Storms versus tsunamis: Dynamic interplay of sedimentary, diagenetic, and tectonic processes in the Cambrian of Montana Brian R. Pratt*
Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada
ABSTRACT The Upper Cambrian Deadwood Formation of north-central Montana is composed mainly of fine-grained calcareous sandstone and intercalated silty shale, and contains scattered intraclastic flatpebble conglomerate. These rocks, typical of many mixed carbonatesiliciclastic, subtidal shelf successions of Proterozoic through Early Ordovician age, are distal storm deposits in which the conglomerates are conventionally thought to record occasional, very high energy events that scoured incipiently cemented layers. In the Deadwood Formation, most sandstone beds have a linear to crudely reticulate pattern of vertical cracks filled with injected silt and clay. The cracks exhibit a consistent northwest-southeast trend, and are very shallow burial deformation structures caused by the passage of seismic waves possibly originating from strong earthquakes in Idaho. Most conglomerates are composed of angular to subangular sandstone intraclasts, the polygonal plan view of which is similar in outline to the areas between cracks in the sandstones. The deformation implied by the cracks, sporadic distribution of conglomerates, high degree of scouring indicated, and angularity of the intraclasts argue that, rather than storms, the conglomerates were generated by occasional tsunamis. The sweep of these tsunamis across the shallow intracratonic sea created extraordinarily strong oscillating bottom currents at the deeper reaches of storm wave base. ‘‘Tsunamites’’ are therefore identified with confidence virtually for the first time in shallow subtidal shelf deposits.
seismic activity can be envisaged. However, almost no examples have been identified beyond the level of suspicion (e.g., Kazmierczak and Goldring, 1978; Duringer, 1984). Nevertheless, many successions exhibit anomalous facies that defy simple bathymetric interpretation (e.g., Molina et al., 1997), and for these a tsunami origin may be realistic. The purpose of this paper is to document briefly the facies preserved in a low-latitude, intracratonic, storm-dominated, shallow-subtidal succession of Late Cambrian age, and to make a case for the occasional but distinctive effect of tsunamis, specifically in generating the flatpebble conglomerate beds. This interpretation should provoke a reconsideration of other units in which the role of tsunamis may have been overlooked. FACIES ASSOCIATION The Deadwood Formation is 250–300 m thick and, although recessive, is locally exposed in the flanks of the Little Rocky Mountains of north-central Montana (where it has been called the Emerson Formation; Knechtel, 1959). The unit spans the late Middle Cambrian to earliest Ordovician (Lochman, 1950; Lochman and Duncan, 1950; Kurtz, 1976), and was deposited in the vast, tropical epeiric sea that covered much of Laurentia (Fig. 1; Lochman-Balk, 1971; Hein and Nowlan, 1998). The lowest interval, the Wolsey Member, consists of
Keywords: storms, earthquakes, tsunami, facies, flat-pebble conglomerates, Cambrian, Precambrian, Proterozoic, Montana. INTRODUCTION The intimate interbedding of higher energy facies with shales is regarded as the classic signature of deposition below fairweather wave base but within storm wave base (e.g., Ager, 1974; Aigner, 1985; Myrow and Southard, 1996; Ferna´ndez-Lo´pez, 1997). Fine-grained hemipelagic and weak storm-influenced sedimentation was punctuated by major storms that are recorded by wave-ripple cross-laminated sandstone, peloidal lime mudstone, or bioclastic packstone from episodic reworking. Interrupting such successions, especially those of Proterozoic and Cambrian to Early Ordovician age, are intraclastic rudstones (flat-pebble, edgewise, or intraformational conglomerates) that compose a conspicuous depositional motif believed to record particularly powerful storms that scoured incipiently lithified layers (e.g., Markello and Read, 1981; Sepkoski, 1982; Wilson, 1985; Whisonant, 1987; Westrop, 1989; Mount and Kidder, 1993; Liang et al., 1993; Sprinkle and Guensburg, 1995). Variation and apparent cyclicity of storm deposits—tempestites—have been interpreted to reflect either trends in proximality of shallow-water environments or fluctuating sea level (e.g., Aigner, 1985; Holland et al., 1997). Recent emphasis on certain enigmatic deformation structures now attributable to earthquakes (Pratt, 1994, 1998a, 1998b, 2001a, 2001b; Neuweiler et al., 1999) points toward the natural step of trying to recognize deposits due to strong wave action and off-surge currents caused by tsunamis. The co-occurrence of tempestites, seismites, and tsunamites should be considered normal on ocean-facing platforms, as well as in intracratonic basins for which intermittent syndepositional *E-mail: [email protected]
Figure 1. Map of central-western North America showing location of Little Rocky Mountains (star), and structural elements inferred to be positive features during Late Cambrian. Diagonally ruled pattern shows presentday outcrop of Mesoproterozoic Belt (Purcell) Supergroup, which formed core of early Paleozoic land area called Montania (Deiss, 1941) and northern extension of Lemhi Arch. Inset map of North America shows distribution of Laurentian Upper Cambrian and outline of Montana. Rose diagram is plot of synsedimentary crack orientations in different beds in Deadwood Formation from three exposures in northern and eastern Little Rocky Mountains; vector mean is 1208–3008.
q 2002 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]
Geology; May 2002; v. 30; no. 5; p. 423–426; 5 figures.
Figure 2. Bedding-plane views of cracked calcareous sandstone beds. Scale is same for all. A: Anastomosing linear. B: Rectangular; blocks tilted 108 to left. C: Irregular polygonal. D: Polygonal; blocks at left side tilted 58 to left.
silty shales with rare, thin lenses of trilobite and linguliform brachiopod coquina. The overlying Park Member consists of thinly interbedded silty shales and calcite-cemented, locally glauconitic, fine-grained sandstone exhibiting plane- and wave-ripple cross-lamination. Pot and gutter casts are present locally, and soles of sandstone beds preserve small tool marks, the bottoms of Arenicolites burrows, and wrinkle marks (Runzel marks, ‘‘kinneyia’’ structures). Intraclastic rudstone lenses, 5–20 cm thick, meters to tens of meters wide, with bases showing as much as ;15 cm relief, are sporadically interbedded. Their stratigraphic distribution is irregular, occurring ;0.5–5 m apart. The succeeding Zortman Member is composed of calcareous siltstone, peloidal lime mudstone, silty shale, and intraclastic rudstone. Most sandstone beds in the Park Member are cut by arrays of vertical cracks that exhibit a crudely linear or reticulate pattern on bedding planes. Subparallel lines ;1–5 cm apart with a consistent northwest-southeast orientation (Figs. 1–3) are dominant. One case was observed where the cracks spiraled in a pinwheel fashion away from the imprint of an overlying pot cast. Cracks are ;1–3 mm wide, parallel sided, and filled with silt and clay injected from both subjacent and superjacent muds. In some beds the cracks are microfaults bounding slightly rotated blocks (Fig. 2, B and D). The intraclasts are identical in composition to the associated sandstones, and are variably imbricated to stacked edgewise. They are an-
Figure 3. Bedding-plane view of calcareous sandstone bed with anastomosing-linear cracks, disrupted and overlain by equant and elongate intraclasts at right and at top. 424
gular to subangular, 0.3–1 cm in thickness, and ;1–5 cm in diameter but occasionally larger; well-defined grading is absent. In plan view (Figs. 3 and 4), most intraclasts are polygonal in outline—many are quadrangular, equant to rectangular—similar to the areas delineated by the crack arrays in the sandstones. No compound intraclasts or internal scour surfaces were observed in the conglomerates. DEPOSITIONAL SETTING The Upper Cambrian, low-relief, intrashelf basin in Montana was a storm-dominated system, which led to low-energy silt and clay sedimentation and fairly distal sandy tempestites. Sediment was probably derived from the Canadian Shield and islands of the Transcontinental Arch (Byers and Dott, 1995). The muddy substrate supported an infauna of worms, plus localized trilobite and brachiopod populations. Calcite cementation in the sandy layers began soon after deposition, and was more or less pervasive on account of the fact that cracks occur throughout. Cracking happened intrastratally, as shown by crosscutting relationships, the influence of overlying sole structures on crack patterns, and both downward and upward mud injection. Rare, very strong erosional events scoured to ;15 cm, creating flat-pebble conglomerates. The lenticular nature of these beds, variable imbrication and edgewise stacking of intraclasts, lack of distinct grading, and absence of allochthonous particles indicate that these intraclasts were eroded and worked in situ by oscillating currents from wave action (see also Whisonant, 1987; Mount and Kidder, 1993). The angularity and size of the intraclasts, and the lack of compound intraclasts, are strong evidence that entrainment was confined in most cases to the single depositional event. ORIGIN OF CRACKS The near-surface, intrastratal origin of the cracks, their remarkably uniform orientation, their filling by injected mud, and the local rotation of the resulting blocks point to an origin by seismic deformation, akin to the basic mechanism proposed for syneresis cracks, molar-tooth structure, and similar crack arrays (Pratt, 1994, 1998a, 1998b, 2001b; Grimm and Orange, 1997; Neuweiler et al., 1999). By this process, seismic waves shocked and shook from time to time the near-surface package of alternating layers of relatively rigid, calcite-cementing sand and soft silt and clay. This motion caused brittle deformation of sand beds and liquefaction and injection of the intercalated muds. Minor rotation took place in the latter phases of deformation or during subsequent events. Crack linearity may have been imposed by the arrival of the S wave, which is the first strong wave, and thus could have been at right angles to the source (Pratt, 1998a, 1998b; see also Brothers et al., GEOLOGY, May 2002
Figure 4. Bedding-plane views of flat-pebble conglomerate beds showing intraclast outline and degree of rounding. Scale is same for all. A: Equant. B: Equant and elongate. C: Equant and elongate. D: Equant.
1996). If so, this could incriminate, as the main locus of seismic wave generation, the Lemhi Arch of Idaho ;600 km to the southwest (Fig. 1), thought to have been one of several intermittently active features in the vicinity (Ruppel, 1986). Such a distance would imply particularly strong earthquakes, perhaps reaching M ø 8 (cf. Allen, 1986), unless seismic waves became somehow amplified through the sediment veneer on Precambrian basement. Although western Laurentia is typically regarded as a tectonically stable area, evidence for strong, synsedimentary earthquake-induced deformation is present in Upper Cambrian strata elsewhere in this region (breccia with sedimentary dikes, northwestern Wyoming—Saltzman, 1999; microfaults and syneresis cracks, northeastern Wyoming—Wilson, 1985; syneresis cracks, southern Saskatchewan—Pratt, 1998a; sedimentary dikes, microfaults, and syneresis cracks, southwestern Alberta—Pratt, 1982, and personal observations). ORIGIN OF FLAT-PEBBLE CONGLOMERATES The widespread cracking of the sandstone beds means that the stratigraphically scattered distribution of flat-pebble conglomerate was not a function of sporadic or patchy cementation that would have permitted only occasional intraclast formation. Furthermore, the uniformity of the host distal tempestites and punctuated aspect of the conglomerates, without associated gradational facies or evidence of protracted reworking, argue that the conglomerates do not represent relative lowering of base level due to episodic sea-level fall. Such coarse beds have usually been interpreted as having been created by the most intense storms. This interpretation is unsatisfactory, however, for two reasons: (1) the resulting conglomerates were generally untouched by subsequent storm activity, which is a climatically improbable situation for a storm-dominated setting; and (2) the relatively infrequent stratigraphic occurrence of conglomerates, compared to the dominance of sandstone tempestites, points to truly rare erosional events, far rarer than expected for major storm activity in any conceivably analogous, tropical region. That strong tsunami waves reach much deeper than storm waves on continental shelves (Coleman, 1968) implicates tsunamis as a viable
alternative to generate erosional events of exceptional magnitude at and beyond storm wave base. The comparative rarity of these events in the Deadwood Formation is consistent with presumed intermittent tectonic activity in an intracratonic setting far from the open ocean and any influence of subduction zones, as well as the effect of only the largest tsunamis. However, the pervasiveness of cracking is evidence that only some of the earthquake-generating fault movements caused tsunamis. For the Mesoproterozoic Belt Basin of western North America, I (1998b, 2001b) called upon off surge coupled with oscillating currents from tsunamis to create deep scour surfaces, transport quartz sand or ooids from shallow water, and produce hummocky cross-lamination. By contrast, the lack of coarse allochthonous particles in the Deadwood Formation suggests that unidirectional currents from tsunami off surge did not have an appreciable effect. Perhaps these currents dissipated among the islands of the Transcontinental Arch. It is worth noting that there is possible evidence of tsunami impact on the Cambrian coast at one of the few places in central Laurentia where shoreline facies are preserved (Fig. 12 of Haddox and Dott, 1990). CONCLUSIONS The sporadic and abrupt interbedding of flat-pebble conglomerate with alternating shale and wave-rippled calcareous sandstone or peloidal lime mudstone is a uniquely Proterozoic and Cambrian-Ordovician association. It demonstrates background hemipelagic settling with frequent, weak storm–induced turbulence followed by incipient calcite cementation of the coarser grained layers. Extensive cracking of these beds in Montana is ascribed to earthquake-induced ground motion. The occasional deep erosion of the weakened substrate that generated the flat-pebble conglomerates must have involved scouring of a degree that far exceeded quotidian processes and was far less common than imaginable for storm activity. For the intracratonic Deadwood Formation, and perhaps other examples, major tsunamis are the logical explanation. Seismites are not just a signature of individual tectonic events of sufficient magnitude, but also reflect certain rheologies that render sediments sensitive to deformation. Similarly, tsunamites reflect the nature
Figure 5. Mechanism of flat-pebble conglomerate formation involving synsedimentary cementation of fine-grained sandstone or peloidal lime mudstone tempestites, earthquake-induced cracking, and tsunami scour. GEOLOGY, May 2002
of the sediment and whether the resulting deposit could be erased afterward by background processes. No doubt tsunamites are much more common in the stratigraphic record than currently appreciated, but very few have been identified with any certainty in environments other than shorelines (e.g., Bourrouilh-Le Jan and Talandier, 1985; Clague and Bobrowsky, 1999; Rossetti et al., 2000) and the deep sea (Cita et al., 1996), as well as areas affected by meteorite impacts (Hassler and Simonson, 2001). In the Deadwood Formation, it is the fortuitous coincidence of epeiric mid-shelf setting, seafloor cementation, and synsedimentary brittle deformation that allowed the passage of the larger tsunami waves to be recorded by flat-pebble conglomerates (Fig. 5). Without the cracking, neither the strongest storms nor most tsunamis likely would have been able to penetrate the armored sea bottom, and quite possibly tsunamis would have left no attributes distinguishable from those of storms. The disappearance of this distinctive facies after the Early Ordovician means that different criteria need to be sought to recognize large tsunamis in younger subtidal shelf strata. ACKNOWLEDGMENTS Teaching field geology in the Little Rocky Mountains provided the inspiration to develop these ideas. Comparative work has been supported by the Natural Sciences and Engineering Research Council of Canada.
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