Earthquakes as Probable Causes of Chaotic and Deformed Stratigraphy in an Ancient River Meander Deposit, Dinosaur Provincial Park, Alberta
Derald G. SmithStephen M. HubbardPeter E. PutnamMilovan FusticDale A. LeckieDavid A. EberthJason M. LavigneChris H. Hugenholtz
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An ancient (Upper Cretaceous, 77-76.5 Ma, Oldman Formation) river meander deposit, exposed in the Steveville Badlands of Dinosaur Provincial Park, AB., exhibits extensive deformed and chaotically bedded strata. The most impressive features are large scale rotations of inclined heterolithic stratified (sandstone and shale) blocks, up to 6 m high and 50 m long, dipping in the opposite direction to that of the lateral accretion trend (Fig. 2). We observe three separate sets of reversely dipping beds along one badland gully, oriented parallel with the direction of lateral accretion. The large reverse cross-stratified structures rest on shale failure planes, suggesting the structures formed as back-rotational slumps of inclined heterolithic strata that slid down an active point bar slope into the channel before it was buried by subsequent lateral accretion sediments. Chaotic and disturbed sandstone and shale blocks, soft sediment deformation, and evidence for sediment foundering in the upper 3 m of point bar stratigraphy are common throughout the ancient meander bend, atypical of meandering river deposits (Fig. 1). Some of the broken and blocky sandstone strata displays a domino-like effect, with all blocks leaning in the same direction. Overturned sandstone beds resting on interpreted failure planes, attributed to slumping, are suggestive of down-slope failures (Fig. 3). Faulting represents the final form of deformation of stratigraphy with displacements of up to 2 m, and the hangingwall always on the channel side of the meander lobe (Fig. 4). We interpret all of these structures as having been caused by large magnitude earthquakes and tremors associated with Laramide thrusting. The three sets of reversed inclined heterolithic strata encased within normal lateral accretion bedding, are interpreted to record three major seismic events, separated by periods of relativeKeywords:
Meander (mathematics)
Slumping
Conglomerate
Point bar
Outcrop
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Soft‐sediment deformation structures from the Alcântara Formation (late Albian to Cenomanian), São Luís Basin, northern Brazil, consist of (1) contorted structures, which include convolute folds, ball‐and‐pillow structures, concave‐up paths with consolidation lamination, recumbently folded cross‐stratification and irregular convolute stratification that grades into massive beds; (2) intruded structures, which include pillars, dykes, cusps and subsidence lobes; and (3) brittle structures, represented by fractures and faults displaying planes with a delicate, ragged morphology and sharp peaks. These structures result from a complex combination of processes, mostly including reverse density gradients, fluidization and liquefaction. Reverse density gradients, promoted by differential liquefaction associated with different degrees of sediment compaction, led to the genesis of convolute folds. More intense deformation promoted the development of ball‐and‐pillow structures, subsidence lobes and sand rolls, which are attributed to denser, and thus more compacted (less liquefied), portions that sank down into less dense, more liquefied sediments. Irregular convolute stratification that grades into massive beds would have formed at periods of maximum deformation. The subsidence of beds was accompanied by lateral current drag and fluid escape from water‐saturated sands. In addition, the fractures and faults record brittle deformation penecontemporaneous with sediment deposition. All these mechanisms were triggered by a seismic agent, as suggested by a combination of criteria, including (1) the position of the study area at the edge of a major strike‐slip fault zone that was reactivated several times from the Albian to the Holocene; (2) a relative increase in the degree of deformation in sites located closer to the fault zone; (3) continuity of the deformed beds over large distances (several kilometres); (4) restriction of soft‐sediment deformation structures to single stratigraphic intervals bounded by entirely undeformed strata; (5) recurrence through time; and (6) similarities to many other earthquake‐induced deformational structures.
Cenomanian
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Geophysical and subsurface geologic data suggest that the Avak structure, which underlies the Arctic Coastal Plain 12 km southeast of Barrow, Alaska, is a hypervelocity meteorite or comet impact structure. The structure is a roughly circular area of uplifted, chaotically deformed Upper Triassic to Lower Cretaceous sedimentary rocks 8 km in diameter that is bounded by a ring of anastomosing, inwardly dipping, listric normal faults 12 km in diameter. A zone of gently outward-dipping sedimentary country rocks forms a discontinuous ring of within the peripheral ring of normal faults. Beyond these anticlines, the sedimentary rocks are almost flat-lying. Basement consists of strongly deformed Ordovician and Silurian argillite. Density and acoustic impedance con rasts between the argillite and the overlying strata produce gravity and seismic-reflection signatures that define a ring of anticlines around the disturbed zone and a structural high surrounded by an annular structural low at its center. In the adjacent Barrow gas fields, the tops of the informally named Neocomian unit and the gas-producing Lower Jurassic Barrow sand (local usage) lie at average subsea depths of 488 m and 670 m, respectively. In the Avak 1 well, drilled on the central high, the pebble shale and the Barrow sand lie near the surface, documenting more than 500 m of relative uplift at the high. The cores in this well have steep dips (30-90 degrees), mixed breccia with Franklinian argillite clasts 10 and 90 m above basement, quartz grains with shock mosaicism and multiple sets of shock lamellae, oriented concussion fractures in sand-size quartz grains, and shatter cones resembling those found in the peripheral zones of well-documented impact structures. In addition, above-background levels o fractured quartz grains in Barrow sand were found as far as 19 km beyond the rim of the Avak structure. Data concerning the age of the Avak structure are not definitive. If submarine landslide deposits in the upper part of the Aptian and Albian Torok Formation, in the subsurface 200 km to the east, were triggered by the Avak event, then the Avak meteorite struck a submerged marine shelf about 100 + or - 5 Ma. However, the impact features found at Avak (shatter cones, concussion fractures, shock lamellae and shock mosaicism in quartz grains, and widespread cataclasis) characterize the distal zones of meteorite impact structures. Fused rocks, plastic deformation, and shock-metamorphic minerals found in more proximal zones of impact structures are apparently missing. These observations, and the lack of Avak ejecta in cuttings and cores from the Torok Formation and Nanushuk Group (Albian to middle Cenomanian) in surrounding test wells, indicate that the impact event postdated these beds. In this case, the Avak meteorite struck a Late Cretaceous or Tertiary marine shelf or coastal plain between the Cenomanian (ca. 95 Ma), and deposition of the basal beds of the overlying late Pliocene and Quaternary Gubik Formation (ca. 3 Ma).
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Abstract A variety of unusual early post‐depositional deformation structures exist in grainstone and flat‐pebble conglomerate beds of Upper Cambrian strata, western Colorado, including slide scarps, thrusted beds, irregular blocks and internally deformed beds. Thrusted beds up to tens of centimetres thick record thrust movement of a part of a bed onto itself along a moderate to steeply inclined (15° to 40°) ramp, locally producing hanging wall lenses with fault‐bend geometries. Thrust plane orientations are widely distributed, and in some cases nearly oppositely oriented in close proximity, indicating that they did not form as failures acted upon by gravity forces. Irregular bedded to internally deformed blocks are isolated on generally flat upper bedding surfaces. These features represent parts of beds that detached, moved up onto and some distances across, the laterally adjacent undisturbed bed surfaces. Deformation of thin intervals of mud on the ocean floor by moving blocks rules out the possibility of storm‐induced deformation, because the mud was not eroded by high shear stresses that would accompany the extremely large forces required to produce and move the blocks. Finally, internally deformed beds are characterized by large blocks, fitted fabrics of highly irregular fragments and contorted lamination, which represent heterogeneous deformation, such as brecciation and liquefaction. The deformation structures were produced by earthquakes linked to the reactivation of Mesoproterozoic, crustal‐scale shear zones in the central Rockies during the Late Cambrian. Analysis of the deformation structures indicates very large body forces and calculated earthquake‐generated ground motion velocities of ca 1·6 m sec −1 . These correspond to moment magnitudes of ca 7·0 or more and a Mercalli Intensity of X+. These are the only known magnitude estimates of Phanerozoic (other than Quaternary) large‐intensity earthquakes for the Rocky Mountain region, and they are as large as, or larger than, previous estimates of Proterozoic earthquakes along these major shear zones of the central Rockies.
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Structures within the Franciscan Complex of northern California that resulted from stratal disruption of a sequence of debris flow-emplaced diamictites and mediumto thick-bedded turbidites indicate that early bedding-parallel extension occurred in response to downslope creep of inner trench slope sediments. Diamictite contains detached segments of thin-bedded turbidites, slump folds and trace amounts of greenstone &nd chert. Turbidites contain load casts and flame structures which give downcurrent (downslope?) direction. The sedimentary rocks were deformed prior to lithification in response to both independent and dependent particulate flow, which resulted in boudinage and formation of small extensional faults and vein arrays. Faults are listric and most abundant in the upper laminated divisions of beds. They commonly become bedding-parallel within lower massive or graded divisions. Bed segments were translated and rotated along faults in the downslope direction. Curvilinear arrays of clay and silt-filled veins, arranged in conjugate sets at moderate to high angles to bedding, abound in the central portions of beds beneath faulted upper divisions, and in hanging-wall regions above faults in lower divisions. Veining and faulting occurred concurrently and resulted in differential extension of upper verses lower portions of beds. The passage of an early-formed fault or vein through an entire bed is uncommon. Diamictite deposition and downslope creep of trench slope sediments occurred prior to emplacement of the Crescent City olistostrome, which is up to 600 m thick, which stratigraphically overlies the rocks described herein and which crops out for 12 km along the coast between Crescent City and Point St. George. Sediment creep may in part have triggered the olistostrome event by oversteepening part of the trench slope.
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Slumping
Graded bedding
Debris flow
Growth fault
Turbidity current
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ABSTRACT As Interstate Highway 10 (I-10) transects the Edwards Plateau northwest of San Antonio, Texas, its roadcuts augment natural outcrops allowing detailed examination of stratigraphic and structural relations. Overall, strata are nearly horizontal, with hills capped by Cretaceous Edwards Group carbonates and younger strata, and exposures of older Cretaceous clastics and locally Paleozoic strata in deep river valleys. Between Kerrville, Junction, and Sonora, though the bulk stratigraphic package remains nearly horizontal, roadcuts reveal intense brecciation and deformation, including local overturning. The causal mechanism is generally interpreted to be gypsum dissolution and resultant welding of Edwards Group strata. A distinct zone of brecciation and related deformation occurs at the same stratigraphic position for over 60 miles along I-10. The basal contact is sharp, marked by horizontal, undisturbed strata below, and intense brecciation above. Original bedding is unrecognizable for several feet above the contact. Proceeding upsection, large blocks of strata several feet across occur in chaotic arrangement. Bedding becomes progressively more coherent upsection, but is still intensely folded and faulted. Deformation decreases upsection, until overlying strata appear undisturbed, nearly horizontal. Structural details of withdrawal synclines provide outcrop analogues to similar structures in the Gulf of Mexico salt basin. A nearby quarry and well control indicate presence of gypsum layers within Edwards Group strata. No gypsum occurs in I-10 roadcuts, but macroscopic and petrographic evidence indicates its former presence. The occurrence of peat deposits and caliche over structural lows, and the breccia's commonly open framework favor relatively recent (Quaternary) gypsum dissolution, although some dissolution may have occurred much earlier.
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ABSTRACT An impressive oriented clastic dike swarm (> 165 dikes) occurs within 3.25 m of strata of a submarine-fan turbidite succession of the Upper Cretaceous Nanaimo Group, British Columbia. The dikes are remarkably parallel in orientation, are consistently subvertical, and strike southwest. Slump-fold vergence, paleocurrent indicators, and other evidence indicate that local paleoslope strike was northwest, with a downslope direction to the southwest. Thus the clastic dikes strike downslope, indicating that extension occurred perpendicular to the regional downslope direction, not parallel as generally expected. Two possible models are proposed: (1) Rapid downslope compression caused by sudden loading, perhaps due to slumping events or deposition of thick turbidity current beds. This downslope compression caused minor extension perpendicular to the downslope direction, resulting in small extensional joints that accommodated clastic injection from buried, overpressured sands. (2) Chaotic sedimentary breccias laterally adjacent to the dike swarm formed as part of a large slumping event, which created a local slump channel. Minor slumping into the main channel from the sides caused extension parallel to the channel-margin slope and formation of the clastic dikes parallel to the channel margin but perpendicular to the regional slope. Oriented clastic dike swarms may represent a new type of paleoslope indicator, but they may be created by a number of processes and should therefore be used only in conjunction with supporting evidence.
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Structures within the Franciscan Complex of northern California that resulted from stratal disruption of a sequence of debris flow-emplaced diamictites and medium- to thick-bedded turbidites indicate that early bedding-parallel extension occurred in response to downslope creep of inner trench slope sediments. Diamictite contains detached segments of thin-bedded turbidites, slump folds and trace amounts of greenstone &nd chert. Turbidites contain load casts and flame structures which give downcurrent (downslope?) direction. The sedimentary rocks were deformed prior to lithification in response to both independent and dependent particulate flow, which resulted in boudinage and formation of small extensional faults and vein arrays. Faults are listric and most abundant in the upper laminated divisions of beds. They commonly become bedding-parallel within lower massive or graded divisions. Bed segments were translated and rotated along faults in the downslope direction. Curvilinear arrays of clay and silt-filled veins, arranged in conjugat e sets at moderate to high angles to bedding, abound in the central portions of beds beneath faulted upper divisions, and in hanging-wall regions above faults in lower divisions . Veining and faulting occurred concurrently and resulted in differential extension of upper verses lower portions of beds. The passage of an early-formed fault or vein through an entire bed is uncommon. Diamictite deposition and downslope creep of trench slope sediments occurred prior to emplacement of the Crescent City olistostrome, which is up to 600 m thick, which stratigraphically overlies the rocks described herein and which crops out for 12 km along the coast between Crescent City and Point St. George. Sediment creep may in part have triggered the olistostrome event by oversteepening part of the trench slope.
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Slumping
Graded bedding
Growth fault
Debris flow
Turbidity current
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Anticline
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Stylolite
Bedding
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Siliciclastic
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Cores from a deep well at the crest of a dome about 2½ miles in diameter near Peoria, Illinois, show a normal sequence of Paleozoic strata down to the Upper Ordovician Maquoketa Shale, but the underlying 1,500 feet of rocks are intensely disordered. Middle and Lower Ordovician strata normally underlying the Maquoketa are not recognized in the disturbed zone. However, the core penetrated several jumbled blocks of Cambrian formations uplifted about 1,000 feet above their normal stratigraphic position. The disturbed section consists chiefly of brecciated dolomite, with minor amounts of compact, mylonitic sandstone, and contorted beds of red and green shale. Dark argillaceous bands and shale partings show many small-scale faults. Dips are randomly oriented and attain up to 90° throughout much of the section, but in the lower 400 feet of the well the apparent bedding planes dip southeast and the angle decreases with depth from 70° to 40°. The chaotic condition of the pre-Maquoketa rocks suggests that this structure was caused by a violent explosion which probably took place in early Cincinnatian time. The writers believe the explosion was the result of meteorite impact. Thinning and arching of strata overlying the breccia indicate that there was gradual and continuous structural development of the dome from Late Ordovician to at least Pennsylvanian time and perhaps to the present.
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