New descriptions of the cap dolostone and associated ... · 2011 New descriptions of the cap...

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The Geological Society of AmericaField Guide 21

2011

New descriptions of the cap dolostone and associated strata, Neoproterozoic Pocatello Formation, southeastern Idaho, U.S.A.

Carol M. DehlerUtah State University, Department of Geology, 4505 Old Main Hill, Logan, Utah 84322-4505, USA

Kathleen AndersonDepartment of Geological Sciences, Brigham Young University, S389 ESC Provo, Utah 84602, USA

Robin NagyUtah State University, Department of Geology, 4505 Old Main Hill, Logan, Utah 84322-4505, USA

ABSTRACT

An ~90-m-thick interval of mixed siliciclastic-carbonate strata, including a cap dolostone unit, overlies diamictite of the upper Scout Mountain Member of the Pocatello Formation in the Fort Hall Mine area south of Portneuf Narrows, south-eastern Idaho, and is ≤ ca. 665 Ma. Six facies comprise this interval: silty sandstone (reworked diamictite matrix), laminated dolomite, dolomite and sandstone, sand-stone, dolomite-chip breccia, and argillite and limestone. Sedimentary structures and bedding geometries of facies indicate paleoenvironments ranging from below storm wave base to upper shoreface. The edgewise, mounded, and parallel-bedded dolomite-chip breccia indicates slope failure and reworking of the lower shoreface during large storms.

Facies relationships allow generalized division of these strata into three units. The lowermost unit, Unit A, comprises intimately interbedded laminated dolomite (“cap dolostone”), dolomite-chip breccia, and sandstone facies and is 17 m thick. Unit A apparently grades upward into Unit B, a 45-m-thick interval of the sandstone facies. Unit C, 28 m thick, rests sharply on Unit B, and comprises a basal laminated dolomite facies and the limestone and argillite facies. Units A and B may indicate a regressive wave-dominated coast that was infl uenced by large storms (highstand systems tract). Unit C indicates near storm wave base deposition and an overall deepening, as shown by dark argillite beds of the overlying upper member of the Pocatello Formation (transgressive systems tract).

δ13C and δ18O values from dolomite and limestone samples of Units A and C are similar to values from local, regional, and transglobal cap carbonate intervals. δ13C values range from −1.9 to −5.6‰ and δ18O values range from −10.2 to −17.4‰, with no systematic correlation between C- and O-isotope values. δ13C values are consis-tent with previously reported values from the Pocatello Formation and are similar to

Dehler, C.M., Anderson, K., and Nagy, R., 2011, New descriptions of the cap dolostone and associated strata, Neoproterozoic Pocatello Formation, southeastern Idaho, U.S.A., in Evans, J.P., and Lee, J., eds., Geologic Field Trips to the Basin and Range, Rocky Mountains, Snake River Plain, and Terranes of the U.S. Cordil-lera: Geological Society of America Field Guide 21, p. 181–192, doi:10.1130/2011.0021(08). For permission to copy, contact [email protected]. ©2011 The Geological Society of America. All rights reserved.

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INTRODUCTION

Neoproterozoic cap carbonate units, which overlie glacial and subaqueous mass fl ow deposits worldwide, are interpreted to indicate the recovery from glacial conditions of controver-sial severity (e.g., Hoffman et al., 1998; Fairchild and Kennedy, 2007; Allen and Etienne, 2008; LeHeron et al., 2011). Cap dolostones (the basal transgressive tract of the cap carbonate unit) with distinctive lithologic and geochemical similarities to the type Marinoan carbonate (Keilberg member, Maiberg For-mation, Namibia; Hoffman, 2011) are presumed to be 635 Ma everywhere and thus are used to mark the base of the Ediacaran Period of the latest Neoproterozoic (Condon et al., 2005; Knoll et al., 2006). The Marinoan cap dolostone is typically pink to tan, has an average global thickness of 18.5 m, and may contain some or all of the following, depending on water depth: parallel- and (or) cross-laminated microcrystalline dolomite, peloids, geop-lumb stromatolites and(or) tubestone, sheet cracks, giant wave ripples, teepee structures, barite and aragonite crystal fans, and negative δ13C values that decrease upward (e.g., Hoffman et al., 2007; Hoffman, 2011). “Sturtian” cap carbonate units, which are interpreted to predate the Marinoan glaciation and range in age from 660 to 720 Ma (Hoffman and Li, 2009), are less well stud-ied. These cap units are organic-rich, dark gray, iron-rich, contain rhythmic and anastomosing laminae, and have δ13C values >0 (Kennedy et al., 1998). Despite the importance of cap carbonate units in understanding deep-time paleoclimate and their potential utility as time lines, few alleged “Sturtian” cap units have been well described and almost no cap units have been placed in a solid geochronologic framework.

Previous work on the cap dolostone and associated strata of the Pocatello Formation indicates that they represent deposi-tion at ca. 665 Ma (Fanning and Link, 2004, 2008; Dehler et al., 2009), yet the lithologic and isotopic character of the strata are similar to alleged Marinoan cap carbonate units (Lorentz et al., 2004; Kirkham et al., 2009; Anderson and Dehler, 2010), only one of which has been directly dated at 635 Ma (Doushantuo For-mation; Condon et al., 2005). This discrepancy has implications for our understanding of the number and nature of post-glacial alkalinity events, and for testing the snowball Earth hypothesis (Hoffman et al., 1998).

The cap dolostone and associated strata in the Pocatello For-mation are important because they represent one of few cap car-bonate units in western Laurentia and one of the only cap inter-vals with geochronologic control. Until now, most information on the Pocatello cap interval has come from two localities, north and immediately south of the Portneuf Narrows (also known as

Portneuf Gap), Idaho (Locations 1 and 2, Fig. 1). However, the sections at these two localities are incomplete. The northern sec-tion lacks a cap dolostone, and the southern section, although it does exhibit a 1-m-thick cap dolostone, is only ~10 m thick and is poorly exposed.

Herein we present preliminary facies, stratigraphic, and stable isotopic data from newly discovered exposures of the cap dolostone and associated strata of the Pocatello Formation, south of Portneuf Gap in the Fort Hall area (Locations 3 and 4, Fig. 1). The results from these new measured sections mod-ify previous stratigraphic and sedimentologic interpretations, which are key to understanding this cap carbonate unit and how it relates to others.

Scout Mountain Member of the Pocatello Formation

The Pocatello Formation is a >1.5-km-thick mixed silici-clastic, carbonate, and volcanic unit that is exposed for 100 km along strike between the Pocatello area of southern Idaho, south to the Utah portion of Cache Valley (Fig. 1). The forma-tion comprises 3 members, has no exposed base, and is overlain by the Blackrock Canyon Limestone (Link, 1983). The lowest member, the Bannock Volcanic Member (200–450 m thick), is composed of mafi c metavolcanic rocks (pillow lava, agglomer-ate, and greenstone) and volcaniclastic rocks (Link et al., 1994; Keeley and Link, this volume). This unit grades upward into the Scout Mountain Member (~<800 m thick), which contains a wide variety of lithotypes including quartz, arkosic, and lithic arenite, argillite, clast-supported orthoconglomerate, intra- and extra-basinal diamictite, and subordinate limestone, and dolo-stone. This unit grades upward into the informal upper member of the formation, which comprises (>600 m thick) of laminated argillite with thin sandy beds.

The Pocatello Formation is interpreted to represent one of many rift basins that were fed by glaciers along the western Cor-dilleran margin in late Neoproterozoic time (Stewart, 1972; Link, 1982). The Bannock Volcanic Member mafi c volcanic rocks are thoeliitic-alkaline to alkaline in composition and indicate intra-plate volcanism, which is consistent with a rift basin setting (Harper and Link, 1986). The Scout Mountain Member records rapid deposition of immature sediments, including interpreted glacial and resedimented glacial deposits. Evidence for a glacial origin is limited, but the lateral extent of the deposits (100 km), rare striated clasts, and correlation with glacial deposits along the Cordilleran margin argue for a glacial origin (e.g., Crittenden et al., 1983; Link et al., 1994). The carbonate and sandstone units in the upper Scout Mountain Member indicate tidal, shoreface, and

values from the alleged Marinoan Noonday Formation in Death Valley, California, and the Marinoan Maieberg Formation in Namibia.

Collective data from the cap dolostone and associated strata of the Pocatello Formation suggest protracted mixed siliciclastic-carbonate deposition on a storm- dominated shelf at ca. 665 Ma.

Cap dolostone and associated strata, Neoproterozoic Pocatello Formation 183


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Pocatello Formation upper member unit B--quartzite unit A--phyllite

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Pocatello Formation Bannock Volcanic Member

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Strike and dip of overturned bedding

Strike and dip of vertical bedding

Anticline axial trace: dashed where approximate,dotted where concealedOverturned anticline axial trace : dashed where approximate,dotted where concealed

SYMBOLS

MAP UNITS

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Figure 1. Geologic map showing distribution of Pocatello Formation members in the Portneuf Gap area. Measured sections 1 through 4 are shown with red lines. The “upper sandstone and limestone unit” of the Scout Mountain Member is the focus of this study. Map modifi ed from Rodgers et al. (2006).

184 Dehler et al.


deeper water conditions (Link, 1983). The upper member of the Pocatello Formation is interpreted to indicate an overall deepen-ing caused by post-glacial eustatic rise (Link et al., 1994).

CAP DOLOSTONE AND ASSOCIATED STRATA OF THE FORT HALL AREA

What has traditionally been called the cap dolostone of the Pocatello Formation is a 1-m-thick “pink dolostone,” which sits

in sharp contact on a prominent extrabasinal diamictite known as the “upper diamictite” and is located south of the Portneuf Gap (Link, 1982, terminology; Section 2 in Figs. 2, 3, and 4). Newly explored outcrops along strike of this cap dolostone to the south reveal a thicker section of cap dolostone, which is intimately inter-bedded with dolomite-chip breccia and sandstone (Sections 3 and 4 in Figs. 2, 3, and 4). North of Portneuf Gap, the cap dolostone is absent and instead, a meter-thick “dolomite-chip breccia” lies directly on the upper diamictite (Link, 1982, terminology).

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Figure 2. Stratigraphic column showing upper Neoproterozoic (Cryogenian and Ediacaran) strata of southeastern Idaho (left) and gener-alized stratigraphy of the Pocatello Formation, and location of the cap-carbonate sequence in the upper Scout Mountain Member. Modifi ed from Fanning and Link (2004). The 705 Ma maximum depositional age is reported from Keeley and Link (this volume). The 680 and 665 Ma maximum depositional ages are re-ported from Dehler et al. (2009).



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186 Dehler et al.


North Fort Hall Section 3

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FACIES

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Figure 4. Examples of lateral and vertical facies changes and depositional environments of the basal cap carbonate interval in the South Gap area. Note the lateral discontinuity of different carbonate facies. See Figure 1 for location of measured sections.



Cap-Carbonate Interval Geochronology

Zircon U-Pb geochronology from volcaniclastic units in the cap dolostone and associated strata indicates deposition ≤665 Ma. A green reworked tuffaceous siltstone with abun-dant euhedral, zoned igneous zircons lies near the top of Unit B (~45 m from base of interval, North Gap locality; Figs. 1, 2, and 3) and yields a U-Pb age of 667 ± 5 (sensitive high-resolution ion microprobe [SHRIMP] concordia age; Fanning and Link, 2004, 2008). Igneous zircon grains, also euhedral and zoned, from fi ne-grained sandstone interbeds in the basal cap dolostone interval, Unit A (4 m above base of interval of the Fort Hall section; Figs. 2 and 3) yield a similar age of 666 ± 6 Ma (SHRIMP concor-dia age; Dehler et al., 2009), which provides the understanding that the cap dolostone and associated strata are ca. 665 Ma or younger. The upper diamictite matrix immediately below the cap dolostone interval at the Fort Hall locality yields a maximum depositional age of ca. 680 Ma, as does an arkosic sandstone unit ~110 m from the base of the Scout Mountain Member (SHRIMP concordia ages on youngest populations; Fig. 2; Dehler et al., 2009). The basal Scout Mountain Member yields a maximum depositional age of 705 ± 5 Ma (SHRIMP concordia age; Keeley and Link, this volume). Regionally, the upper age constraint on the cap dolostone and associated strata is a ca. 580 Ma Rb-Sr date on the Browns Hole Formation in northern Utah, which cor-relates with local strata that is ~2 km upsection (Christie-Blick and Levy, 1989).

The young zircon populations in the cap dolostone and asso-ciated strata are interpreted to have been derived from approxi-mately syndepositional volcanism and thus likely very closely bracket maximum depositional ages. This interpretation is based on the progressively younger zircon ages found upsection in the Pocatello Formation, lack of young zircons in overlying forma-tions, and correlation to other dated post-glacial deposits. The maximum depositional age of the lowermost Scout Mountain Member is 705 Ma ± 5 Ma (Keeley and Link, this volume). The maximum depositional age for the diamictite and lower units in the overlying Scout Mountain Member is ca. 680 Ma and the maximum depositional age of the cap dolostone and associated strata, Units A and B, in the upper Scout Mountain Member is 666 Ma ± 6. This younging of ages (from 705 Ma to 680 Ma to 666 Ma) would be more likely preserved if volcanism occurred as sediments were being deposited. Young zircon grains are stratigraphically constrained to the Pocatello Formation. Detrital zircon geochronologic analyses conducted on overlying Neo-proterozoic and Cambrian units in the region show a dearth of 705–665 Ma grain populations (Stewart et al., 2001; Keeley et al., 2009; Reed et al., 2010; Link et al., 2011). The 666 Ma date of the cap dolostone interval from the Pocatello Formation also overlaps in age with two other post-glacial deposits. The post-glacial black shale of the Aralka Formation, Australia is 657 ± 5 Ma (Re-Os on black shale, Kendall et al., 2009), and the cap carbonate in the Datangpo Formation, south China is 663 ± 4 Ma (U-Pb zircons in ash, Zhou et al., 2004). On the other hand, as

described below, the cap dolostone and associated strata share sedimentological and isotopic similarities with Marinoan and alleged Marinoan cap-carbonate intervals worldwide, most of which, however, lack good geochronologic control.

Facies Analysis and Paleoenvironmental Interpretation

Seven facies are described herein and include diamictite, silty sandstone (reworked diamictite matrix), laminated dolo-mite, dolomite and sandstone, sandstone, dolomite-chip breccia, and argillite and limestone facies (Table 1).

The upper diamictite in the newly described sections (Local-ities 3 and 4, Figs. 1 and 2) was only examined immediately below the contact with the overlying cap interval. The chaotic fabric of the clasts and lack of glacial indicators suggests sub-aqueous mass fl ow deposition, although the sediments were likely derived from glaciers (Link, 1982; Crittenden et al., 1983). The water depth is diffi cult to discern due to a lack of sedimen-tary structures, but this facies rests on laminated argillite that rep-resents deep-water deposition.

The silty sandstone facies rests sharply on the diamictite and grades upward into the laminated dolomite facies. The silty sand-stone facies takes on the same color as the underlying matrix of the diamictite, is a calcareous silty sandstone with granules and pebbles of dolomite, and shows local symmetrical ripplemarks and laminations. This facies is similar sedimentologically to the overlying laminated dolomite facies (Table 1) and is interpreted to represent reworking of the diamictite during initiation of cap dolostone deposition. Although it is not yet known how much time might be missing on this reworked surface, it represents reworking of the underlying diamictite matrix and mixing with the dolomite facies.

The laminated dolomite (“pink dolostone” of Link, 1982) is very thinly bedded and becomes thinly bedded and grades into the dolomite and sandstone facies upsection on a meter scale (Fig. 4). Sedimentary structures in the laminated dolomite facies indicate deposition below storm wave base where it is purely dolomitic, to deposition near and above fair weather wave base where it is interbedded with rippled sandstone with rare hummocky cross stratifi cation (interbedded dolomite and sandstone facies). Link (1982) interpreted this facies to indicate deposition in a tidal fl at, but did not discount the potential of a deep-water origin. Within Unit A, this facies is considered to be the cap dolostone (Figs. 3 and 4). A similar dolomite facies appears at the base of Unit 3, yet is not considered part of the cap dolostone because it is separated by a potential bounding surface, lacks abundant lamination, and has different weathering characteristics (Figs. 3 and 4).

The dolomite-chip breccia facies occurs twice in meter-thick, laterally discontinuous beds, and can be traced laterally into the sandstone and dolomite facies (Fig. 4). Breccia clasts are com-posed of laminated dolomite and the matrix is sandstone, both of which are directly sourced from the sandstone and dolomite facies. Breccia clasts are platy and range in size from a few centimeters to a meter (long axis). Some of the breccia shows chaotically oriented

188 Dehler et al.


clasts including edgewise clasts, rosettes of clasts, and breccia mounds. Another subfacies of breccia has bedding parallel clasts in a sandy matrix, indicating reworking. Some of the clasts are bent to folded, indicating they were soft during transport.

The dolomite-chip breccia facies is very similar to intrafor-mational breccia units of the Upper Cambrian to Lower Ordovi-cian Snowy Range Formation, Wyoming (Myrow et al., 2004) and the Lower Cambrian Sellick Hill Formation, South Aus-tralia (Mount and Kidder, 1993), which were both interpreted to indicate mass fl ow and gravity slide processes on the lower shoreface, caused by large storms. Mount and Kidder (1993) sug-gest that the storms must have been very extreme to produce the observed chaotic fabrics. At the North Gap locality, the dolomite-chip breccia facies is exposed in a solitary 1-m-thick interval and rests directly on the diamictite. Link (1982) interpreted the brec-cia here to indicate a transgressive lag and that it was unconform-able with the laminated dolostone (in the cap dolostone interval). Now that these two facies are known to intercalate (Fig. 4), no unconformity is necessary between the two facies.

The sandstone facies is a dominantly massive subfeldspathic sandstone that is crudely and thickly bedded. Rare groove casts, plane beds, parting lineations, and trough crossbedding are pres-ent and, considering association with other facies, likely repre-sent the shoreface/foreshore environment. Link (1987) also inter-preted this facies to indicate a shallow marine to beach setting. At

the North Fort Hall locality, the basal sandstone has a channel-ized base. Trough crossbedding and rip ups of laminated dolo-mite are also observed at the base (Fig. 4). This facies has a few decimeter-scale interbeds of green argillite, some of which have yielded organic-walled microfossils (Fig. 5).

The argillite and limestone facies consists of covered inter-vals with subcrops of limestone and argillite. The argillite is green to brown and laminated. The limestone is bluish gray and shows laminations, replaced aragonite fans, and climbing ripples. The replaced aragonite fans are continuous on a decimeter scale and range in height from 2 mm to 2 cm. Lorentz et al. (2004) discovered replaced aragonite fans in this same facies and strati-graphic interval north of the Portneuf Narrows (Section 1, North Gap Section, Figs. 1 and 3). Because there is no indication of exposure and this facies is overlain by a signifi cant thickness of argillite of similar character, it likely indicates deeper water deposition. Other workers have interpreted aragonite fans in Neoproterozoic deposits to indicate deep water (and maximum fl ooding) (e.g., Hoffman, 2011).

Stratigraphy

An ~90-m-thick interval of strata rests sharply on the upper diamictite in the Fort Hall area south of Portneuf Gap. This inter-val is mapped as the “upper sandstone and limestone unit” of the

TABLE 1. FACIES WITHIN AND BELOW THE CAP-CARBONATE INTERVAL IN THE PORTNEUF GAP AREA noitaterpretnI noitpircseD seicaF

Diamictite Matrix: purple-gray, silt to coarse sand, poorly sorted. Clasts: granule to boulder, dominantly quartzite with lesser volcanic clasts.

Glaciomarine and reworked glacial deposits (Link, 1982).

Silty sandstone facies Dm-thick interval of calcareous siltstone, sand and granules. Purple-gray color, thin bedding, normal grading, sinuous symmetric and interference ripplemarks.

Reworked diamictite matrix.

Laminated dolomite Tan-pink color, thin bedding with internal lamination, peloidal dolomicrite to dolosiltite. Mud-draped symmetric and interference ripples, rare hummocky cross-stratification.

Lower shoreface to offshore, near or below fair-weather wave base, rare combined flow, density currents, microbial influence, suspension settling.

Interbedded dolomite and sandstone

Laminated dolomite facies rhythmically interbedded with fine-grained sandstone in thin to medium beds. Amalgamated and solitary hummocky cross-stratification (<20 cm high, 1 m wide), rare dolomite-chip breccia in hummocks, rare m-thick channel forms, symmetric and interference ripples, positive flute casts.

Upper to lower shoreface, near fairweather wave base, combined flow, density currents.

Sandstone Fine- to medium-grained sand, planar medium to very thick beds. Rare trough crossbedding. Small dolomite intraclasts (avg. diameter 3 cm) at base of some beds. Possible swaley beds. Few beds of green argillite and volcaniclastic material. Load, flute, groove casts, dish structures, parting lineations. Channelized base at North Fort Hall locality. Organic-walled microfossils.

Upper shoreface to foreshore or beach?

Dolomite-chip breccia Thickly bedded. Clast-supported. Clasts composed of laminated dolomite facies, typically tabular (<1 m on

long axis), syndepositional deformation. Fabric types: chaotic, fan-shaped, bedding-parallel. Scours, rare onlapping relationships, breccia mounds.

Deposits from large storms, combined flow, mass flow, gravity slide processes causing failure of lower shoreface.

Argillite and limestone facies

Meter-thick intervals of green and brown argillite interbedded with medium beds of bluish-gray limestone. Horizontal laminations, climbing ripples. Minor quartz silt component. “Ghost” aragonite fans (<3 cm tall).

Relative deepest water. Offshore. Suspension settling and crystal fan growth.



upper Scout Mountain Member on the Inkom 7.5′ quadrangle (Fig. 1; Rodgers et al., 2006). This interval is the focus of this study because it not only captures lateral and vertical facies changes of the cap dolostone, it also includes dolomite and limestone units higher in the section that may be related to the underlying cap dolostone unit. The top of the interval of study coincides with the contact between the Scout Mountain Member and the overly-ing upper member of the Pocatello Formation (Figs. 1, 2, and 3). The upper member is a relatively homogeneous argillaceous and sandy unit that is hundreds of meters thick (Link, 1982).

Facies relationships allow generalized division of these strata into three units. The lowermost unit, Unit A, comprises intimately interbedded laminated dolomite (cap dolostone), dolomite-chip breccia, and sandstone facies and is ~17 m thick. The relationship between the laminated dolomite and the other facies indicates dynamic mixed siliciclastic-carbonate deposi-tion that was alternating between below fair weather wave base to below storm wave base. The facies indicating deeper water

(laminated dolomite and sandstone and dolomite facies) are more common in the lower part of Unit A, whereas sandstone with hummocky cross-stratifi ed sandstone is higher in Unit A, indicat-ing an overall shallowing upward trend (Figs. 3 and 4).

Unit B is apparently gradational with underlying unit A and is a 45-m-thick interval of the sandstone facies. The sandstone within this unit is similar to the sandstone beds below except it is signifi cantly thicker, relatively homogeneous, and it is mas-sive for the most part. It is also different in that it contains sedi-mentary structures such as soft sediment deformation features, parting lineations, plane beds, faint rare crossbeds, and rare dolomite-chip lags. These features indicate the likely shoreface part of the wave-dominated system, whereas the interbedded sandstone facies below likely indicates deposition below fair weather wave base.

Unit C, 28 m thick, rests sharply on Unit B, and begins with a basal laminated dolostone facies that is 6 m thick. Above this, interbedded argillite, some sandstone, and medium beds of

Figure 5. Representative organic-walled microfossils (“acritarchs”) from green argillite of the Scout Mountain Member of the Pocatello Forma-tion (sample SMM-08-02), including: spine-bearing acritarchs (A, B); textured vesicles (C), and a smaller specimen comprising many minute vesicles (cf. Bavlinella faveolata) (D). Fossils were recovered via standard hydrofl uoric acid maceration techniques. Although these specimens have not yet been assigned a taxonomic identity, they likely represent marine eukaryotes (e.g., Vidal and Knoll, 1983; Servais, 1996). Scale bar is 10 microns for A–D.

190 Dehler et al.


limestone are present. This unit was only measured up to the last exposed limestone bed, which also marks the top of the Scout Mountain Member (Link, 1982; Rodgers et al., 2006). The change back to carbonate and argillite indicates an overall deep-ening and likely marks a fl ooding surface.

Correlation of the Fort Hall section with the other sections (Figs. 3 and 4) shows Unit A becoming more sandy toward the north and probably thinning below Unit B to a few meters at the North Gap locality. Unit B appears to maintain about the same thickness between the southernmost and the northernmost sec-tion. The basal dolomite unit of Unit C is not present in the North Gap section and limestone is more abundant (Fig. 4).

Stable Isotope Analyses

Dolomite and limestone samples from Units A and C were microdrilled to obtain powder for carbon- and oxygen isotope analyses. Methods of sample preparation and analyses are those followed in Dehler et al. (2005).

Resulting δ13C values range from −1.9 to −5.6‰ and δ18O values range from −10.2 to −17.4 ‰. A crossplot of these values from the South Gap localities shows overlap with values previ-ously reported by Lorentz et al. (2004) from the North Gap area, yet our values are typically more positive (Fig. 6). This difference in values is likely due to the fact that the majority of their samples came from Unit 3 equivalent in the North Gap area (Fig. 3), which has been partially altered to marble, and therefore, those values would be expected to partly refl ect metamorphism and show more negative values (e.g., Kaufman and Knoll, 1995). Values are also consistent with previous values reported by Smith et al. (1994).

The range of values reported herein is similar to those reported from the Noonday Formation of Death Valley, southeastern Cali-fornia, and from the Maiberg Formation of Namibia. The Sentinel Peak (cap dolostone) and lower Radcliff members of the Noonday Dolomite show a range of δ13C values from −1.8 to about –6.1‰ and δ18O values of −5 to about −15‰ (Petterson et al., 2011). Cap carbonate δ13C values from the Maieberg Formation in Namibia range from −3 to −5‰ and δ18O values are between −5‰ and −10‰ (Hoffman et al., 1998). The range of δ18O values from both the Pocatello Formation and the Noonday Dolomite are relatively low and could be an indication of meteoric diagenesis or meta-morphism (e.g., Kaufman and Knoll, 1995).

DISCUSSION

If there were no age constraints on the Pocatello cap dolos-tone and associated strata and no other reported ca. 660 Ma dates from other diamictite-cap carbonate bearing successions (Zhou et al. 2004; Fanning and Link, 2008; Kendall et al., 2009), the newly described sections would likely be considered 635 Ma in age because of their similarities to the Namibian and Doushan-tuo cap carbonate successions and other alleged Marinoan suc-cessions. However, the available geochronology, in combination with the geologic context, suggests that the Pocatello strata were deposited ca. 665 Ma. This interpretation has implications for using cap carbonate units as timelines (Halverson, 2006; Corsetti and Lorentz, 2006) and brings into question the use of the Mari-noan cap carbonate as the global stratotype section and point of the basal Ediacaran (Knoll et al., 2006). Unless the geochrono-logic context of a cap-carbonate unit is well constrained, or until

Figure 6. Isotope crossplot comparing carbon and oxygen isotope values from the South Gap area (this study) and the North Gap area (Lorentz et al., 2004). Note that the North Gap locality values are relatively more negative than those at the South Gap locality and are probably refl ecting diagenetic alteration or meta-morphism of the North Gap carbonates.



cap-carbonate units can be demonstrated to be coeval, caution must be taken in assuming synchroneity of deposition.

The sedimentology of the basal Pocatello cap dolostone unit indicates large storm events. In the basal (Marinoan) cap sequences on other continents, sedimentary structures interpreted to be giant wave ripples may also indicate infrequent storms with sustained high velocity winds (Allen and Hoffman, 2005). Inter-preted storm features in the Pocatello Formation may be a dif-ferent expression of the same type of post-glacial process. The Pocatello Formation cap dolostone unit, if ca. 665 Ma, indicates that there were at least two paleoclimate episodes (665 Ma, 635 Ma) that experienced similar post-glacial processes (e.g., large storms) resulting in geologically similar sequences.

Most diamictite-cap-carbonate contacts described in the lit-erature are interpreted to be gradational, despite many of them having sharp contacts (e.g., Fairchild and Kennedy, 2007). The Pocatello Formation diamictite-cap dolostone contact appears to be unconformable; it is sharp with a reworked zone of diamic-tite matrix above the contact. The presence of an unconformity is supported by a jump in young detrital U-Pb age populations across the contact, from ca. 680 Ma below, to ca. 666 above (Fan-ning and Link, 2004, 2008; Dehler et al., 2009; Keeley and Link, this volume). This relationship may have implications for our understanding of post-glacial processes.

Facies associations between the laminated dolomite of Unit A (cap dolostone) and other facies including the dolomite-chip breccia, sandstone and dolomite, and sandstone facies indicate that cap-carbonate deposition was not laterally continuous on a basinal scale, but rather these carbonates were deposited in a dynamic environment, mixing with siliciclastic sediment. Fur-thermore, Units A through C show negative C-isotope values like cap-carbonate units elsewhere, yet likely record at least one bounding surface. These points suggest that the cap dolostone and related strata of the Pocatello Formation record a protracted and dynamic history of mixed siliciclastic-carbonate deposition (cf. Kennedy and Christie-Blick, 2011).

CONCLUSIONS

Lithofacies, paleoenvironmental interpretation, and isoto-pic values are similar to other cap intervals that are dated as, or interpreted to be, Marinoan (635 Ma) in age, yet there are many arguments for the Pocatello cap interval to be ca. 665 Ma. Obser-vations from this interval contrast with generalizations about cap carbonate successions that include: (1) the contact between the underlying diamictite unit and the cap dolostone may be an unconformity; (2) the cap dolostone interfi ngers with several other facies, including siliciclastic facies; and (3) there may be at least one bounding surface within the overall post-diamictite interval. Because other post-glacial strata date to ca. 660 Ma, it may be that this is a 665 Ma cap carbonate interval that looks very similar to Marinoan cap carbonate intervals. These fi ndings have implications for using cap carbonates as global marker units and for interpreting them to indicate global alkalinity events.

DIRECTIONS TO CAP SEQUENCE EXPOSURES

From Interstate Highway 15, northbound or southbound, take the Fort Hall Mine/Landfi ll exit (Exit 63), ~3 mi southeast of Pocatello. Head south on Fort Hall Mine Road. Cross the rail-road tracks and the Portneuf River. Continue for 0.2 mi to Port-neuf Road. Cross this road and drive straight for 0.6 mi. At this point, take a left onto the dirt road (before the Fort Hall Mine Landfi ll entrance). Proceed along this road. It will curve to the south, up Fort Hall Mine Canyon, and will turn into an all-terrain vehicle track (1 mi). Park here and walk up road that leads south up to Fort Hall Mine to the east, until you are above the mine. Here will be a faulted section of the cobble conglomerate in the Scout Mountain Member. Walk up the ridge and, once you reach the diamictite, veer to the north toward the drainage. To see the units below in this member, walk up next drainage to north to fi nd the “lower diamictite” (correlative with Oxford Ridge Diamictite (see Keeley and Link, this volume). The upper diamictite forms a cliff near the top of the ridge. Sharply overlying the diamictite is the cap carbonate interval. The climb is about ~800 vertical feet total. Walk exposures along strike to the north to next ridge to see lateral facies changes in the cap sequence.

ACKNOWLEDGMENTS

This research was supported by National Science Foundation grant EAR-0819759. Field work was provided by Scott Sallay, Hans Anderson, Kelly Bradbury, Dawn Hayes, Esther Kings-bury, Liz Petrie, Dan Rybczynski, Eva Lyon, and Sienna and other fi eld dogs. Ryan Petterson and Maya Elrick provided helpful reviews. Mark Fanning, Paul Link, and Adolph Yonkee provided helpful comments.

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