Tectonically induced change in lake evolution recorded by seismites in the Eocene Green River Formation, Wyoming

Tectonically induced change in lake evolution recorded byseismites in the Eocene Green River Formation, Wyoming

Bal�azs T€or}o, Brian R. Pratt and Robin W. RenautDepartment of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon SK S7N 5E2, Canada

ABSTRACT

Seismogenic sedimentary structures – seismites – provide an

opportunity to understand the effects of syndepositional tec-

tonics on lacustrine sedimentation. The lowermost Wilkins

Peak Member (Eocene Green River Formation, Wyoming, USA)

contains laterally extensive intervals displaying folds, load

structures, mixed brittle–ductile deformation, microfaults,

breccias and sedimentary dykes, which formed intrastratally

in a low-energy, sublittoral lacustrine environment. They

developed in situ by liquefaction and plastic or brittle behav-

iour of semi-lithified sediment, or by remobilization and injec-

tion of granular material. Their morphological attributes and

lateral extent, zonation in deformation intensity and the

depositional setting all imply deformation triggered by earth-

quake-induced stresses in shallow-buried sediment. Their

stratigraphic distribution coincides with a shift in hydrological

conditions from a balanced-filled to an underfilled lake type,

independent of climate change. This study is the first to

ascribe seismites to a pulse of tectonic activity that caused a

synchronous change in lacustrine evolution.

Terra Nova, 00, 1–7, 2015

Introduction

Although lakes are relatively short-lived sedimentary basins, their isola-tion and dominantly low-energysetting mean they can be high-resolu-tion archives of palaeoenvironmentalchanges, such as climate, geomor-phology, hydrology and their tec-tonic milieu (Cohen, 2003; Renautand Gierlowski-Kordesch, 2010). Theconceptual subdivision, whereby vari-ations in facies and their stackingpatterns define three major categories– overfilled, balanced-filled and un-derfilled – reflects primarily the rela-tive contributions of climate andaccommodation in lake evolution(Carroll and Bohacs, 1999; Bohacset al., 2000, 2003).Lacustrine deposits may have sus-

ceptible rheologies ideal for record-ing large-magnitude seismic events(e.g. Sims, 1975; Ricci Lucchi, 1995).Earthquake-induced sedimentarystructures (‘seismites’) may provide apowerful tool to better understandthe contribution and significance oftectonic events in the evolution oflacustrine systems, especially those intectonically active regions. Althoughseismites have been reported exten-

sively (e.g. Moretti and Sabato, 2007;Beck, 2011), direct linkage with syn-tectonic changes of lake-basin typehas not yet been attempted.The Green River Formation

(Eocene) is one of the best-studiedancient lacustrine deposits (e.g.Smith et al., 2008). It accumulated ina tectonically active region wheresynchronous faulting has beeninferred (e.g. Pietras et al., 2003).However, in situ sedimentary defor-mation features have not beenrecorded and integrated with theregional-scale evolution of the lacus-trine system. Using seismites pre-served in the Green River Formationof south-western Wyoming, thisstudy documents for the first timethe sedimentary record of syndeposi-tional tectonic activity that alsoinduced changes in the hydrologicalbalance of a lake basin.

Geological setting

The Greater Green River Basin isflanked by the Sevier Fold and ThrustBelt to the west and by Precambrian-cored uplifts on its northern, easternand southern margins, and is subdi-vided by intrabasinal arches, all ofwhich were periodically active duringthe waning stages of the LaramideOrogeny (e.g. Dickinson et al., 1988;DeCelles, 1994) (Fig. 1A). Theirmovements modified the regionaldrainage, geomorphology, gradientsaround the palaeolakes and locations

of depocentres (Roehler, 1992, 1993;Pietras et al., 2003; Carroll et al.,2006, 2008).Lake Gosiute occupied the Greater

Green River Basin and passedthrough hydrologically open toclosed and back to open conditionsduring its history (Roehler, 1993).These stages are recorded by distinctfacies associations: the fluvial-lacus-trine (overfilled) and fluctuating pro-fundal (balanced-filled) Tipton andLaney members, and the evaporitic(underfilled) Wilkins Peak Member(Carroll and Bohacs, 1999) (Fig. 1B).These changes reflect the longer terminterplay between climatic and tec-tonic controls under an overall warmclimate, which led to variable sedi-ment supply, accommodation andwater influx (e.g. Smith et al., 2008).This study focuses on the transi-

tion between the Rife Bed of the Tip-ton Member and the overlyingWilkins Peak Member in south-east-ern Bridger Basin (Fig. 1A,B). It isrecorded by a rapid facies changeover 2–3 m, from organic-rich oilshale to organic-poor calcareousmudstone and siltstone, marking ashift to more saline and alkaline lakechemistry (e.g. Pietras and Carroll,2006).

Sedimentary facies

At five outcrops near the contempo-rary depocentre (Fig. 1A) the lower-most Wilkins Peak Member is

Correspondence: Bal�azs T€or}o, Depart-

ment of Geological Sciences, University

of Saskatchewan, 114 Science Place,

Saskatoon, SK S7N 5E2, Canada. Tel.:

+1 306 241 5424; e-mail: balazs.toro@

usask.ca

© 2015 John Wiley & Sons Ltd 1

doi: 10.1111/ter.12150

organized into repetitive 3–5-m-thickpackages (‘cycles’ of Pietras andCarroll, 2006). These are dominatedby parallel-laminated carbonatemudstone, with upward-increasingintercalation of wavy- or lenticular-laminated calcareous siltstone; oilshale is present at the base andcalcareous sandstone at the top.Sandstones display wave-ripple cross-lamination and, less commonly, com-bined-flow ripple cross-lamination.There is a southward-coarseningtrend, marked by increased intercala-tion of very fine- to medium-grainedsandstone.These sediments were deposited in

the shallow profundal to sublittoralzones of Lake Gosiute, below stormwave base (parallel-laminated car-bonate mud with variable organic

content) or between fair-weatherwave base and storm wave base (car-bonate mud, silt and sand) (Roehler,1990; Fig. 2). Sedimentary deforma-tion features are abundant only inthe lowest two cycles (Fig. 3A). Thestudied interval lacks indicators of aplaya-lake setting, such as evaporitemineral casts, polygonal desiccationcracks or pedogenic modifications,all of which are common higher inthe Wilkins Peak Member (e.g.Pietras and Carroll, 2006).

Deformation features

Deformation is present in five or sixhorizons, separated from each otherby undeformed strata with eithersharp or gradual boundaries. Thestyle of deformation is fairly uniform

in the northern area, consisting ofmillimetre- to centimetre-scale com-posite brittle and ductile features thatform crumpled horizons in interca-lated siltstone and mudstone(Fig. 3B). Individual deformed inter-vals up to 80-cm thick can be tracedlaterally for >20 km and includefolds, load structures, microfaults,breccias and sedimentary dykes. Ingeneral, breccias and dykes are morecommon in the southern locations.Folds are irregularly spaced <25 cm

apart, up to 15-cm high and 10-cmwide and exhibit variable geometry,with open to tight and recumbentshapes (Fig. 3B,D). Successive bedswithin a single deformed interval com-monly show different fold geometries,orientation and wavelength.Mudstone interbeds have irregular

thickness and are typically highlycontorted or disrupted and churned.Millimetre- to centimetre-scale loadstructures are common at their bases,penetrating the underlying foldedand faulted siltstone (Figs 3C,D and4A). These structures are up to 5 cmin diameter, filled by fragmentedmaterial in a mud-rich matrix, andirregularly spaced 1–10 cm apart.Sets of laminated siltstone and

sandstone are commonly offset bymicrofaults with high-angle normal,reverse and thrust geometries. Differ-ent types of faults, with variably dip-ping fault planes and various lengthsand throws, may co-exist in a singlebed (Figs 3D,E and 4B). Displace-ment ranges from a few millimetresto 5 cm, and microfaults die outboth upwards and downwards. Theyoccur in intervals 1–40-cm thick,which are overlain by contortedmudstone beds (Fig. 3B,D).Breccias in siltstone and sandstone,

interbedded with mudstone, formirregular bodies up to 1 m in size,whose boundaries with undeformedlaminae are either sharp or grada-tional (Fig. 4C,D). A single intervalhosts several breccia bodies andother deformation features, and canbe traced laterally at least 50–100 m.The top and base of the host intervaltypically pass sharply into unde-formed beds. The breccias consist ofrandomly oriented clasts up to 10long, with angular to subangularedges that are locally folded andfaulted. The clasts commonly ‘float’in a structureless mixed muddy

(A) (B)

Fig. 1 (A) Regional geological map of the greater Green River Basin, southwesternWyoming (modified after Smith et al., 2008). Studied locations: SJ – Slippery JimCanyon; MF – Middle Firehole Canyon; FC – Firehole Canyon; SC – Sage Creek;RC – Red Creek. (B) Simplified stratigraphy, study interval (white star) and lake-type stages of the Green River Formation (modified after Smith et al., 2008, 2010).Tw – Wasatch Formation (main body), Twn – Niland Tongue; Twc – CathedralBluffs Tongue, Tg – Green River Formation, Tglu – Luman Member, Tgts –Scheggs Bed, Tipton Member, Tgtr – Rife Bed, Tipton Member, Tgw – WilkinsPeak Member, Tgll – lower LaClede Bed, Laney Member, Tgl – upper LaCledeBed, Laney Member, Tb – Bridger Formation, Twa – Washakie Formation.

2 © 2015 John Wiley & Sons Ltd

Green River Formation seismites • B. T€or}o et al. Terra Nova, Vol 0, No. 0, 1–7

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matrix. In places, breccia bodies havean obliquely oriented, crudely curvi-linear shape, resembling sedimentarydykes (Fig. 4C,D).Sedimentary dykes are abundant as

intrusions up to 3-m long and 5-cmwide (Fig. 5A) filled with homoge-nized lime mud, silt or a mixture withsand- and pebble-sized mudstoneclasts (Fig. 5B). Dykes display rag-

ged, folded and bifurcating linearpaths with sharp boundaries in bothcross-section and plan view. At dis-tinct lithological boundaries theycommonly show step-like offsets. AtRed Creek, they have two prominentorientations (~005° and ~120°). AtMiddle Firehole Canyon, many ofthem follow normal microfault planesand penetrate downwards through

several, locally folded, siltstoneinterbeds (Fig. 5C). The upper termi-nation of most dykes is against paral-lel- or wavy-laminated siltstone orsandstone, or, in some places, mud-stone. The overlying bed locallyshows folding, irregular thickness orbrecciation. Where visible, dykestaper downwards, indicating that theywere sourced from above.

Fig. 2 Schematic depositional model for the southern Greater Green River Basin (modified after Roehler, 1993; Renaut andGierlowski-Kordesch, 2010) (not to scale). The seismites described here occur in sublittoral calcareous siltstone deposits (star).Slope gradient was of the order of 0.02–0.5° (Surdam and Wolfbauer, 1975; Roehler, 1990). The diagram is vertically exagger-ated to show facies relationships.

(A) (B) (C)

(D) (E)

Fig. 3 (A) Stratigraphic section at FC with five deformed intervals (A–E). (B) Composite deformation feature with irregularfolding and brecciation, sandwiched between less deformed and microfaulted intervals (interval C; MF). (C) Thin-sectionphotomicrograph showing load structure filled with brecciated laminated mudstone (interval A; SJ). (D) Load structure com-posed of disrupted and churned mudstone, with folds, microfaults and millimetre-scale dykes below (arrow) (interval B; SJ).(E) Thin-section photomicrograph showing normal microfault in intercalated siltstone and sandstone with isolated andupward-protruding dykes (arrows) (interval E; MF).


Terra Nova, Vol 0, No. 0, 1–7 B. T€or}o et al. • Green River Formation seismites

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Discussion

Deformation mechanism andrheology

Soft-sediment deformation featuresare mostly related to loss of shearstrength in unconsolidated to semi-consolidated deposits through lique-faction and/or fluidization driven byinternal and external forces includingdensity contrast, elevated pore pres-sure and physical stress (e.g. Owen,1987). The style of deformation, inturn, is controlled by the rheologicalproperties of the host sediment andthe magnitude of the force exerted onit (e.g. Pratt, 1998; Spalluto et al.,2007).All the deformation features in the

studied interval are interpreted tohave formed intrastratally at shallowburial depth, as shown by the lack oftruncation by erosive lake-floor pro-cesses and the common gradual tran-sition into undeformed beds bothupwards and downwards. Becausenearly the full spectrum of deforma-tion styles are seen in the same bedand the structures pass laterally intoeach other, a common genesis isimplied. Comparable structures havebeen widely reported from marineand lacustrine units (e.g. Rodr�ıguez-Pascua et al., 2000; Bachmann andAref, 2005; Berra and Felletti, 2011;El Taki and Pratt, 2012).The ductile features (folds and

load structures) formed by liquefac-tion and squeezing of semi-cohesivesediments, whereas brittle features(microfaults, breccias) formed inmore cohesive sediments. The litho-logical heterogeneity resulted in dif-ferent mechanical properties, variableresponses to the same event, and for-mation of composite structures. Dis-rupted muddy layers indicate a lowerresistance to liquefaction. Interca-lated silt- and sand-rich beds com-monly show abundant microfaultsand are overlain by plasticallydeformed beds, suggesting a moreconsolidated state of the underlyinglayers and simultaneous brittle andplastic deformation.The irregular geometry of the brec-

cias, the flat and non-erosive bed tops,and the transitional boundaries toundeformed host sediment all indicatethat brecciation resulted from partial

(A) (B)

(C) (D)

Fig. 4 (A) Load structure associated with dykes (arrows) (interval B; SJ). (B) Inter-calated siltstone and mudstone showing an array of reverse microfaults with amud-filled dyke (black arrow) and brecciation (left side) (interval C; FC). (C) Obli-que breccia body within disrupted siltstone (interval C; SC). (D) Tracing of (C),showing the breccia body (light grey) surrounded by disrupted laminae (dashedlines) and ‘floating’ clasts (dark grey).

(A) (B)

(C)

Fig. 5 (A) Two cross-cutting sets of metre-scale dykes (arrows) in laminated car-bonate mudstone (interval E; RC). (B) Thin-section photomicrograph showing abreccia-filled dyke penetrated in turn by a folded silt-rich dyke (middle) (intervalE; RC). (C) Intercalated mudstone and siltstone with a gently folded siltstone bed(black arrow) cut by a dyke (white arrow) following a normal fault plane (intervalE; MF).



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liquidization of the carbonate mud in-terbeds and brittle failure of the cohe-sive, coarser grained layers withoutsignificant transportation (Kahle,2002; El Taki and Pratt, 2012). Wherebreccias fill sedimentary dykes, agenetic sequence of temporally closelyspaced events can be inferred, includ-ing (1) liquefaction, (2) brecciationand homogenization and (3) remobili-zation through injection.The intrastratal formation of

sharp-walled sedimentary dykes indi-cates shrinkage and brittle failure ofsemi-lithified sediments with concom-itant injection of granular material indirections defined by the stress field(Pratt, 1998; Jolly and Lonergan,2002). Thus, these dykes are not nep-tunian dykes formed by the passiveinfilling of pre-existing fissures.Although downward emplacementis dominant, lateral or upward injec-tion may have occurred in someinstances (e.g. Rodr�ıguez-Pascuaet al., 2000).

Trigger mechanism

A number of natural trigger mecha-nisms have been invoked to generatecomparable deformation structures,including wave action, sedimentloading, drag by currents, ice-induced stresses and earthquakes.Consequently, positive identificationof the most likely origin is often per-ceived to be elusive (Owen et al.,2011a).The mid-latitude palaeogeographi-

cal location and the warm palaeocli-mate (e.g. Roehler, 1993; Wilf, 2000)exclude glaciotectonics. The tranquildepositional setting and lack of mass-transport or channel deposits rule outoverloading and drag by bottom cur-rents. Shear stresses associated withtidal currents (e.g. Greb and Archer,2007) are unlikely in lakes. The faciesevidence and the inferred low slopegradient (~0.02–0.5°; Surdam andWolfbauer, 1975; Roehler, 1990) pre-clude deformation from breaking orpounding waves (Dalrymple, 1979;Nataraja and Gill, 1983; Alfaro et al.,2002) or gravitational instability.Moreover, most such triggers wouldgenerate only localized features andcannot explain a lateral extent insome cases of >20 km.A desiccation origin for the sedi-

mentary dykes is rejected based on

the depositional context, the overallsheet-like geometry and oriented nat-ure of the dykes, the lack of evidenceof subaerial exposure, and theabsence of fabrics that would beexpected if the cracks were open andfilled passively from above by sub-aerial or fluvial processes (e.g. Nealet al., 1968; Eugster and Hardie,1975; Harris, 2004).The observed associations and

morphologies of the deformationstructures are best explained as theresult of liquefaction, fluidizationand shear stresses induced by shak-ing during syndepositional earth-quakes. The cyclic and chaoticstresses related to the repeated pas-sage of seismic waves during spo-radic events explain the: (1)unoriented and contorted nature ofthe folded and faulted structures; (2)recurrence of deformed intervalsbound stratally by undeformed beds;(3) superposition of brittle and duc-tile structures in rheologically hetero-geneous sediment; and (4) largelateral extent. The southwardincrease in the relative proportion ofbreccias and sedimentary dykesmight be related to changes in faciesand rheology, or to higher deforma-tion intensity and, in turn, proximityto the source of the earthquakes.

Implications

Three nearby fault zones were activeduring deposition of the lowermostWilkins Peak Member at ~51.5 Ma:the Sevier Fold and Thrust Belt,110–140 km to the west (Coogan,1992); the Laramide Wind RiverThrust system, 95–130 km to thenorth (Pietras et al., 2003); and theUinta Thrust system, 15–50 km tothe south (Roehler, 1993; Bradley,1995; Fig. 1A). Considering modernand ancient analogues, and empiricalrelationships between magnitude andepicentral distance for liquefaction(e.g. Galli, 2000; Monecke et al.,2004; Castilla and Audemard, 2007;Berra and Felletti, 2011), the seis-mites identified here were most likelytriggered by earthquakes with a mag-nitude ≥5 and epicentral intensity≥VI, sourced at the northern front ofthe Uinta Uplift (Figs 1A and 2).The lateral zonation of deformedbeds further supports this interpreta-tion.

The shift from a balanced-filled toan underfilled state that occurred atthe boundary of the Tipton and theWilkins Peak members has beenascribed to increasing aridity (e.g.Bradley and Eugster, 1969; Roehler,1993) or tectonic isolation of the lakebasin (Pietras et al., 2003). The sedi-mentary deformation features, as astratigraphic archive of syndeposi-tional earthquakes, imply that, ratherthan being climatically forced, thechange in palaeohydrological condi-tions of Lake Gosiute was indeedtectonically induced through thediversion of the regional drainagesystem.

Conclusions

Lacustrine deposits of the EoceneGreen River Formation of south-wes-tern Wyoming exhibit laterally exten-sive sedimentary deformationstructures in the boundary intervalbetween the Tipton and Wilkins Peakmembers. These are interpreted asseismites formed in situ by selectiveliquefaction processes and elevatedshear stresses in rheologically hetero-geneous, sublittoral deposits undershallow burial. Their morphologicalcharacteristics, the depositional set-ting, and the zonation of deformationintensity imply that deformation wastriggered by earthquakes with a mag-nitude of ≥5, related to the activity ofbasin-bounding faults of the UintaUplift to the south. For the first time,such seismites have been linkeddirectly to a syntectonic change inlake-basin type, in this case from bal-anced-filled to underfilled.Syndepositional tectonism poten-

tially influences the hydrology,hydrogeology, watershed geology,spring activity and local climate.Thus, it ultimately affects lake chem-istry and sedimentation, especially inregions of active tectonics, such asrift systems or orogenic belts (e.g.Owen et al., 2011b; Lee et al., 2013;Cohen et al., 2015). This study dem-onstrates that seismites should bemore widely utilized to assess thecontribution of tectonics in lake evo-lution, not only in other sub-basinsof the Green River Formation, butalso in other modern or ancient lakedeposits where lacustrine conditionschanged in a tectonically activesetting.



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Acknowledgements

This research was supported by NSERCDiscovery Grants (to B.R. Pratt, R.W.Renaut) and student research grants fromAAPG, IAS, SEPM and GSA (to B.T€or}o). We thank both referees plus theeditor and associate editor for comment-ing on the manuscript, and S. Dasgupta,E. Wiens and J.J. Scott for discussion andhelp in the field.

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Received 26 November 2014; revised

version accepted 10 March 2015



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Tectonically induced change in lake evolution recorded by seismites in the Eocene Green River Formation, Wyoming

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