TIDAL-BUNDLE SEQUENCES IN THE JORDAN SANDSTONE … · 2004-02-18 · TIDAL-BUNDLE SEQUENCES IN THE JORDAN SANDSTONE (UPPER CAMBRIAN), SOUTHEASTERN ... same facies in outcrops of lesser

JOURNAL OF SEDIMENTARY RESEARCH, VOL. 73, NO. 3, MAY, 2003, P. 354–366Copyright q 2003, SEPM (Society for Sedimentary Geology) 1527-1404/03/073-354/$03.00

TIDAL-BUNDLE SEQUENCES IN THE JORDAN SANDSTONE (UPPER CAMBRIAN), SOUTHEASTERNMINNESOTA, U.S.A.: EVIDENCE FOR TIDES ALONG INBOARD SHORELINES OF THE SAUK

EPICONTINENTAL SEA

CARL H. TAPE,1* CLINTON A. COWAN,1 AND ANTHONY C. RUNKEL2

1 Geology Department, Carleton College, One North College Street, Northfield, Minnesota 55057, U.S.A.e-mail: [email protected]

2 Minnesota Geological Survey, 2642 University Avenue, St. Paul, Minnesota 55114-1057, U.S.A.

ABSTRACT: This study documents for the first time tidal bundling ina lower Paleozoic sheet sandstone from the cratonic interior of NorthAmerica, providing insights into the hydrodynamics of ancient epicon-tinental seas. The Jordan Sandstone (Upper Cambrian) in the UpperMississippi Valley contains large-scale planar tabular cross-sets withtidal-bundle sequences, which were analyzed in detail at an exceptionalexposure. Tidal-bundle sequences (neap–spring–neap cycles) were de-lineated by foreset thickening–thinning patterns and composite shaledrapes, the latter of which represent accumulations of mud during theneap tides of neap–spring–neap tidal cycles. Fourier analysis of thebundle thickness data from the 26 measurable bundle sequences re-vealed cycles ranging from 15 to 34 bundles per sequence, which sug-gests a semidiurnal or mixed tidal system along this part of the LateCambrian shoreline.

We extend the tidal interpretation to widespread occurrences of thesame facies in outcrops of lesser quality, where the facies is recogniz-able but too few bundles are exposed for tidal cycles to be measured.By doing so, this study shows that tidally generated deposits have asignificant geographic and temporal extent in Upper Cambrian strataof central mid-continent North America. The deposition and preser-vation of tidal facies was related to the intermittent development ofshoreline embayments during transgressions. The tidally dominateddeposits filled ravined topographies that were repeatedly developed onthe updip parts of the shoreface. Resulting coastal geomorphologies,accompanied perhaps by larger-scale changes in basinal conditionsand/or configuration, led to changes in depositional conditions fromwave-dominated to tide-dominated.

Outcrops of the Jordan Sandstone tidal facies in the Upper Missis-sippi Valley represent the farthest inboard recorded transmission ofocean-generated tides in the Laurentian epicontinental seas, demon-strating that tidal currents were significant agents in the transport ofsand along the far cratonic interior shorelines of Cambrian NorthAmerica. The results of this study improve the facies-level understand-ing of the genesis of sheet sandstones. Furthermore, tidalites docu-mented here occur in a specific position within a sequence stratigraphicarchitecture for the Jordan Sandstone. This provides a framework tocompare these ancient deposits and processes to younger (e.g., Car-boniferous) epicontinental systems where stratal and sediment dynam-ics are better documented.

INTRODUCTION

The hydrodynamics of ancient epicontinental (literally, on top of thecontinent) seas are poorly known, in part because of the lack of suitablemodern analogues for such vast, low-gradient, flooded areas. The classiclower Paleozoic ‘‘sheet sandstones’’ of North America are the prototypicalexamples of nearshore deposits in epicontinental seas and have long vexedsedimentologists. Extreme textural and mineralogical maturities and vastgeographic extents are among the significant obstacles to the interpretation

* Present address: Department of Earth Sciences, Oxford University, Parks Road,Oxford, OX1 3PR, U.K.

of these units. Interpretations of sheet sandstones in the literature havetended to be conflicting and speculative. Early scrutiny of these unitsspawned a model of epeiric (synonymous with epicontinental) sedimenta-tion dominated by wave action and devoid of tides (e.g., Irwin 1965) thatis still reproduced in modern textbooks (e.g., Boggs 2001). Irwin (1965)proposed that inboard shorelines of these exceptionally broad seas weretideless because of frictional damping of the tidal wave (Friedman andSanders 1978; Hallam 1981), and there remains a widely held conceptionthat tidal currents were insignificant in the transport of sand here (e.g.,Johnson and Baldwin 1996, p. 271–272). In contrast, Klein (1982) sug-gested that tidal currents were an integral part of epeiric-sea processes,dominating deposition of lower Paleozoic strata regionally across the cra-tonic interior during the culmination of major transgressions. The supportfor regionally significant tidal currents in the model proposed by Klein(1982), however, was based on a peritidal misinterpretation for a wide-spread facies that is now considered to be dominated by hummocky cross-strata and deposited on a wave-dominated offshore shelf (Dott and Bour-geois 1982; Runkel 1994; Hughes and Hesselbo 1997; Mahoney et al.1997; Runkel et al. 1999). More recent studies of lower Paleozoic cratonicsandstones have only tentatively attributed rare features to the action oftides—for example, local occurrences of shale drapes, reactivation surfaces,flaser bedding, herringbone cross-strata, and cracked shale (Driese et al.1981; Dott et al. 1986; Terwindt 1988; Haddox and Dott 1990; Runkel1994; Byers and Dott 1995). These features are no longer considered to beby themselves diagnostic of tidal activity (Nio and Yang 1991b). Myrow(1998) described large-scale features that he interpreted as representingtidal sand waves in the Cambrian Sawatch Quartzite; this deposit, however,is known from a single area of the ancient North American craton in Col-orado. No studies of Cambrian or Ordovician cratonic sheet sandstonesfrom the continental interior offer compelling evidence for significant tidalactivity by documenting tidal bundling in a facies that is regionally exten-sive.

The prevailing view that cratonic sandstones of the Sauk and Tippecanoesequences (Sloss 1963, 1988; Bunker et al. 1988) are significantly differentin origin from younger deposits stems not only from their sheet geometry,but to a large degree from a paucity of reported facies in lower Paleozoicsuccessions of the cratonic interior that could confidently be attributed totides. By contrast, wave-generated facies in these successions are well doc-umented, and this has imparted a bias in the literature (Dott and Bourgeois1982; Runkel 1994; Byers and Dott 1995; Runkel et al. 1999). The his-torical lack of recognition of tidal facies in mid-continent Cambrian andOrdovician sheet sandstones stands in marked contrast to work on younger,nearshore epicontinental deposits. Most notable of these are later Paleozoicand Mesozoic strata, in which tidalites (i.e., units displaying tidal bundlesor tidal rhythmites) are reported to be plentiful and widespread. Examplesinclude Carboniferous strata in regions just inboard of the Appalachian–Ouachita belt (Heckel 1986; Kvale et al. 1989; Brown et al. 1990; Archeret al. 1995; Miller and Eriksson 1997) and strata deposited in the vastepicontinental Cretaceous Interior seaway of western North America(McCrory and Walker 1986; Mellere and Steel 1995; Willis and Gabel2001).

In this study, we apply criteria diagnostic of sustained tidal current ac-

355CAMBRIAN TIDAL BUNDLE SEQUENCES

FIG. 1.—A) Generalized large-scale facies map of Cambrian North America (Laurentia). Traditional facies belts and paleogeography after Aitken (1966), Lochman-Balk(1971), and Dott and Batten (1981). Box outlines area enlarged in Part B (Upper Mississippi Valley). B) Approximate outcrop belt in the Upper Mississippi Valley forthe Jordan Sandstone (gray area). The study site at Homer, Minnesota, is indicated by the H; solid circles indicate other Jordan Sandstone outcrops where similar tidalfacies are recognized; other letters correspond to outcrops photographs in Figures 5B and 12 (M 5 Mankato, W 5 Weaver, R 5 Rushford).

FIG. 2.—Schematic stratigraphic attributes and generalized facies in the St. Lawrence Formation and overlying Jordan Sandstone. The St. Lawrence and Jordan formationsoccur across the Upper Mississippi Valley as a relatively thin, widespread sheet with a lower interval composed of facies deposited below fair-weather wave base, and anupper interval composed of facies deposited above fair-weather wave base. The heterolithic facies association occurs within the upper Jordan–St. Lawrence succession andis demonstrated here to have a tidal signature. The St. Lawrence–Jordan sheet is internally composed of seaward-shingled parasequences deposited during a prolongedrelative fall in sea level. Individual parasequences are bounded by ravinement surfaces that developed during relatively minor episodes of landward retreat. The JordanSandstone is capped by a regional sequence boundary that everywhere separates it from overlying mixed carbonate–siliciclastic units deposited during continental trans-gression in the Ordovician.

tivity (Nio and Yang 1991b)—notably, measured periodicities in foresetthickness patterns attributable to neap–spring–neap tidal cycles—to a prev-alent facies in the Sunwaptan-age Jordan Sandstone of the Upper Missis-sippi Valley, U.S.A. The results demonstrate for the first time that tidalprocesses were important in the deposition of a sheet sandstone in the farinterior of the craton. Our investigation also improves our facies-level un-derstanding of the genesis of sheet sandstones, i.e., the discrimination oftidal versus wave-dominated processes, and of the larger-scale stratigraphicand paleogeomorphic controls on these processes.

SEDIMENTOLOGIC AND STRATIGRAPHIC CONTEXT

Cambrian strata in the Upper Mississippi Valley (UMV) (Fig. 1) weredeposited largely on a texturally graded, siliciclastic-dominated shelf (Run-kel 1994; Byers and Dott 1995; Runkel et al. 1999). The Sunwaptan-ageSt. Lawrence and Jordan formations together constitute a classic exampleof deposits that accumulated in such a setting during a relative fall in sealevel (Fig. 2). The upper St. Lawrence Formation and the lower JordanSandstone are composed largely of very fine- to fine-grained, locally shaly,

356 C.H. TAPE ET AL.

FIG. 3.—Generalized stratigraphic column of the Jordan Sandstone in southeasternMinnesota (after Runkel et al. 1999).

silty and/or calcareous sandstone dominated by hummocky and swalycross-strata (Fig. 3). These strata were deposited below fair-weather wavebase, largely under the influence of combined-flow currents during storms(Runkel 1994; Byers and Dott 1995; Hughes and Hesselbo 1997; Runkel2000). The upper Jordan Sandstone is composed chiefly of fine- to coarse-grained sandstone dominated by dune-scale, high-angle cross-stratification,and is markedly more complex and variable compared to the underlyingstrata (Fig. 3). The upper Jordan Sandstone was deposited above fair-weath-er wave base, as evidenced by the deposits of dunes and, locally, beachswash. Detailed facies descriptions of the St. Lawrence–Jordan successionin the UMV are given by Runkel (1994), Byers and Dott (1995), andHughes and Hesselbo (1997).

Recent studies have shown that the St. Lawrence and Jordan formationstogether constitute a regressive succession (Fig. 2) (Runkel 1994; Runkelet al. 1999; Hughes and Hesselbo 1997; Runkel 2000). Regression and/orprogradation was interrupted repeatedly by relatively minor episodes ofshoreface retreat that are expressed as ravinement surfaces associated withlandward shifts in lithofacies (Runkel 1994; Hughes and Hesselbo 1997).At a larger scale, these ravinements and associated shifts in facies defineseaward-shingled parasequences displaying hundreds of kilometers of of-flap across a maximum flooding surface developed on the underlying Fran-conia Formation (Runkel 1994; Hughes and Hesselbo 1997; Runkel et al.1999; Runkel 2000). Offlap spanned at least seven Late Cambrian and EarlyOrdovician conodont zones; the most landward parasequences are at leastas old as Proconodontus muelleri in age, the most seaward are Hirsuto-dontus simplex or younger in age (Runkel et al. 1999; J.F. Miller, unpub-

lished conodont identifications). The relatively thin but widespread St.Lawrence–Jordan sheet that covers much of the UMV is capped by a re-gional sequence boundary that separates it from overlying mixed carbon-ates–siliciclastics that were deposited during continental transgression inthe Early Ordovician (Runkel 1994; Runkel et al. 1999; Smith et al. 1996).

This research focuses on the sedimentologic attributes of the upper Jor-dan Sandstone (Fig. 3). We have significantly refined previous work thatdiscriminated two general facies associations for these strata in southeasternMinnesota and Wisconsin (Runkel 1994; Byers and Dott 1995; Runkel2000): (1) a trough cross-stratified association that contains relatively littleshale, and (2) a heterolithic association dominated by tabular sets of cross-strata, but with a substantial component of shale, and containing a diversesuite of sedimentary structures.

The trough cross-stratified facies association of the upper Jordan is char-acterized by fine- to coarse-grained sandstone bodies dominated by sets oftrough cross-strata 10–40 cm thick, subordinate tabular to wedge sets oftangential cross-strata and, more rarely, horizontal to low-angle, planar-laminated sandstone; symmetrical ripples and Skolithos burrows are com-mon. Runkel (1994) and Byers and Dott (1995) ascribe deposition of thisfacies association to wave-dominated, middle to upper shoreface and fore-shore environments. The trough cross-strata that dominate this associationare interpreted to record migration of 3D dunes; planar-laminated sandstoneis interpreted as beach swash lamination.

The heterolithic facies association is characterized by meter-scale cosetsthat are internally composed of thin to thick, tabular to wedge-shaped setsof tangential to angular cross-strata in medium- to coarse-grained sand-stone. Foresets in adjacent tabular to wedge-shaped sets may show oppos-ing dip directions. Thin (, 10 cm) shale beds, laminae, and intraclasts arecommon locally. Both symmetric and asymmetric ripples are common inthe finer sand fraction. This facies association is attributed by Runkel(1994) and Byers and Dott (1995) to deposition at, or adjacent to, the LateCambrian shoreline, possibly as part of sand waves, deltas, or complexbars. These workers tentatively suggested that the sedimentologic attributesof this facies association were more indicative of tidal currents than ofwave activity.

METER-SCALE CROSS-STRATA OF THE HETEROLITHIC FACIES

ASSOCIATION

Description

The focus of this study is an exceptional exposure of the heterolithicfacies association (Figs. 3, 4). Bundles were measured in a cross-set thataverages 0.9 m thick and is exposed for approximately 100 m laterally(lateral terminations of the set are not exposed) (Fig. 4). A small area ofpoor exposure separates the cross-set into a Left Section (18.0 m long) anda Right Section (13.5 m long). After cleaning the outcrop, we measuredthe thickness of the foresets in a direction normal to the foreset dip, at aheight midway between the lower and upper bounding surfaces of thecross-set. We designated foresets of zero thickness as those which werepresent at lower heights of the cross-set but absent at the measuring height.

The heterolithic facies association here is 12 m thick. The measured setis the uppermost of three that compose a coset; overlying cross-sets aresimilar to the set of interest although their foresets dip in the oppositedirection (north). The upper boundary of the heterolithic facies associationat this exposure corresponds to the pebbly sandstone that marks a regionalsequence boundary capping the Jordan Sandstone across the UMV (Runkel1994; Runkel et al. 1999) (Figs. 2, 3); the lower boundary is a sharp,irregular surface with a minimum 0.8 m of relief. This lower surface isinterpreted as a ravinement surface and truncates trough to transitional swa-ly-trough cross-strata (cf. Datta et al. 1999) deposited in a middle to uppershoreface environment.

The cross-set of interest is a medium- to coarse-grained quartz arenite


FIG. 4.—Upper section of the Jordan Sandstone at Homer, Minnesota, showing a 20-m portion of the meter-thick, tabular cross-bed set of interest, which extends laterallyapproximately 100 m. Foresets in the cross-set dip toward the south; thick (7 cm) foresets are conspicuous to the left of the person. Composite shale drapes (see text)weather recessively. The underlying cross-sets are similar to the set of interest but have not been scraped clean. Foresets in the overlying cross-sets dip to the north. Silverdiscs (dots along upper bounding surface of set of interest) mark composite shale drapes (neap intervals).

FIG. 5.—Photographs of relatively thick, graded foresets. Diffuse internal laminae are attributed to fallout of suspended sand during unsteady flow conditions (waningebb current). A) From study outcrop at Homer, Minnesota (ruler increment 5 1 cm). B) From same facies in Mankato, Minnesota (hammer is 30 cm tall).

that locally contains a significant fraction of clay- to silt-size sediment.Foresets range in thickness from a few millimeters to 7.3 cm, and have anaverage true dip of 258 to the south (about 1708). The mean thickness ofthe 526 measured foresets from the Right Section and Left Section is 1.96 1.2 cm. Thickness of foresets varies across the length of the exposure:

foresets of similar thickness are bunched together, lending a repeated thick-ening–thinning pattern along the length of the set. Many of the thickerforesets show pronounced normal grading, and some have diffuse internallaminae (Fig. 5). Small (a few millimeters thick, , 2 cm in length) shaleintraclasts are common in the bottomsets or lower reaches of relatively


FIG. 6.—Photograph of a composite shale drape consisting of up to six, mm-thickshale layers alternating with millimeter-thick sandstone layers (ruler increment 5 1cm). Burrows (arrows) are associated with composite shale drapes, which mightsuggest a relatively long period (a few days) of quiet water, such as during the neaptides, that allowed infauna to colonize the dune.

FIG. 7.—A) Photograph of flood current rippledefined by small-scale, reverse-dipping cross-strata in the bottomset of the large-scale tabularcross-set. Foresets in the flood current ripplecontain abundant shale rip-up clasts and dip inthe opposite direction from the large-scale, ebbcurrent foresets (see Fig. 11, Stage 3). The floodcurrent ripple is draped above and boundedbelow by shale. B) Line sketch of photograph inPart A (ruler increment 5 1 cm). Dotted linesare generalizations of foresets.

thick foresets. Reactivation surfaces with observable discordance are rareand limited to the upper part of the cross-set.

Shale drapes are common on foresets. Drapes merge at the toe of the setto form a shale bed 1 to 14 cm thick. The abundance and thickness ofshale drapes vary across the set and show an inverse relationship to foresetthickness: drapes are rare and thin in intervals of thick foresets, and abun-dant and thick in intervals of thin foresets. In particular, fewer than 5% offoresets . 3 cm thick are draped, and more than half of foresets , 1 cmthick are draped. Where the thinnest (; 1 mm) sandstone foresets occur,there is a clustering of shale drapes, and we refer to these closely-spaceddrapes as ‘‘composite drapes’’ (Fig. 6). Drapes rarely extend to the upperbounding surface of the cross-set.

Reverse-dipping (northward or paleo-landward), centimeter-scale cross-strata are present in the cross-set. Some of these ripple cross-strata arepresent along the bottomsets of the large-scale cross-set and are interbeddedwith composite shale drapes (Fig. 7), whereas others occur along the lowerreaches of the foresets (Fig. 8). The latter lack shale and differ only ingrain size and/or packing from the large-scale cross-bedding that encasesthem.

Burrows are locally abundant (Fig. 6) and include Arenicolites, Diplo-craterion, Chondrites, Skolithos, and a possible Monocraterion. Arenicol-ites and Diplocraterion have subtle but discernible linings of finer-grainedmaterial, and are filled by a ferruginous-dolomitic mudstone to siltstone.Burrows are most abundant in intervals of relatively thin foresets and abun-


FIG. 8.—A) Photograph of cross-bedding (grayarea in line sketch) defined by small-scale,reverse-dipping cross-strata on the lower slipfaceof ebb current dunes. We interpret this cross-bedding to be the result of migration of ripplesup the slipface of the large dunes in response toback-eddy currents from flow separation duringthe peak ebb-current stage (see Fig. 11, Stage 1).Note the well-defined foresets in the large-scalecross-set and the shale rip-up clasts accentuatingsome the foresets within both the flow-separationripple cross-strata and the large-scale cross-strata. Ruler is situated within a composite shaledrape (increment 5 1 cm). B) Line sketch ofphotograph.

dant shale drapes (i.e., composite shale drapes), and penetrate downwardfrom foreset surfaces (Fig. 6). In contrast, burrows are sparse in intervalsof relatively thick foresets and are dominated by Chondrites.

General Interpretation

The 100-m-long cross-set of interest records the migration of a large-scale, subaqueous dune (cf. Ashley 1990). From the inferred trend of thepaleoshoreline (Runkel 1994; Byers and Dott 1995), the dominant paleo-flow direction (southward) is offshore. We attribute the conspicuous normalgrading and vague internal laminae in many foresets to fallout of suspendedsand during unsteady flow conditions. Each shale drape indicates still waterconditions and settling of suspended fines; composite shale drapes thusrecord repeated episodes of quiescence. Reactivation surfaces may indicateunsteady to possibly reverse flow or truncation by wave action. Unsteadyand reversing flow conditions and quiescent periods are characteristic of,but not exclusive to, tidal environments (Dalrymple and Rhodes 1995).

Tidal Interpretation

Qualitatively, patterns of increasing and decreasing foreset thickness andthe distribution and configuration of shale drapes are suggestive of neap–spring–neap tidal cycles in sedimentation (Visser 1980; Boersma and Ter-windt 1981). Quantitatively, Fourier analyses of sedimentary laminae thick-ness data have been used to demonstrate tidal influence on sedimentation(e.g., Kvale et al. 1999). Below, a comparative treatment of the cross-setis followed by a Fourier analysis of the foreset thickness data.

Comparative Analysis of Foresets and Sedimentary Structures.—Ahistogram of foreset thicknesses (Fig. 9A) resembles the neap–spring–neaptidal rhythms reported in other studies of ancient and pre-modern tidaldunes (e.g., Visser 1980; Allen and Homewood 1984; Driese 1987). Thecomposite drapes record repeated slack-water periods and are associatedwith the thinnest foresets in the cross-strata set (Fig. 9A). This suggeststhat they may represent deposition during neap tides, when tidal currentswere at their most sluggish (minimum sand transport, maximum mud de-


FIG. 9.—A) Thickening and thinning pattern of tidal-bundle thickness and the position of composite shale drapes for the cross-set of interest. Histogram shows thecomplete data set (526 foresets). The gap of poor exposure corresponds to a 16-m interval of approximately 150 unmeasured bundles within 12 bundle sequences. Eachline below the horizontal axis indicates the position of a composite shale drape with respect to the bundles (foresets). B–C) ‘‘Sliding window’’ Fourier analysis for theLeft Section and Right Section data sets (see text). Periods for the peaks in the Left Section range from 15 to 34 bundles; the maximum resolved peak in the Right Sectioncorresponds to 29 bundles. D–F) Cross-sections of the contour plots in Parts B and C. The dashed lines in Part D indicate the range between ideal Cambrian diurnal (f 51/15) and semidiurnal (f 5 1/30) tidal cycles. G) Number of tidal bundles per sequence for the 26 sequences in the cross-set of interest (data set shown in Part A). Thereare 20 6 7 bundles per sequence. Dashed lines indicate ideal diurnal (15) and semidiurnal (30) systems.

position and preservation). Thicker foresets with little to no shale representdeposition during spring tides, when currents were most vigorous. Underthis interpretation, a foreset (or foreset-drape couplet where shale is present)is a tidal bundle, i.e., the sediment deposited during the dominant currentstage of an ebb–flood cycle. The interval between two successive compositedrapes is a tidal-bundle sequence, i.e., the amount of sediment depositedduring one neap–spring–neap cycle (Fig. 10). The 3-cm-thick compositeshale drape shown in Figure 6 represents a relatively long period of time(several days) in relatively sluggish waters. In contrast, a single 7-cm-thickindividual foreset (Fig. 5) represents deposition during a single ebb–floodcycle (about 12 or 24 hours) during spring tides, when current velocitiessufficient to transport medium to coarse sand were sustained for longerperiods of time.

We attribute the relatively thick, normally graded foresets (Fig. 5) tosand fallout during the deceleration stage of the ebb current. The absenceof sediment representative of ebb acceleration suggests that deposition onforesets was initiated only during or near peak flow conditions and contin-ued during subsequent deceleration. Mud draping the dune at the start ofthe ebb–flood cycle may also have inhibited acceleration-phase sand ero-sion. We interpret the diffuse, thin, internal laminae within the graded fore-sets to be the results of avalanches and/or minor pulses of unsteady flow

that accompanied sand fallout during the deceleration stage of the ebbcurrent.

Large dune fields, especially those in tidal regimes, are under the influ-ence of complex currents. Some of the centimeter-scale cross-strata withlandward (northward) paleoflow in the set of interest are similar to floodcurrent ripples documented by Fenies et al. (1999, fig. 7), who studiedmodern tidal deposition in the Gironde estuary of France (Fig. 7). Others,such as those in Figure 8, may be associated with recirculatory flow fromflow separation associated with large-scale dunes. Figure 11 shows ourinterpretation of ripple cross-strata in relation to the migration of the dune.

Trace fossils suggest that burrowing organisms became established dur-ing sluggish neap periods, when minimal sand transport allowed coloni-zation of the stagnant slipface of the dune. The association of burrows withneap-tide foreset bundles is not well documented in the literature, and mayprove to be a useful criterion for recognition of tidal current activity inother ancient facies that are dominated by shale-poor cross-stratified sand-stone.

For the cross-set studied here, ebb currents were markedly dominant overflood currents (e.g., pronounced large-scale ebb-current cross-bedding withminor flood current cross-bedding; rare reactivation surfaces). Overlyingcross-sets with foresets dipping northward preserve a record of opposing


FIG. 10.—A) Photograph of a a tidal-bundlesequence. A bundle sequence is recognized asthe interval of sandstone between two compositeshale drapes (arrows). Fingers span a toeset witha composite shale drape that contains eightmillimetric shale laminae with interlayeredsandstone. Several of the thicker bundles shownin this sequence are capped by single shaledrapes. B) Schematic of a tidal-bundle sequencein the study area. A bundle sequence is thesediment deposited by a migrating dune duringone complete neap–spring–neap tidal cycle(about 14 days), whereas a tidal bundle consistsof the sediment deposited during a single ebbstage. Relatively thin bundles are depositedduring neap tides, relatively thick bundles aredeposited during spring tides. Bundle thicknessis measured perpendicular to the dip of theforesets along a horizontal guided medialbetween the upper and lower bounding surfaces.C) Histogram of bundle thickness for the bundlesequence in Part A. Note the thick–thinalteration, which is suggestive of a diurnalinequality.

dominant paleoflow direction, reflecting a change in local geomorphologythat caused landward (flood)-directed tidal currents to become dominant atthis location. Such changes are common in tidally dominated systems (Dal-rymple 1992), and their record in the heterolithic facies association of theJordan Sandstone in the context of larger-scale depositional features is dis-cussed below.

Fourier Analysis of Foreset Thickness Data.—The thickening andthinning pattern of the foresets (tidal bundles) is quantitatively revealed ina spectral analysis of the data (Fig. 9). We used a discrete Fourier transformfunction in a Matlab script that effectively slides a window of a specifiedwidth across the data set in specified increments. Each snapshot through awindow provides a frequency spectrum (Figs. 9D–F), and the spectra arethen stacked and contoured to provide a thorough look at the entire dataset (Figs. 9B, C). In the contour plots, the peaks are normalized to themaximum height of any single peak within the individual spectra. Theperiods of the peaks with relative spectral power . 0.5 are labeled.

The contour plot of the Left Section data reveals cycles with periods of

15, 16, 22, 27, 31, and 34 bundles (Fig. 9B). A typical individual spectrumfrom the Left Section contains a peak at about 15 bundles and a peak at27–33 bundles (Fig. 9E). The right half of the Right Section (windowmidpoint . bundle #162) contains a peak at 29 bundles; the left halfcontains a peak with a period of 110 bundles (Fig. 9C). No peaks withperiods of approximately 2 bundles (f 5 0.5), corresponding to a diurnalinequality (de Boer et al. 1989), were observed in either data set. We alsoemployed the spectral methods of Archer et al. (1995) and did not detecta diurnal-inequality signal.

What periods would we expect tidal cycles to have during Cambriantime? Analyses of more continuous tidal rhythmite deposits from lower-energy, offshore settings have calculated an increase in the length of thesynodic month since the Neoproterozoic as approximately one day (Table1; Sonnet and Chan 1998; Williams 2000). Such paleo-tidal studies supportmodern laser-ranging measurements that indicate an increasing Earth–Moon distance of a few centimeters per year (Dickey et al. 1994). Thus,in a Cambrian tidal system we would expect approximately 30 or 15 ebb–


FIG. 11.—Schematic of dune migration during one ebb–flood cycle, based on thesedimentary structures observed in the cross-set of interest. During the peak-ebb-current stage (Stage 1), the dune migrates, and back-flow ripples form on the lowerslipface (A in figure). During the peak flood current stage (Stage 3) ripples (B)migrate across the bottomset of the dune, and the mud and some of the sand fromthe upper slipface of the dune is eroded to form a reactivation surface (C). Whenthe ebb current resumes (bottom of diagram, repeat Stage 1) at the start of a newebb–flood cycle, mud is removed from the upper reaches of the dune (cf. Visser1980).

TABLE 1.—Selected paleoastronomical values determined from Neoproterozoicrhythmites in Australia, in comparison with modern astronomical values (after

Williams 2000).

ParameterNeoproterozoic

(;620 Ma) Modern

A Length of solar day (hours)B Lunar days per synodic monthC Lunar days per neap–spring–neap cycle (51/2 3 B)D Solar days per synodic monthE Synodic months per yearF Solar days per year (5 D 3 E)G Hours per year (5 A 3 F 5 constant)**

21.9 6 0.429.5 6 0.5*14.8 6 0.3*30.5 6 0.513.1 6 0.1*400 6 78,766.7**

24.0028.5314.2729.5312.37

365.248.766.7

* Primary value determined directly from rhythmite data (Williams 2000).** Value G, the period of the Earth’s rotation about the Sun, is assumed to be the same during the Neo-

proterozoic as it is now.

TABLE 2.—Expected number of tidal bundles per sequence for a Cambrian tidalsystem.

Tidal SystemTidal Character

F-value**Ebb–Flood Cycles

Per FortnightTidal BundlesPer Sequence*

semidiurnalsemidiurnal–mixeddiurnal–mixeddiurnal

F , 0.250.25 , F , 1.5

1.5 , F , 3.0F . 3.0

3030 (variable)15 (variable)15

3015–3015–30

15

* Assumes preservation of every dominant current stage of the ebb–flood cycles that exceeds the thresholdvelocity of sediment transport (cf. Nio and Yang 1991b). For the mixed systems, this represents a maximumrange.

** The tidal character F-value determines the type of tidal system: F 5 (K1 1 O1)/(M2 1 S2), where K1and O1 are diurnal tidal constituents and M2 and S2 are semidiurnal constituents (Lisitzin 1974). (Note thatthis value is not calculated for the rocks in this study.)

flood cycles per fortnight; the corresponding sediment record would be 30bundles per sequence for a Cambrian semidiurnal system and 15 for adiurnal system (Table 2). In a mixed system, which exhibits a diurnalinequality, a range of approximately 15–30 bundles should be expected,because not all of the dominant currents of the ebb–flood cycles would bestrong enough to transport sediment and deposit tidal bundles.

The number of bundles per sequence in the cross-set is consistent withthe expected approximate range of 15–30 for a Cambrian tidal regime (Fig.9G). Because each bundle sequence corresponds to a fortnight, the 26 bun-dle sequences represent approximately thirteen Cambrian months. The pres-ence of different cycles in the bundle-thickness data (Figs. 9B–F) and dif-ferent counts of bundles per sequence (Fig. 9G) could be attributed to oneor a combination of the following scenarios: (1) a mixed tidal system, (2)the occasional removal of bundles by storm-generated currents, (3) the lackof tidal-bundle deposition during certain neap intervals (bedform does notmigrate, because of diminished tidal currents), or (4) error in identifyingtidal-bundle boundaries. The first three of these possible scenarios wouldcause the number of bundles per sequence to be shorter than the numberof ebb–flood cycles per fortnight. The bundle sequences (Fig. 9G) andcycles (Fig. 9B) with . 30 bundles are probably longer, because of error

in defining false tidal-bundle boundaries. The peak at 110 bundles in Figure9C is poorly resolved, and we disregard it as a forcing tidal cycle.

A spectral peak with a period of about 30 bundles is the predominanttidal cycle in the data set (Fig. 9B, C). This suggests that the tidal depo-sitional system was predominantly semidiurnal, whereby each day the ebbcurrent (deceleration stage) from each of the two ebb–flood cycles depos-ited a tidal bundle. Two interpretations of the tidal system are possible: (1)a semidiurnal system with no diurnal inequality, or (2) a semidiurnal–mixedsystem with a diurnal inequality. For the first case, one could argue thatall the bundle sequences with , 30 bundles (Fig. 9G) and the cycles withperiods , 30 bundles (Fig. 9B, C) represent truncated semidiurnal cycles.Data analysis of the bundle thickness patterns indicates no diurnal inequal-ity, i.e., no thick–thin–thick pattern in the bundles. Brest, France, is a mod-ern example of a semidiurnal system with no diurnal inequality (Kvale etal. 1999). The second case suggests that the abundance of sequences andcycles with , 15 bundles (Figs. 9B, G) indicate the influence of a diurnalpattern in a semidiurnal–mixed system, where the diurnal inequality waspresent, but its record in the sediment was, for the most part, obscured byrandom variation (Nio and Yang 1985). We favor the second interpretationand note that modern semidiurnal systems typically exhibit a diurnal in-equality (Kvale et al. 1999).

Analyses of modern tides (water level) have provided insight into therelationship between the tidal system and the forcing astronomical cycle(Archer et al. 1995; Kvale et al. 1999). One observation is that not allneap–spring–neap cycles are linked with phase changes of the Moon (syn-odic month). For example, the neap–spring–neap cycle in some diurnalsystems is due to declinational changes of the Moon with respect to theEarth (tropical month) (Kvale et al. 1999). In our case, the periods of ,30 bundles per fortnight represent ebb-current deposition in a predomi-nantly semidiurnal system. Thus, the neap–spring–neap cycles in the cross-set are indeed linked with the phase change of the moon: spring tides occurwhen the Earth–Moon–Sun are in syzygy, neap tides during quadrature.


FIG. 12.—Field photographs of the heterolithic facies associations with recognizable tidal bundling. A) Quarry in Mankato, Minnesota (farthest west occurrence, labeledM in Figure 1B). This location shows meter-scale cosets separated by bounding surfaces inclined seaward (southward) at angles ranging from 5 to 158. B) Close-up ofMankato quarry face showing conspicuous tidal bundling in the heterolithic facies association. C) Outcrop near Rushford, Minnesota (R in Figure 1B). D) Outcrop nearWeaver, Minnesota (W in Figure 1B).

LARGER-SCALE DEPOSITIONAL CONTEXT OF TIDAL FACIES

The heterolithic facies association can be recognized in a number ofoutcrops distributed regionally across the UMV (Figs. 1B, 12). These oc-currences range in age across at least five conodont zones (Runkel et al.1999; J.F. Miller, unpublished conodont identifications). Mesoscale strati-graphic relationships in the best preserved and exposed examples of theheterolithic facies association in the UMV have sedimentologic attributessuggestive of deposition during shallowing conditions, and show aggra-dational to progradational geometries regionally. Cross-strata sets typicallydisplay an upward decrease in set thickness and an increase in grain size.Where the heterolithic facies association is not directly capped by the re-gional Cambrian–Ordovician unconformity, cosets are locally overlainsharply by planar-laminated, medium- to coarse-grained sandstone, whichapparently records the establishment of a swash zone (Runkel 1994). Cross-strata sets with tidal bundles like those at the outcrop near Homer are alsowell exposed in a quarry near Mankato, Minnesota (Fig. 12A, B), wherethey occur as meter-scale cosets separated by bounding surfaces inclinedseaward (southward) at angles ranging from 5 to 158. Similar tidally gen-erated cross-strata cosets arranged in seaward-stepping fashion have beendocumented in other ancient sandstones and in modern environments, andthey have been interpreted variably as the product of deposition in mi-grating-sand-wave, bar, and delta-front systems (cf. Nio and Yang 1991a,fig. 14; Dalrymple 1992, fig. 18; Mellere and Steel 1995; Willis et al. 1999).Variability in the dominance of ebb versus flood currents, recorded at sev-eral exposures of the heterolithic facies association, is common in suchsystems, and is a response to changes in the geometry of mesoscale accre-tionary features, as well as to changes in even larger-scale coastline geo-morphology (Dalrymple et al. 1990; Morales 1997).

Facies shown in this study to be tidal in origin invariably rest upon asharp, irregular surface developed on middle to upper shoreface deposits.Such contacts are ravinement surfaces, previously recognized as a commonfeature in the Jordan Sandstone in the UMV (Runkel 1994). The surfacesmay be flat or display broad swales with relief of one to two meters. Somesurfaces are highly irregular, displaying as much as tens of centimeters ofsharp relief developed on underlying sandstone. Associated ichnostructuresare dominated by vertical burrows that locally are markedly wider and moredeeply penetrating than those of the Skolithos ichnofacies characteristic ofUpper Cambrian shoreface lithofacies in the UMV (e.g., Runkel 1994;Byers and Dott 1995). Surfaces are typically overlain by a poorly sorted,locally burrow-mottled, fine- to coarse-grained sandstone lag containingintraclasts composed of shale, siltstone, and sandstone. Large (tens of cen-timeters) angular intraclasts composed of fine- to coarse-grained sandstoneare locally common, some with recognizable cross-stratification and tracefossils that are absent in the surrounding matrix. The preservation of suchlarge, angular, coarse-grained intraclasts attests to the partial lithificationof the shoreface and foreshore deposits that were eroded during ravinement.

Shoreface retreat and ravinement in this succession were first interpretedin the lower interval of the St. Lawrence–Jordan by Runkel (1994; i.e., his‘‘transgressive tongues’’), however, that study did not explore the relation-ship between ravinement and facies in the upper Jordan Sandstone, as de-scribed here. This study establishes the presence of ravinement surfacesseparating wave-generated facies from tidally generated facies, and sug-gests that episodes of shoreface retreat played an important role in thegeneration of tidal deposits in this sheet sandstone. That is, tidally domi-nated deposits filled a ravined topography that was repeatedly developedon the updip parts of the shoreface. Resulting coastal geomorphology, ac-


FIG. 13.—Depositional and sequencestratigraphic model for the St. Lawrence–Jordansuccession, with focus on the genesis of facieswith a distinct tidal signature. A) Progradation oflargely wave-dominated facies during a sustainedrelative fall in sea level. B) Relative rise in sealevel and ravinement, leading to the localdevelopment of shallow, flooded embaymentsalong the coastline and intraclastic lag deposits(clasts). Embayments are relatively protectedfrom wave action, and have an increased tidalprism. C) Progradation during relative fall in sealevel. Embayments along the coastline are filledwith the heterolithic facies association, recordingthe dominance of tidal currents in transportingand depositing sediment. Continued progradationleads to the eventual return to a fully wave-dominated shoreface. D) Relative rise in sealevel and ravinement, leading again to the localdevelopment of shallow, flooded embaymentswith an increased tidal prism. E ) Progradationduring relative fall in sea level; embayment filledwith the heterolithic facies association. HCS-SCS5 hummocky and swaly cross-stratification.

companied perhaps by larger-scale changes in basinal conditions and/orconfiguration, led to at least local changes in depositional conditions fromwave-dominated to tide-dominated. This geomorphology may have been amore irregular shoreline with greater submarine topographic relief whencompared to wave-dominated shorelines. The development of such a shore-line produced the coastal geometry and local accommodation space nec-essary to create and preserve the tidal deposits.

Particularly compelling evidence that upper Jordan Sandstone tidal facieswere deposited in protected coastal settings (presumably carved by marineravinement) is that shale is relatively abundant in this facies for a UMVcratonic sheet sandstone. An alternation along the Cambrian paleocoastlinebetween (1) embayments where tidal deltas and/or sand waves were de-posited and (2) stretches of wave-dominated strandline where relativelymud-free wave-generated deposits dominated would explain the spatial andtemporal distribution of facies in the upper interval of the Jordan Sandstoneacross the UMV.

DISCUSSION

The Jordan Sandstone is the most cratonward record of epicontinentalsedimentation preserved for Cambrian North America (Lochman-Balk1971). Fundamental to the identification of tidal bundles in the JordanSandstone were two features that are not common for lower Paleozoic sheetsandstones of this area, and may explain why tidal bundles have not beenhitherto recognized. First, exceptional lateral exposure provides the dataset required for measurements of tidal periodicities. Secondly, an unusuallyhigh shale content for nearshore facies accentuates the foreset thicknesspattern and provides a cipher for the distribution of time through the cross-set. Recognition of tidal bundling in a sheet sandstone deposited along thefar inboard shoreline of the vast early Paleozoic epicontinental sea broadens

our understanding of the sedimentary and hydrodynamic processes in epi-continental settings. In particular, the tidal deposits in the Jordan Sandstoneprovide a much-needed gauge for the relative importance of tides vs. waveaction along the Cambrian paleoshoreline.

The tidal facies documented here recur in stratigraphically predictablefashion, and are distributed in sufficient geographic and chronostratigraphicextent (Runkel et al. 1999) to have constituted a fundamental componentof the St. Lawrence–Jordan depositional system in the UMV. This suggeststhat inboard early Paleozoic seas may be more comparable to youngernearshore epicontinental seas than commonly characterized. A similar jux-taposition of tidal facies on top of wave-dominated nearshore deposits isincreasingly recognized in ancient nearshore marine siliciclastic sectionsdeposited in a variety of settings (e.g., Greb and Archer 1995; Prave et al.1996; Cross 1998). Nearshore deposits of the Cretaceous Interior seawaycontain wave- and tide-generated facies akin to those in the Jordan Sand-stone (e.g., Mellere and Steel 1995; Mellere 1996; Meyer et al. 1999; Bhat-tacharya and Willis 2001; Seidler and Steel 2001; Willis and Gabel 2001).As with the Upper Cambrian of the UMV, the recurrence of tidal facies inthese Cretaceous strata is typically attributed to the intermittent develop-ment of shoreline embayments in which the tidal prism is increased duringa flooding event that marks a turnaround from regressive to transgressiveconditions (e.g., Seidler and Steel 2001; Mellere 1996). Although the pre-cise mechanism responsible for the development of such embayments isdebated, over the past ten years most sequence stratigraphic analyses oftidal facies have attributed their genesis largely to fluvial incision duringlowstands that immediately preceded flooding. The stratigraphic and sedi-mentologic context of the tidal facies in the Jordan Sandstone instead favorsdeposition in embayments developed by ravinement processes during flood-ing, similar to the scenario proposed by Bhattacharya and Willis (2001)


for parts of the Upper Cretaceous Frontier Formation in the Powder Riverbasin of Wyoming. During greenhouse times, such as the Late Cambrianand Cretaceous, the shoreface retreat necessary to cause repeated ravine-ment may have been driven by mechanisms other than glacioeustasy, suchas variations in sediment supply, subsidence, or climate.

The most thoroughly studied epicontinental tidal deposits in North Amer-ica are perhaps the rhythmites of the Carboniferous inboard of the Ouach-ita–Appalachian belt (e.g., Heckel 1986; Kvale et al. 1989; Kvale and Ar-cher 1990; Martino and Sanderson 1993; Greb and Archer 1995; Tessieret al. 1995; Miller and Eriksson 1997; Kvale et al. 1999). At our presentlevel of understanding, the Late Cambrian shoreline in the UMV is thoughtto have had none of the influences believed responsible for the proliferatetidal deposits in the Carboniferous (see review in Archer 1998). Theseinclude (1) icehouse glacioeustasy (tens of meters of sea-level fluctuationwithin the Milankovitch band), (2) densely vegetated tropical coastal plainsthat, together with high-amplitude glacioeustatic sea-level changes, createda coastline riddled with incised valleys that acted to focus and enhancetidal flow, (3) an exceptionally large open ocean (Panthalassa) that mayhave increased tides globally, and (4) an oceanic trough-like basin adjacentto the tidal-prone coasts (i.e., the Appalachian Basin). Furthermore, it isworth revisiting the traditional view that Cambrian–Ordovician NorthAmerica was surrounded by vast, shoal-water carbonate banks hundreds ofkilometers wide (Dott and Batten 1981), which would have damped thepropagation of the tidal wave onto the Cambrian continental interior.

Few studies of tidal wave propagation onto the epicontinental shelf arebased on hydrodynamic principles. Calculating bottom slope stability foran epicontinental sea, Keulegan and Krumbein (1949) concluded that thepresence of tidal currents of sufficient strength to transport sand near theshores of early Paleozoic epicontinental seas was ‘‘perhaps doubtful’’ (p.860). Irwin (1965) suggested that these epicontinental seas were too shal-low to transmit significant tidal energy from the open oceans to the floodedcontinental interiors. These theories have seen limited development (e.g.,Hallam 1981), and the modeling of tidal wave propagation through theNorth American epicontinental sea remains open for study. The identifi-cation of ancient tidal systems from various areas of North America, to-gether with paleogeographic reconstructions of the sea-floor topographyand paleocoastline, should provide constraints for the modeling problem(Archer 1998).

Our study shows that tides were indeed significant agents of sedimenttransport along inboard areas of the Cambrian paleocoastline. Thus, theJordan Sandstone represents the farthest recorded propagation of the tidalwave in the North American epicontinental seas (Cambrian or Carbonif-erous). This study presents evidence against the notion of tideless sedi-mentation in the Cambrian–Ordovician epicontinental seas of the midcon-tinent, leading one to question the validity of the assumption of the shal-lowness of these seas (Irwin 1965). Paleo-water depth is difficult to deter-mine from ancient deposits, but it must have been sufficiently great to allowresonance with the open ocean tides.

CONCLUSIONS

Conspicuous tidal-bundle sequences (semidiurnal or mixed) within awidespread facies of the Sunwaptan Jordan Sandstone were analyzed atone superb outcrop in the Upper Mississippi Valley. The occurrence of thisfacies throughout the Upper Mississippi Valley indicates that tidal currentswere significant agents in the transport of sand along the far inboard shore-lines of Cambrian North America. Our depositional and sequence strati-graphic model for deposition of the upper Jordan Sandstone, therefore, isas follows (Fig. 13). Shoreface ravinement led to the development of coast-al embayments that increased the tidal prism and afforded protection fromwave action to the extent that: (1) nearshore deposition of suspended sed-iment increased, (2) tidal currents were strong enough to transport coarsesand, and (3) tidal sand dunes, such as the one responsible for the cross-

set measured in this study, migrated undisturbed for relatively long timeintervals (e.g., 13 Cambrian months). Ravinement created the geomor-phology and topography that allowed both creation and preservation ofextensive sets of cross-strata generated by relatively large tidal dunes. Rec-ognition of a widespread tidal record in the Upper Cambrian of the cratonicinterior of North America and its recurrence in a specific, ravinement-related sequence stratigraphic position strengthens our understanding ofthese deposits that have long been considered enigmatic.

ACKNOWLEDGMENTS

Thoughtful and thorough reviews by Bob Dalrymple, Erik Kvale, and Paul Myrowsignificantly improved this manuscript. We also acknowledge the support of theCarleton College Geology Department, the Kresge Foundation, and the MinnesotaGeological Survey.

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Received 6 June 2001; accepted 16 September 2002.

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TIDAL-BUNDLE SEQUENCES IN THE JORDAN SANDSTONE … · 2004-02-18 · TIDAL-BUNDLE SEQUENCES IN THE JORDAN SANDSTONE (UPPER CAMBRIAN), SOUTHEASTERN ... same facies in outcrops of lesser