed to several factors such as the presence of a vegetation cover that stabilized the superficial sediments, inhibited erosion after faulting, and provided an easily recognizable, bioturbated soil marker horizon. Episodic yet relatively low-energy, flood-related deposition was sufficiently rapid to bury the fault scarp before it suffered significant degradation. Although the sediments indicate deposition in shallow water, periods of subaerial exposure may be inferred from the vertebrate and myriapod footprints. It is therefore surprising that rainfall and weathering did not cause more extensive degradation of the fault scarp. This may be due to (i) low rainfall (although the sediments contain no evidence of desiccation), (ii) effective sediment binding by plant roots, (iii) very rapid burial of the exposed fault scarp, and (iv) shelter provided by a vegetation canopy. Of these factors, (iii) and (iv) may have been the most important. Dewatering structures in relation to sedimentary environment Although lacustrine sediments are an important component

FIG. 8. Type 2 sandstone dikes in the upper part of unit G. The dikes consist of silt-free, fine- to medium-grained sandstone generated through the elutriation of the host sediment. Note the frequent changes in thickness and orientation of the dikes.

of the sequence illustrated in Fig. 2, they are rare in the Tynemouth Creek Formation as a whole. Similarly, examples of sediment remobilization related to dewatering are scarce outside this sequence. The playa lake sediments were deposited in

CAN. J .

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by 199.201.121.12 on 06/05/13 For personal use only.

9 12

EARTH

a relatively low-lying, poorly drained area in which the groundwater table lay close to o r above ground level and the sediments were saturated for long periods of time. These conditions rendered the sediments particularly susceptible to remobilization through liquefaction and water escape. Comparison of the dewatering structures with both recent and ancient analogues suggests that they formed in response to seismic shocks. In view of the tectonic setting of the area, close to a major transcurrent fault, it is perhaps surprising that seismically induced structures are not more common in the Tynemouth Creek Formation. This apparent anomaly may reflect the relatively good drainage of the alluvial sediments, which reduced their susceptibility to liquefaction, rather than a lack of triggering earthquakes. Only in poorly drained areas such as playa lakes did the sediments contain sufficient pore water to undergo substantial liquefaction as a result of seismic shocks.

Acknowledgments This paper was prepared during tenure of a Natural Environment Research Council/North Atlantic Treaty Organization (NERC/NATO) postdoctoral fellowship, and this support is gratefully acknowledged. The manuscript was greatly improved by the constructive criticism of Murray D'Orsay, Martin Gibling, Mike Mayall, Ron Pickerill, Kevin Pickering, and Roger Walker. I thank Jack Whonvood for preparing the photographs and the Ewing family of Tynemouth Creek for their hospitality during fieldwork. R. K. 1984. ArthroBRIGG~, D. E. G., PLINT,A. G., and PICKERILL, pleura trails from the Westphalian of eastern Canada. Palaeontology, 27, pp. 843-855. CASE, E. C. 1895. On the mud and sand dikes of the White River Miocene. American Geologist, 15, pp. 248 - 254. DILLER,J. S. 1889. Sandstone dikes. Bulletin of the Geological Society of America, 1, pp. 41 1-442. DOIT, R. H. 1966. Cohesion and flow phenomena in clastic intrusions (abstract). American Association of Petroleum Geologists Bulletin, 50, pp. 610-61 1. EIBY,G. A. 1980. Earthquakes. Van Nostrand Reinhold Co., New York, NY, 209 p. FULLER,M. L. 1912. The New Madrid earthquake. United States Geological Survey, Bulletin 494, 119 p. GOODMAN, R. E., and APPUHN, R. A. 1966. Model experiments on the earthquake response of soil-filled basins. Geological Society of America Bulletin, 77, pp. 1315- 1326. HARMS,J. C. 1965. Sandstone dikes in relation to Laramide faults and stress distribution in the southern Front Range, Colorado. Geological Society of America Bulletin, 76, pp.98 1- 1001. HAYASHI, T. 1966. Clastic dikes in Japan. Japanese Journal of Geology and Geography, 37, pp. 1-20. HESSE,R., and READING,H. G. 1978. Subaqueous clastic fissure eruptions and other examples of sedimentary transposition in the lacustrine Horton Bluff Formation (Mississippian) Nova Scotia, Canada. In Modern and ancient lake sediments. Edited by A. Matter and M. E. Tucker. International Association of Sedimentologists, Special Publication 2, pp. 241 -257. JOHNSON, H. D. 1977. Sedimentation and water-escape structures in some Late Precambrian shallow marine sandstones from Finnmark, north Norway. Sedimentology, 24, pp. 389-41 1. KEPPIE,J. D. 1982. The Minas Geofracture. In Major structural zones and faults of the northern A~~alachians. Edited bv P. St-Julien and J. Beland. Geological ~ssodiationof Canada, special Paper 24, pp. 263 -280.

SCI. VOL. 22.

1985

MARSCHALKO, R. 1965. Clastic dikes and their relations to synsedimentary movements (flysch of central Carpathians). GeologickC price, 36, pp. 139-148. MAYALL, M. J. 1979. Sedimentology of the Rhaetic (Upper Triassic) in S. W. Britain. Ph.D. thesis, University of Reading, Reading, England. 1983. An earthquake origin for syn-sedimentary deformation of a late Triassic (Rhaetian) lagoonal sequence, southwest Britain. Geological Magazine, 120, pp. 613 -622. MCCULLOCH, D. S. 1968. Slide-induced waves, seiching and ground fracturing caused by the earthquake of March 27. 1964 at Kenai Lake, ~ L s k aunited . States ~ e b l o ~ i cSurvey, al Professional Paper 543A. NAGTEGAAL, P. J. C. 1963. Convolute lamination, metadepositional ruptures and slumping in an exposure near Pobla de Segur (Spain). Geologie en mijnbouw, 42, pp. 363-374. NEWSOME,J. F. 1903. Clastic dikes. Bulletin of the Geological Societv of America. 14. DD.227-268. PETERSON,G. L. 1968. Flo;v'structures in sandstone dikes. Sedimentary Geology, 2, pp.177-190. PICARD,M. D., and HIGH,L. R. 1972. Criteria for recognising lacustrine rocks. In Recognition of ancient sedimentary environments. Edited by J. K. Rigby and W. K. Hamblin. Society of Economic Paleontologists and Mineralogists, Special Publication 16, pp. 108- 145. PICKERING, K. T. 1983. Small-scale, syn-sedimentary faults in the Upper Jurassic 'Boulder Beds'. Scottish Journal of Geology, 19, pp. 169-181. PLINT, A. G., and VAN DE POLL, H. W. 1982. Alluvial fan and piedmont sedimentation in the Tynemouth Creek Formation (Lower Pennsylvanian) of southern New Brunswick. Maritime Sediments and Atlantic Geology, 18, pp. 104- 128. 1984. Structural and sedimentary history of the Quaco Head area, southern New Brunswick. Canadian Journal of Earth Sciences, 21, pp. 753-761. PLINT,A. G., RYAN,R. J., and VAN DE POLL, H. W. 1983. The distribution, biota and stratigraphy of a Windsor Group limestone (Mississippian) and associated sediments in the Quaco Head area of New Brunswick. Maritime Sediments and Atlantic Geology, 19, pp. 107-115. SEILACHER, A. 1969. Fault-graded beds interpreted as seismites. Sedimentology, 13, pp. 155- 159. SIEH,K. E. 1978. Prehistoric large earthquakes produced by slip on the San Andreas Fault at Pallett Creek, California. Journal of Geophysical Research, 83, pp. 3907 -3939. STEEL,R. J., and AASHEIM,S. M. 1978. Alluvial sand deposition in a rapidly subsiding basin (Devonian, Norway). In Fluvial sedimentology. Edited by A. D. Miall. Canadian Society of Petroleum Geologists, Memoir 5, pp. 385 -412. TRUSWELL, J. F. 1972. Sandstone sheets and related intrusions from Coffee Bay, Transkei, South Africa. Journal of Sedimentary Petrology, 42, pp. 578-583. VAN LOON, A. J., and WIGGERS,A. J. 1976. Metasedimentary 'graben' and associated structures in the lagoonal Almere Member (Groningen Formation, the Netherlands). Sedimentary Geology, 16, pp. 237-254. WEBB,G. W. 1969. Palaeozoic wrench faults in the Canadian Appalachians. American Association of Petroleum Geologists, Memoir 12, pp. 754-786. WILSON,J. T. 1962. Cabot fault, an Appalachian equivalent of the San Andreas and Great Glen faults and some implications for continental displacement. Nature (London), 195, pp. 135- 138. WINSLOW,M. A. 1983. Clastic dike swarms and the structural evolution of the foreland fold and thrust belt of the southern Andes. Geological Society of America Bulletin, 94, pp. 1073-1080.