348
RESEARCH REPORT
Origin and Significance of Tube Structures in
Neoproterozoic Post-glacial Cap Carbonates: Example
from Noonday Dolomite, Death Valley, United States
FRANK A. CORSETTI
Department of Earth Science, University of Southern California, Los Angeles, CA 90089; E-mail: [email protected]
JOHN P. GROTZINGER
Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139
Unusual carbonate fabrics characterize carbonate strata that cap globally distributed Neoproterozoic glacial deposits. In addition to recently described crystal fans (pseudomorphic after aragonite, Grotzinger and Knoll, 1995;
James et al., 2001; Hoffman and Schrag, 2002; RodriguesNogueira et al., 2003; Corsetti et al., 2004), tubestones
constitute a particularly enigmatic texture found in some
cap carbonates (Fig. 1). The tubestones, first described
from Death Valley, California (Hazzard, 1937; Johnson,
1957), currently are known from four regions (Death Valley, Brazil, northern Namibia, and southern Namibia—
Cloud et al., 1974; Hegenberger, 1987; Hoffman et al.,
2002; Hoffman and Schrag, 2002; Marenco and Corsetti,
2002; Rodrigues-Nogueira et al., 2003). Cloud et al. (1974)
comprehensively reviewed various hypotheses for tubestone origin and systematically rejected metazoan burrows, columnar stromatolites, the inter-column spaces
that separate columnar stromatolites, solution pipes, and
root casts as possible tube-forming agents. Fluid escape
was suggested to be the mechanism most consistent with
their observations. Hegenberger (1987) concurred with
this interpretation for similar Neoproterozoic tubestones
found in Namibia (Bildah Member, Witvlei Group; Maieberg Formation, Otavi Group), but favored a gas-escape
origin, where gas presumably was supplied via the decay
of microbial mats. Recently, this idea has been modified
and included into Neoproterozoic low-latitude glacial-aftermath scenarios where the tubes are purported to result
from CO2 degassing (Hoffman et al., 1998), or, alternatively, methane bubbled from gas hydrates disassociated during post-glacial warming (Kennedy et al., 2001). More recently, it has been suggested that the tubes may result
from growth of microbialites/stromatolites (Woods and Bottjer, 2000; Hoffman et al., 2002).
The hypotheses currently under consideration for tubestone formation (fluid escape/methane escape, unusual
stromatolite form) require different environmental circumstances to occur, and thus would each predict significantly different conditions in the Neoproterozoic post-glacial oceans. Because much of our understanding of Neoproterozoic climate change, and thus the purported evolutionary consequences of low-latitude glaciation, originates
from the post-glacial cap carbonates (e.g., Kaufman et al.,
1997, Hoffman et al., 1998; Kennedy et al., 2001), any insight into cap-carbonate formation may provide additional
environmental constraints on the potentially extreme glacial conditions. A better understanding of tubestone formation will help delineate the effects of low-latitude glaciation upon the biosphere.
Here, a detailed meso- and microscopic analysis of the
tubestones from the Noonday Dolomite, eastern California, is presented that suggests the tubestones originated
via upward propagation of an unusual stromatolite morphology. Field observation, inspection of polished slabs,
Copyright Q 2005, SEPM (Society for Sedimentary Geology)
0883-1351/05/0020-0348/$3.00
PALAIOS, 2005, V. 20, p. 348–362
DOI 10.2110/palo.2003.p03-96
The Neoproterozoic Noonday Dolomite (Death Valley, USA),
a post-glacial cap carbonate, contains closely packed, meter-long, cm-wide, tube-like structures that define the vertical accretion direction. Similar tubestones are known from
post-glacial cap carbonates in Namibia and Brazil. In vertical cross section, the tubes average 2 cm in diameter, pinch
and swell greatly along their length, may bifurcate and coalesce, and are filled with brown laminated micrite/microspar where best preserved. The tubes do not root or terminate in a particular layer and are randomly distributed
where present. The laminated host rock is composed of an
early lithified, microclotted fabric with framework void
space filled with sparry dolomite cement. The contact between the tube fill and the host rock is diffuse and feathered;
commonly, wisps of laminated host rock cross the tube fill
and bridge between adjacent stromatolitic structures, compartmentalizing the tubes.
The tubes likely result from the contemporaneous interplay between microbialite growth and sedimentation/cementation, rather than fluid or gas escape, as demonstrated
by the compartmentalization by bridging laminae. Vertical
cross sections resemble inter-column depressions that form
between columnar stromatolites. Bed-parallel sections,
however, reveal that the tube structures represent isolated,
sediment-filled depressions within a continuous layer of
stromatolite. The genesis of this unusual stromatolite morphology is likely related to highly supersaturated seawater
in the aftermath of low-latitude glaciation in Neoproterozoic time. Similar tube-forming microbialites are known
from alkaline lake systems such as Lake Turkana, Pavilion
Lake, and paleo-Lake Gosuite (Green River Formation). The
tubestones are interpreted to represent a rarely attained
end-member in stromatolite morphospace, likely associated
with anomalously high carbonate supersaturation.
INTRODUCTION
NEOPROTEROZOIC TUBESTONES FROM DEATH VALLEY
349
FIGURE 1—Typical Neoproterozoic tubestones from the Noonday Dolomite, Death Valley (A–C) and the Maiberg Formation, Otavi Group, Namibia
(D, E). (A) Poorly preserved tubestone, lower Noonday Dolomite, Nopah Range, typical of the Death Valley area; tubes are filled with late-stage
coarse spar (see Fig. 2 for locality information). (B) Well-preserved tubestone from the lower Noonday Dolomite, Winters Pass area, vertical cross
section; tubes are filled with dark micrite/microspar. Note where the lighter colored host rock bridges across the darker tube fill (arrows). (C) Horizontal
cross-section of (B). (D) Vertical cross section of tubestone, Maieberg Formation, Namibia, for comparison to the Noonday Dolomite tubestones. Scale
5 2.8 cm across. (E) Horizontal cross section, large clast of Maieberg Formation in Mulden Formation, Namibia. Scale 5 2.8 cm across.
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CORSETTI AND GROTZINGER
and thin-section analysis of well-preserved samples allow
further refinement of the observations made by previous
workers (e.g., Cloud et al., 1974) and development of additional constraints on the formation of the tube-like
structures. In addition, the tubestone stromatolite morphotype is not unique to Neoproterozoic post-glacial cap
carbonates, but is known from other marine and non-marine carbonate paleoenvironments, and may provide a
useful paleoenvironmental indicator.
Geologic Background
The Neoproterozoic Noonday Dolomite crops out in the
Death Valley area of southeastern California. It rests
conformably upon the uppermost glacial diamictites of
the Kingston Peak Formation, unconformably upon older
parts of the Kingston Peak Formation and older strata,
and non-conformably upon older gneissic/granitic basement rocks (Wright and Troxel, 1966; Stewart, 1970; Williams et al., 1974; Wright et al., 1978; Wright et al., 1984;
Miller, 1985; Prave, 1999; Corsetti and Kaufman, 2003;
Fig. 2). The basal portion was deposited upon a flooding
surface across these various formations during post-glacial transgression. The Noonday Dolomite is divided informally into the lower ‘‘algal’’ member and the upper
‘‘sandy dolomite’’ member (Stewart, 1970). The lower and
upper members are separated by a sequence boundary
with as much as 200 meters of regional incision (Summa,
1993).
The tubes, which are the focus of this study, commonly
are found in association with stromatolitic domal buildups
in the lower Noonday Dolomite (Cloud et al., 1974; Williams et al., 1974; Wright et al., 1978), although they do occur where dome development is not obvious. Cloud et al.
(1974) observed domes up to 120 m wide by ;40 meters
high, and Williams et al. (1974) and Wright et al. (1978)
sketched large domes up to 600 m wide and 200 m high.
The authors have observed domes with at least 10 meters
of synoptic relief based on the geometry of flanking beds.
Bedding dips as much as 708 to 808 on the flanks of the
domes and, in rare cases, is vertical to slightly overturned
(Fig. 3). Summa (1993) suggested that a sequence boundary separating the lower from upper members is coincident with the upper surface of the domes and that the apparent relief on the largest domes was enhanced by incision; thus, larger domes may be present, but are not unambiguously primary features and may be an artifact of
post-depositional erosion. In addition, rare examples of
similar tube structures have been noted in the upper
Noonday Dolomite in the absence of dome formation.
The age and tectonic setting of the Noonday Dolomite
are uncertain. The Noonday Dolomite is situated many
hundreds of meters above a 1080 Ma diabase sill that intrudes the lower part of the underlying Crystal Spring
Formation (Heaman and Grotzinger, 1992) and several kilometers below the Precambrian–Cambrian boundary in
the Lower Wood Canyon Formation (Corsetti and Hagadorn, 2000). Thus, the Noonday Dolomite is broadly constrained to have been deposited between 1080 Ma and 544
Ma. The Noonday Dolomite shares similar post-glacial
stratigraphic position, carbon-isotopic profile, and unusual lithofacies (tubestones, sheetcrack cements) with other
Neoproterozoic post-glacial cap carbonates around the
FIGURE 2—(A) Generalized locality map, Noonday Dolomite, Death
Valley. Shaded area represents the outcrop pattern of Neoproterozoic–Lower Cambrian strata (after Stewart, 1970); dashed line demarks the outcrop extent of Noonday Dolomite. Map coordinates for
the following localities are provided in the text: BM: Black Mountains;
GC: Galena Canyon, Panamint Range; IH: Ibex Hills; SNR: Southern
Nopah Range; KR: Kingston Range; WP: Winters Pass area. (B) Generalized stratigraphic column for the Death Valley succession (after
Corsetti and Kaufman, 2003, and references therein).
world, and likely was formed sometime between ;750 Ma
and ;585 Ma (Corsetti, 1998; Corsetti et al., 2000; Corsetti and Kaufman, 2003). Chemostratigraphic studies alternately have correlated the Noonday with Sturtian-age
(;700 Ma) strata (Corsetti, 1998; Corsetti et al., 2000;
Corsetti and Kaufman, 2003) and Marinoan-age (;600
Ma) deposits (Heaman and Grotzinger, 1992; Prave,
1999), respectively, but the correlations are equivocal because carbon-isotopic profiles are oscillatory. The tectonic
setting of the Noonday Dolomite also is unclear. The un-
NEOPROTEROZOIC TUBESTONES FROM DEATH VALLEY
351
FIGURE 3—Noonday Dolomite tubestone associated with the flank of a large stromatolitic structure, southern Nopah Range locality; thin
dashed line denotes stromatolitic lamination. The strata dip approximately 458 to the north (to the right in this photo); the field of view has been
rotated to restore the original vertical orientation of the tubes. No deflection of the stromatolitic lamination is noted across the tubes. The darker
material filling the tubes (tube fill) also occurs in layers that parallel the stromatolitic lamination (blanketed layer). Lens cap is approximately 6
cm wide.
derlying Kingston Peak Formation was deposited in extensional basins, likely related to rifting of the North
American craton in Neoproterozoic time (Miller, 1985;
Heaman and Grotzinger, 1992; Prave, 1999). The Noonday Dolomite seals previously active basin-bounding normal faults (Wright et al., 1974), but faults of syn-Noonday
age may have been active (Wright et al., 1974; Walker et
al., 1986; Summa, 1993).
OBSERVATIONS OF NOONDAY TUBESTONES
Detailed observations were made at the meso- and microscopic scale throughout the outcrop area of the Noon-
day Dolomite (Fig. 2). The tubestones were studied in detail in the Winters Pass area (35846.11192115845.1159),
the Southern Nopah Range (35849.42192116805.5609),
and the Galena Canyon area (36800.94892116855.8879).
The best-preserved samples originate from the Winters
Pass section (Fig. 2), where the Noonday Dolomite rests on
thin glacial tillites of the underlying Kingston Peak Formation and/or metamorphic basement rocks; dome formation is not obvious at this locality. The new observations build from preceding studies (e.g., Cloud et al., 1974;
Wright et al., 1978; Woods and Bottjer, 2000; Marenco and
Corsetti, 2002).
352
Mesoscopic and Microscopic Features in the Lower
Noonday Dolomite
Host Rock: In hand sample, the host rock (the strata in
which the tubes are found) is composed entirely of dolomite and has a micro-clotted, or pseudo-peloidal texture
(sub-millimeter cream-colored opaque clots surrounded by
clear spar, e.g., Cloud et al., 1974; Fig 4A). Varying proportions of opaque microclots (Fig. 4B) and translucent spar
define millimeter- to centimeter-scale lamination (Fig.
4C). Irregular to flat-lying laminae commonly show slight
convex-upward arching between tubes. Similar observations were made by Hoffman et al. (2002) for the Namibian
examples. Laminae contain no microclastic constituents
and differ markedly from the tube fill (discussed below),
suggesting that the microclotted texture accumulated via
in-situ precipitation (see discussion in Grotzinger and
Knoll, 1999).
In thin section, microclotted textures consist of nearly
spherical, dark micrite microclots surrounded by clear
spar (Fig. 4B). The microclots are remarkably consistent
in size where well developed (90–110 mm in diameter, but
a range of 50–200 mm has been observed). Wright et al.
(1978) suggested that this fabric is similar to the microclotted fabric Dzhelindia, which is thought to represent a
calcimicrobe of some kind. Where poorly developed, microclots appear to amalgamate into an irregular mass of micrite in which poorly defined microclots are still visible;
this is the most common fabric noted in the host rock
where a range of preservation is observed. The micritic microclots are the basic structural elements of two largerscale dendrolitic (Fig. 4D) and saccate (Fig. 4E) textures,
which grade into each other (Fraiser and Corsetti, 2003).
In some areas, microclots aggregate to form isolated
dendrolites to dendrolite clusters (Fig. 4D; discussed in detail in Fraiser and Corsetti, 2002; 2003) that comprise a
significant component of the lower Noonday Dolomite in
some localities (e.g., Galena Canyon area). This fabric is
most common on the flanks of certain domes where dips
exceed 408, although it also occurs within the central portions of the domes and in the absence of dome formation
(Fraiser and Corsetti, 2002; 2003). Individual dendrolites
are on a millimeter scale. Dendrolitic structures are overgrown by void-filling cements consisting of fine, isopachous dolomite microspar that grades into a coarser dolomite
spar, interpreted as early marine cements. The preservation of the relatively open framework of delicate dendrolitic structures implies early cementation. The fabric is
reminiscent of Epiphyton, though of larger scale, and is
similar to what others have termed bacterial bushes (e.g.,
Chafetz and Guidry, 1999). Epiphyton commonly is interpreted as a calcareous alga or cyanobacterium (e.g., Pratt,
1984).
In rare cases, microclots comprise thin micritic walls
(50–300 mm) that form irregular lobate chambers (Fig.
4E). The chambers are commonly between 0.5 and 1 mm in
diameter. The walls commonly are lined with poorly preserved isopachous fibrous or bladed cement, and the pores
occluded with drusy spar. Rarely, the saccate structures
link to form irregular chains. The chambers commonly
display a decrease in size along the length of the chain,
perhaps reflecting trends in their three-dimensional composite geometry in thin section rather than actual mor-
CORSETTI AND GROTZINGER
phology. This fabric is reminiscent of Renalcis—another
form commonly interpreted as a calcareous alga or cyanobacterium (e.g., Pratt, 1984)—although it is considerably
more irregular and larger than typical Renalcis. It is more
comparable to structures described from the Paleoproterozoic Odjick Formation (Hofmann and Grotzinger, 1986)
and Neoproterozoic Little Dal Group of northwest Canada
(Turner et al., 1993; Turner et al., 2000a; b).
The origin of the microclots is uncertain. Similar fabrics
have been interpreted to result from the degradation and
early cementation of microbial communities (Turner et al.,
1993; Turner et al., 2000a; Turner et al., 2000b), and the
gross morphology resembles fabrics described by others as
bacterial in origin (e.g., Chafetz and Guidry, 1999). No microbial remains, such as microbial filaments or coccoids,
have been observed within the microclots of the Noonday
Dolomite. However, simple coccoidal microfossils (;5 mm
in diameter) are present along tube margins in transported blocks of the lower Noonday Dolomite (Fraiser and Corsetti, 2003, fig. 3F, p. 382) located in the Neoproterozoic
Ibex Formation (interpreted by Wright et al., 1984, as a
basinal facies of the Noonday Dolomite).
Tubes: Tube structures form a conspicuous component
of the Noonday Dolomite (Fig. 1A–C). Although the tubes
have been observed to pinch and swell dramatically along
their length, in general, they display a mean diameter of
;2.0 cm (61.2 cm; Figs. 5 and 6). Most tubes are between
20 and 100 cm long, although tubes as long as 200 cm have
been reported from Namibia (Hegenberger, 1987). Much
shorter tubes also have been observed (Fig. 6), with some
as short as 1 cm. Rarely, the tubes bifurcate and join with
no apparent change in width between the parent and
daughter tubes (Fig. 6). In horizontal cross section, tubes
have irregular shapes (Figs. 1C; 5B). Weathering, however, gives the false impression that the tubes are perfectly
circular in cross section. Tubes commonly are spaced 1–2
cm apart, as if developed on a lattice, although excursions
to mm-spacing and dm-spacing are observed. Some examples are extraordinarily closely packed, with thin, millimeter to sub-millimeter septae of host rock between them
(Fig. 6). Distribution of tubes on bedding planes varies
from random to a more regular, almost hexagonal geometry. Tubes have not been observed to originate from, or terminate at, a common surface.
Previous workers described up to four different types of
tubes, differentiated based on tube fill: gray tubes, brown
tubes, spar-filled tubes, and ghost tubes (Cloud et al.,
1974). Gray tubes and brown tubes are identical in thin
section, and are filled with micrite and/or microspar (color
differences reflect later staining and do not represent significant compositional differences). Spar-filled tubes commonly are filled with late-stage sparry cement (cf., Fig.
1A); in most cases, the spar nucleates from obvious dissolution surfaces where previous phases were dissolved.
Ghost tubes occur where the host rock and the tube fill do
not retain contrasting color, and appear most commonly
where widespread secondary alteration is present. All previously described tubes are presumed here to share a common origin, with external differences resulting from differential preservation and diagenesis. It is possible that other Neoproterozoic tubestones may be different from the
tubes described here; however, all tubestones the authors
NEOPROTEROZOIC TUBESTONES FROM DEATH VALLEY
353
FIGURE 4—Tubestone host rock (the rock between the tube-structures) (A) Polished slab demonstrates the peloidal-like nature of host rock.
(B) Photomicrograph of peloidal host rock. Micritic microclots were precipitated in situ and are surrounded by coarser clear spar. (C) Thin
section demonstrating the stromatolitic nature of host rock. Laminations vary in size but are typically ;750 mm and are composed of dense
versus sparse layers of micritic microclots. (D) Thin section of host rock where dendrolitic fabric is well developed (see Fraiser and Corsetti,
2003). The peloidal fabric is identical, but in this example, it is organized into a dendrolitic fabric, whereas in Fig. 4B it is unorganized,
demonstrating that the peloids formed via in-situ precipitation. E) Saccate arrangement of peloids, not unlike Renalcis, but less well-organized
and less consistent in diameter.
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CORSETTI AND GROTZINGER
would suggest that the microclots commonly are more
closely packed near the margins, but the absence of deformed microclots suggests that no physical compaction
occurred. Rather, some samples of the host rock display
dendrolitic growth at the tube margin (Fig. 8B). In thin
section, delicate laminae composed of microclots surrounded by spar protrude undisturbed into the micritic
tube fill (Fig. 8C, D). Very commonly, the host-rock protrusions bridge the tube entirely (Figs. 1B; 5; 6; 7A, E; 8C, D).
Examination of successive slabs demonstrates that the
host-rock protrusions commonly seal, and therefore compartmentalize, the tubes; thus, the tubes did not stand
open for their entire length. The maximum length any
tube could have stood open is the vertical distance between the host-rock bridges. In some examples, this distance is quite long (nearly a meter); however, in most examples, the average distance between bridges is only a few
centimeters (e.g., Fig. 1B). Very little compaction affected
the bridge structures (Fig. 8D), implying early lithification. Although no microfossils have been preserved in
these samples, the bridges have a lamination texture consistent with the former presence of microbial mats, which
may have transiently colonized the tube fill. The presence
of the bridging laminae and the fact that the tubes were
compartmentalized are critical to the interpretation of the
tube structures.
FIGURE 5—Cleaned specimens of the Noonday Dolomite tubestone.
(A) Vertical section of the cleaned surface. From this vantage, the
tubestone resembles typical columnar stromatolites that even appear
to branch (arrow). In some areas, the host rock bridges the tubes. (B)
Plan view of A. Note that the tubes are not necessarily round in cross
section and look unlike the typical space between columnar stromatolites.
have observed in Death Valley and Namibia clearly share
a common origin.
In the best-preserved examples, the tube fill is laminated (cf., Figs. 1, 5–7), where laminae are of sub-millimeter
scale and are defined by minute changes in crystal size
(e.g., Fig. 7B, C). Rare laminae display normal, if microscopic, grading. Initially, lamination in the tubes mimics
the substrate upon which it originates; if it originates
where the host rock displays convex-up lamination, the
tube fill initially will display this geometry, as if blanketing the underlying structure (Marenco and Corsetti,
2002). Over the length of the tube, however, the laminae
develop a characteristic concave-up geometry. The brown
micrite from an individual tube commonly extends laterally within the host rock for a distance of several centimeters and conforms to the local curvature of laminae within
the host rock (Figs. 3, 7A). In some areas, mm- to cm-scale
layers of brown micrite identical to the tube fill are present within the host rock and are not associated with
tubes (Fig. 3).
Host Rock/Tube Fill Relations: The contact between the
micritic tube fill and the host rock appears abrupt in outcrop. When examined more closely, the tube-fill/host-rock
contact is relatively diffuse (Fig. 8A). The host rock and
tube fill interfinger on the millimeter scale (Fig. 8A–D).
Cloud et al. (1974) suggested that the host rock was more
compacted near the tube margins; recent observations
INTERPRETATION OF THE HOST
ROCK/TUBE ASSOCIATION
Tubestones as Stromatolites
In vertical cross sections, the tube/host-rock system
bears a striking resemblance to a series of stromatolitic
columns (i.e., host rock) separated by inter-column fill (i.e.,
tubes; Fig. 1B, D; 5A; 6; 7A), as recognized by Cloud et al.
(1974). Synoptic relief of this host-rock stromatolite was
low; tracing a convex arching lamination from edge to edge
reveals only a centimeter or less of relief above the surrounding seafloor. This amount of relief is mirrored in the
inter-column fill (tube) as a concave lamination composed
of darker micrite. In this two-dimensional view, the Noonday host rock is interpreted convincingly as a cluster of columnar stromatolites with very low synoptic relief separated by inter-column fill (the tubes). The host-rock bridges simply represent a period of more robust colonization by
mats, perhaps when sediment flux was relatively lower (a
common occurrence in some stromatolites). Where the
tubes are filled by sediments that extend out across the
stromatolitic surface, sediment flux may have been relatively higher, outpacing the growth of the stromatolite.
Where primary marine cements fill the tubes, the tubes
were simply sealed before they could be filled with sediment, suggesting very low sediment flux (a similar explanation was proposed by Woods and Bottjer, 2000).
It is important to note that the Noonday Dolomite stromatolitic laminae commonly have flat or very mildly convex margins, in contrast to ordinary columnar stromatolites (but see an example from the Otavi Group, Namibia,
where the stromatolitic laminae are strongly convex: Hoffman et al., 2002; Hoffman and Schrag, 2002). Thus, when
individual laminae are traced out towards the inter-stromatolitic depressions (i.e., the tubes), the laminae appear
NEOPROTEROZOIC TUBESTONES FROM DEATH VALLEY
355
FIGURE 6—Tubestone from the Winters Pass locality demonstrates that tubestones need not be composed of meter-scale tube-like structures.
Host-rock bridges are apparent where thin lamina of host rock occludes the tube. The host rock thins dramatically between tubes. Convex,
stromatolitic host-rock laminae between the tubes are found in the lower left. This locality is laterally adjacent to the examples shown in Fig.
1B and C.
to end abruptly. This can create the false impression of
lamination termination via truncation, rather than thinning, which, in turn, can lead to the interpretation that
the tube structures formed by piercement rather than deposition.
Cloud et al. (1974) rejected the stromatolite hypothesis
based on examination of horizontal cross sections (cf., Fig.
5B). In this dimension, the tube/host-rock assemblage
does not conform to the ordinary pattern of columnar stromatolites and inter-column fills. Columnar stromatolites
form isolated concentric circles or ellipses surrounded by
an interconnected network of clastic-textured carbonate.
In contrast, the Noonday Dolomite structures show the opposite relationship—the tubes, as intra-stromatolite depressions, are isolated within an interconnected network
of stromatolite (Fig. 9). In this scenario, the sea floor
would have resembled the dimpled surface of a golf ball.
To quote Cloud et al. (1974, p. 1879),‘‘If the tubes are stro-
matolitic interspaces, they have a unique geometry!’’ The
authors of this report concur.
Thus, the tube/host rock association is best interpreted
as an upward-propagating microbialite with contemporaneous fill. All of the key observations presented above are
explained satisfactorily by this interpretation. The tubefill/host-rock interface represents a dynamic boundary between the stromatolite and the surrounding sediment.
The formation of tubes indicates that intra-stromatolite
depressions were sometimes unfilled, indicating periods of
low sediment flux into the accreting system, and reinforcing the interpretation that these stromatolites grew more
by in-situ precipitation than sediment trapping and binding. Bridging laminae that episodically sealed tubes represent times when microbial mat growth was particularly
robust, or sediment flux particularly low, so that mats
could extend across the open voids or colonize the sediment within depressions. Tube fill was introduced after
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CORSETTI AND GROTZINGER
FIGURE 7—Tube fills. (A) Polished slab with darker-brown tube fill (versus light-colored host rock). Note host rock bridging tube fill (denoted by
‘bridging lam.’). In some areas, the brown tube fill also blankets and follows the stromatolitic structure of the host rock (denoted by ‘a’; cf., Fig. 3). (B)
Photomicrograph of typical micritic tube fill demonstrating concave lamination. (C) Closer view of laminated micritic/microsparitic tube fill. The laminae
are defined by minute changes in crystal size and in some places appear graded. (D) In many cases, the micritic tube fill is replaced by a coarse
spar. In this example, a clear dissolution surface is visible (arrow), suggesting that the original tube fill was dissolved and the void space was filled
with coarse spar. (E) Tube fill in the field (close-up of 1B) from Winters Pass locality demonstrates the color contrast noted between the tube fill and
host rock in the better-preserved samples. Note the light-colored laminae that compartmentalize the tube.
NEOPROTEROZOIC TUBESTONES FROM DEATH VALLEY
357
FIGURE 8—Host-rock/tube-fill relations. (A) Photomicrograph of a tube margin. The contact between the tube fill and the host rock is somewhat
diffuse. (B) Photomicrograph showing apparent growth structures within the host rock lining the margin of the tube. (C) Polished slab showing
a host-rock bridge across the tube fill. The darker tube fill becomes a layer that blankets the host rock towards the top of the image. (D)
Photomicrograph of a host-rock bridge within tube fill. The delicate nature of the host-rock bridge implies early cementation before significant
compaction took place.
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CORSETTI AND GROTZINGER
FIGURE 9—Artist’s rendition of a tube-forming stromatolite. Dark areas represent sediment fill and laminated areas represent stromatolitic
lamination. Note that the tube-forming stromatolite and the more common columnar stromatolite appear similar in vertical cross section but
different in horizontal cross section. In plan view, the tube-forming stromatolite consists of an interconnected network of microbialite with ovate
intra-stromatolite fill (the tubes) while the columnar stromatolite consists of isolated microbialite separated by inter-stromatolite fill. The former
presence of a somewhat tufted mat is envisioned (cf., Woods and Bottjer, 2000). Sediment fills the low areas between the tufts, creating the
base of a tube. The structure is then propagated in the vertical direction with a high degree of inheritance from one layer to the next, creating
the tube-forming stromatolite.
the stromatolite surface had propagated upward by some
amount. Thus, it was supplied infrequently from above,
and occasionally blanketed the growing surface of the stromatolite, in addition to filling the tubes. The rarity of sediment blankets suggests that the flux of loose carbonate
sediment was generally low, with only rare events supplying sediment to the inter-stromatolite depressions.
below the sediment-water interface then would collect
sediment formed at the sediment-water interface (this
was noted by Cloud et al., 1974, but ignored in their final
conclusion). The layers of micrite are best explained as primary sediment layers that merely blanketed the surface of
the growing structure at that time. Thus, a fluid-/gas-escape mechanism is not favored by the present evidence.
Fluid/Gas Escape versus Stromatolites
Tubestone Stromatolite Morphogenesis
Cloud et al. (1974) could not envision a tube-forming
stromatolite, and thus favored a fluid-escape mechanism.
However, if the host rock was unlithified, then greater disruption of the host rock during fluid migration would occur, resulting in the formation of zones of liquefaction, sediment volcanoes, and breccia development where tubes
discharged fluid at the sediment-water interface. Any invading fluid penetrating from below, whether gas or liquid, must have stopped at the first bridging laminae because these, and all their successors, are unbroken. In addition, fluid-escape structures commonly display an upward deflection of host-rock lamination, whereas the
Noonday Dolomite host rock displays a near-flat or convex
geometry at tube margins. Evidence within the host-rock
facies, such as oversteepened margins that require early
cementation for rigidity; the preservation of the open (uncompacted) framework of the micritic microclots and dendrolites composed of microclots (Fig. 4); and the occurrence of drusy, inclusion-rich isopachous rim cements in
larger primary voids that are explained most simply as
early marine cements, points to early lithification that
likely would preclude soft-sediment escape features. The
darker sediment observed within tubes occasionally forms
more extensive layers that extend laterally several centimeters to tens of centimeters parallel to the host-rock lamination (Figs. 3; 7A). While this geometry may suggest
splitting of host-rock laminae via hydraulic pressure created by an invading fluid, it is unclear how a space created
Interestingly, the stromatolite-tube system shows a remarkable degree of inheritance in the growth direction.
Here, inheritance is considered the degree to which the geometry of subsequent laminae in an upwardly propagating stromatolite conforms to the geometry of antecedent
laminae. A high degree of inheritance conveys that the
stromatolite laminae do not change form through time.
Despite a high degree of irregularity along the tube-stromatolite interface, the mean width of tubes and their encompassing stromatolites stays remarkably uniform (a
high degree of inheritance). This property is shared by
other Neoproterozoic tubestones, including the Maieberg
Formation in northern Namibia (Hoffman and Schrag,
2002) and the Witveli Group in southern Namibia. Although inheritance is an attribute that has received little
study, it is logical to propose that if the geometry of a system of stromatolites and their adjacent depressions does
not evolve across a set of successive laminae, then the conditions responsible for their growth did not change in
time—or at least the variability in the set of controlling
factors was not influential in affecting growth. Alternatively, the size and spacing is set early and, for whatever
reason, is resistant to change. Perhaps the micritic sediment initially inhibited growth and this geometry was
propagated upwards, as suggested by Woods and Bottjer
(2000). However, it is unclear why the initial sediment inhibition would form such a regular spacing. Thus, there is
a necessary competition between factors that drive the
NEOPROTEROZOIC TUBESTONES FROM DEATH VALLEY
359
FIGURE 10—Younger analogues for Neoproterozoic tubestones that demonstrate tube-forming microbialites are not limited to Neoproterozoic
post-glacial cap carbonates. (A) Polished upper surface of Cambrian tube-forming thrombolite from the Nopah Formation near Mountain Pass,
Clark Mountains, California. The dark area represents the thrombolite, which forms a Noonday Dolomite host-rock-like network and the lighter
area is filled with grainstone, which fills tubular structures. Scale in mm. (B) Vertical cross section of A. Note striking resemblance to Noonday
tubes. (C) Modern dendritic, tube-forming microbialite from Pavilion Lake, British Columbia; from Laval et al. (2000); structure ;1m across.
(D) Eocene embedment structures (i.e., tubes) within lacustrine stromatolites from the Green River Formation (from Lamond and Tapanila,
2003). Scale bar 5 10 cm.
stromatolite system toward stability, and those that cause
it to change and evolve (Grotzinger and Rothman, 1996;
Grotzinger and Knoll, 1999). For the Noonday Dolomite
and other Neoproterozoic tubestone stromatolites, this ratio of competing factors clearly favored stability, with high
inheritance of shape between laminae as the result.
POSSIBLE TUBESTONE ANALOGUES
The Neoproterozoic tubestones represent a morphologic
end-member where, in plan view, the stromatolite comprises an interconnected network and the intra-stromatolite fill is ovate. However, as an end-member, this implies
that a spectrum of intermediate fabrics may exist, bound
at the opposite extreme by stromatolites represented by
isolated columns and domes surrounded by interconnected inter-column fill. Ancient forms close to the end-member represented by tube structures are known from the
Cambrian of the Great Basin (Cooper, 1989) and paleoLake Gosuite, represented by the Eocene Green River Formation (Awramik, pers. comm., 2003; Lamond and Tapanila, 2003). The Cambrian examples were formed within
small thrombolite mounds in a shallow, subtidal paleoenvironment (Fig. 10A–D). On bedding-plane surfaces, the
host rock is composed of interconnected thrombolitic networks; ellipsoidal areas within the network, analogous to
360
CORSETTI AND GROTZINGER
the Noonday tubes, are filled with grainstone (Shapiro
and Awramik, in press). In vertical cross section, the
thrombolites form narrow pillars separated by interthrombolite depressions filled with laminated grainstone,
and display geometry similar to the Noonday tubes. The
Green River Formation examples formed in an alkalinelake system and recently have been considered embedment structures—the tubes were thought to form when an
unknown metazoan (now-missing) once sat in the stromatolite. The Green River Formation examples easily can
be interpreted as tube-forming stromatolites versus embedment structures. The presence of bridging laminae (cf.,
Noonday tubes) in the Green River examples would seem
to preclude the embedment hypothesis.
Holocene tube-forming stromatolites are known from alkaline lake systems, including the Lake Turkana Basin,
Kenya (Lamond and Tapanila, 2003), and Pavilion Lake,
British Columbia (Laval et al., 2000). The Lake Turkana
Basin tube-forming stromatolites also were described as
embedment structures (Lamond and Tapanila, 2003), an
interpretation considered unlikely given the lack of evidence for metazoan presence. Like the Noonday Dolomite
examples, the Turkana Basin examples reportedly display
bridging laminae and compartmentalized tubes within
the stromatolites. In Pavilion Lake, small, centimeter- to
meter-scale microbial buildups found at 30 m depth display an interconnected morphology in plan view that consists internally of a highly ramified microstructure that is
a strong analog for the dendrites in the Noonday Dolomite
(Fig. 10E; Laval et al., 2000). Morphologic depressions
within the interconnected Pavilion Lake microbialite have
a circular pattern, and therefore would produce tubes if
cut vertically. The combination of macroscopic form (tubeforming framework) and microscopic texture (dendrites)
indicate that the Noonday tube-forming stromatolites
may have formed under similar set of geochemical conditions.
DISCUSSION
Stromatolite growth results from a balance between
growth/decay of microbial mats, mineral precipitation,
and sedimentation; changing any of these parameters will
directly affect stromatolite morphology (Grotzinger and
Rothman, 1996; Grotzinger and Knoll, 1999). The rate of
these processes is particularly important; if the sedimentation rate is too high, the stromatolite will be smothered;
if too low, the mat will accrete without being preserved. In
either of these cases, growth of the stromatolite will cease.
Stromatolite growth models (Grotzinger and Rothman,
1996; Grotzinger and Knoll, 1999) demonstrate that the
interaction of surface-roughening effects (e.g., upward
propagation of microbial mats or carbonate crystals) with
surface-smoothing effects (e.g., episodic sedimentation) is
particularly important in columnar stromatolite morphogenesis. For example, if the sedimentation rate is low
enough, then the smaller topographic features within an
upward-propagating microbial mat will be covered by sediment, leaving the larger features to be reinforced through
time, eventually forming columns. In a two-dimensional,
growth-parallel cross section, the alternation of stromatolite columns and inter-column fills is identical for tubeforming stromatolites and ordinary columnar stromato-
lites; only the three-dimensional expression allows the
Neoproterozoic tube structures to be distinguished from
ordinary stromatolites. Thus, in order to distinguish between the two, it is necessary to study them in three dimensions. Unfortunately, three-dimensional stromatolite
growth models have not been formulated. However, the
parameterization of this next generation of models must
include effects that will limit the lateral growth of the mat
so that it spontaneously gives rise to the depressions that
give the upward-propagating interface the dimpled geometry that is inherited as the structure grows further.
Which factors, either intrinsic (e.g., mat motility, crystal
energetics) or extrinsic (e.g., light, seawater composition,
sediment flux), will be most important in controlling this
pattern is difficult to predict.
A local understanding of tube formation partially may
be enabled by considering some of the global factors that
help illuminate the broader significance of this facies. The
tube-forming stromatolites are particularly well developed within the Neoproterozoic post-glacial cap carbonates, which generally are acknowledged to have formed in
the presence of sea water with unusually high carbonate
saturation (Grotzinger and Knoll, 1995; Kaufman et al.,
1997; Hoffman et al., 1998; Hoffman and Schrag, 2002).
Unusually elevated carbonate-saturation levels are indicated by the occurrence of thick carbonate deposits immediately above glacial rocks that contain locally abundant
pseudomorphs of sea-floor-encrusting aragonite crystal
fans (e.g., Grotzinger and Knoll, 1995; James et al., 2001;
Hoffman and Schrag, 2002; Corsetti et al., 2004). The environmental context of unusually high carbonate saturation is supported by the following observations, all which
imply prolific in-situ precipitation: (1) the host microbialite is constructed of weakly laminated, microclotted micrite and dendrites; (2) the growth of dendrolitic structures is
known only from modern environments containing waters
that are highly oversaturated with respect to calcium carbonate; (3) tubes are only filled episodically, and sediment
layers in the stromatolite host rock are uncommon, pointing to very low sediment flux; and (4) the unusually high
inheritance of form within the stromatolite-tube system is
best interpreted to record high stability within the environment over the period of time represented by the thickness of the tube-bearing interval. It is important to clarify
that this last point must be considered non-dimensionally:
the absolute magnitude of the time-dependent processes
(e.g., rates) is not as important as the relative proportion
between rates of growth processes and rates of environmental change. The environment might be unstable over
short time scales, but if growth rates are fast, then it will
appear as if the system were stable, and vice versa.
Considered collectively, the observations presented
above and inferred processes characterize a growth environment in which the tubes were formed by in-situ precipitation of the host microbialite, which, once nucleated, was
remarkably conservative in its textural evolution. A characteristic morphology, described by the wavelength of tube
spacing, the amplitude of tube dimples (their synoptic relief), and their degree of anisotropy in both plan-view (x,y)
directions is set very early and is not changed over many
growth iterations. Thus, a plan-view cross-section through
the microbialite-tube system just above the base will not
look different from a cross-section near the top. Conse-
NEOPROTEROZOIC TUBESTONES FROM DEATH VALLEY
quently, the key factors for tube growth are embedded
within the formation of the texture at or very soon after
the moment of initiation of growth.
The Noonday tubes resulted from propagation through
time of dimples that formed on the microbially encrusted
sea floor. One simple interpretation of the dimples is that
they represent sites of microbial or chemical inhibition
such that mats or precipitating minerals did not grow. The
question then becomes why the sites of inhibition occurred
where they did, and why were they so regularly limited in
their size? Future advances in understanding the tubestone fabrics will come from further analysis of well-preserved ancient tubestones as well as modern analogs, such
as the microbialites in Pavilion Lake, where intrinsic and
extrinsic growth processes can be ascertained and measured directly. These processes will provide input for algorithmic rule sets of tubestone growth models of stromatolite morphogenesis. Ultimately, these models are limited
in that they account only for the morphogenesis of a peculiar stromatolite form. However, the ability to simulate
this form will depend on the further understanding of the
unique conditions recorded in the aftermath of Neoproterozoic glacial episodes.
CONCLUSIONS
The tubestones are interpreted to represent a rarely attained end-member in stromatolite morphospace. Whereas columnar stromatolites typically occur as isolated
structures within carbonate sediments that accumulate
within inter-stromatolite depressions, the tubestones are
laterally continuous stromatolite sheets that enclose and
isolate intra-stromatolite depressions filled with sediments or void-filling cements. The accreting depositional
interface would have looked similar to the dimpled surface
of a golf ball. Previous interpretations of cap-carbonate
tubestones that involve destructive emplacement of gas
(Hegenberger, 1987; Kennedy et al., 2001) or liquid (Cloud
et al., 1974) are inconsistent with the macroscopic and microscopic textures preserved within the Noonday Dolomite. Tubestone stromatolites feature several important
properties, including weakly developed stromatolitic lamination, well-developed microclotted and, in some cases,
dendrolitic accretion fabrics that likely formed through insitu precipitation, low sediment flux as expressed by incompletely filled intra-column depressions (i.e., the tubes),
and very high inheritance of lamina shape and morphology. The microclotted fabrics and dendrolitic textures are
consistent with precipitation under conditions of elevated
supersaturation with respect to calcium carbonate. Thus,
the presence of tube-forming stromatolites likely indicates
formation in an environment extraordinarily oversaturated with respect to calcium carbonate. This conclusion supports recent models that account for the origin of post-glacial cap carbonates through highly elevated but transient
oversaturation with respect to calcium carbonate (despite
significantly different processes that are invoked to produce oversaturation—compare Grotzinger and Knoll,
1995; Hoffman et al., 1998; Kennedy et al., 2001; Hoffman
and Schrag, 2002). The best modern analogues are found
in alkaline lakes where carbonate precipitation occurs due
to mixing of alkaline lake water and calcium-rich groundwater.
361
ACKNOWLEDGEMENTS
We would like to thank Pedro Marenco, Kate Woods,
Margaret Fraiser, Paul Hoffman, Stan Awramik, Russell
Shapiro, and Dave Pierce for helpful discussions. Kate
Woods graciously provided access to her thin sections, and
Stan Awramik provided access to P. Cloud’s collection at
UCSB. The manuscript benefited from reviews by Linda
Kah, Tony Prave, Brian Jones and John Cooper.
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ACCEPTED DECEMBER 21, 2004