Origin of 3.45 Ga Coniform Stromatolites in Warrawoona Group, Western Australia

Origin of 3.45 Ga coniform stromatolites in Warrawoona Group, Western Australia

H. J. Hofmann* Department of Geology, University of Montreal, P.O. Box 6128, Station A, Montreal, Quebec H3C 3J7, Canada K. Grey Geological Survey of Western Australia, 100 Plain Street, East Perth 6004, A. H. Hickman } Western Australia, Australia R. I. Thorpe Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada

ABSTRACT crofossils constitute the most tangible morpho- bialite (Burne and Moore, 1987, p. 10) refers to logic evidence for early biologic activity on both laminated and unlaminated structures of A new occurrence of conical and branched Earth. More than 30 Archean stromatolite oc- undisputed microbial origin, and thus includes pseudocolumnar stromatolites in Archean currences are now known worldwide (Hofmann, biogenic stromatolites. dolostones in the Pilbara region, Australia, con- 1999), although the biogenicity of those in We here present a first report of a new occur- tributes significant new morphologic informa- rocks older than 3.2 Ga has been questioned rence of stromatolites in dolostone in the War- tion on such structures. These remains are in- (Lowe, 1992, 1994, 1995; Buick et al., 1995). rawoona Group of Western Australia. These are terpreted as probably representing, in part, Alternative explanations feature chemical pre- more convincingly biogenic than previously microbially mediated accretionary growth sur- cipitation and/or soft-sediment deformation as described stromatolites in chert in the same faces in an Archean hypersaline depositional originating causes, with arguments based on succession (Lowe, 1980, 1983; Walter et al., basin. The structures comprise laterally linked analogies with modern abiologic structures, 1980), revealing a nondeformational, accre- pseudocolumns of centimeter width and and on mathematical models (Grotzinger and tionary nature that probably involves biologic decimeter height, with first-order conical lami- Rothman, 1996). activity or biofilms. Microfossils were not ob- nae of as much as 15 cm of synoptic relief and Notwithstanding the fact that stromatolites served. apical angles of 30°– 80°. The conical laminae have often played a prominent role in demon- are modified by a second-order, centimeter- strating the nature and antiquity of the biosphere REGIONAL SETTING AND AGE scale, low-amplitude primary corrugate lami- and the benthos, there is a long-standing contro- nation, with crests and troughs occasionally versy revolving around the definition of the The Warrawoona Group of the Pilbara re- stacked to form satellitic, obliquely directed term stromatolite, and the involvement or par- gion of Western Australia is part of a 10–15- pseudocolumns; bedding surfaces exhibit a ticipation of organisms in the formation of such km-thick Archean volcanic-sedimentary se- preferred direction of elongation of the cones, structures. Although a biologic endowment is quence referred to as the Pilbara Supergroup an orientation that is orthogonal (and unre- readily established in modern stromatolites, (Hickman, 1983). This sequence now forms lated) to the trend of younger folding; the mi- such a contribution for ancient forms is only oc- the greenstone belt of one of the world’s best crostructure is secondary. The stromatolites are casionally demonstrable, because microbial re- exposed granite-greenstone terranes surround- better preserved than those previously known mains are rarely preserved. Various criteria for ing the small town of Marble Bar (Fig. 1). Pre- from chert in the Warrawoona succession. The biogenicity have been circulated (Walter, 1978, cise U-Pb dating of zircons from volcanic units remains exhibit certain distinct morphologic 1983, 1994; Buick et al., 1981), but under the has established that the oldest parts of the suc- attributes corresponding to those in younger most exacting ones, even Proterozoic stromato- cession are older than 3.51 Ga (Buick et al., stromatolites, such as displayed by Thyssagetes lites would mostly not be demonstrably bio- 1995), whereas the De Grey Group, at the top and Jacutophyton, whose biogenicity is gener- genic. Stromatolite specialists have had little of the Pilbara Supergroup, has an age of ally accepted (although difficult to demonstrate difficulty in accepting the vast majority of Prot- 3–2.95 Ga (data from west Pilbara; Nelson, conclusively); the conical Warrawoona forms erozoic stromatolites as biogenic (Walter, 1997). The new locale, here referred to as the may represent the oldest known precursor of 1996). Nevertheless, stromatolites are consid- Trendall locality, MBB 010, is in the Warra- these taxa. ered to be laminated constructs that formed at a woona Group, 50 km west of Marble Bar (Fig. water-substrate interface, and we here charac- 1). The host rocks of these structures are at the INTRODUCTION terize them for practical purposes as morpho- conformable contact between the Panorama logically circumscribed accretionary growth Formation and the Strelley Pool Chert. The The fossil record of the Archean Eon (>2.5 structures with primary lamination that is, or Panorama Formation was dated by Thorpe et al. Ga) is sparse. Stromatolites and very rare mi- may be, biogenic. The more recent term micro- (1992) as 3.46 Ga.

*E-mail: [email protected].

GSA Bulletin; August 1999; v. 111; no. 8; p. 1256–1262; 3 figures.

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V V 120° Fortescue Group (2.77–2.68 Ga) 30 km V V De Grey Group (3.00–2.95 Ga) V V V V V V V Gorge Creek Group V Strelley Pool V

V 21°

V V Sulphur Springs Group (3.24 Ga)

V V V V V V Wyman Formation (3.32 Ga) V VNorth V Pole V V V V V V Strelley Pool Chert and V V Marble Bar Panorama Formation V VV V V V V Warrawoona V Towers Formation

V Group (3.47–

V V V V V Duffer Formation V 3.43 Ga) V V V V V Talga Talga Subgroup V V V MBB 010 V V V V Coonterunah Group (3.51 Ga)

V V V

V V Archean granitoid rocks V V V V V V V (3.50–2.84 Ga) V V V V V V V V V V V V V V V

V V V V V V

V V V V V V V

V V V ° V 22° 119 AUSTRALIA AHH72 04.03.99

Figure 1. Location map; generalized geologic map and stratigraphy. MBB 010 refers to new Trendall stromatolite occurrence. Latitude is in degrees south; longitude, in degrees east.

SOURCE OF MATERIAL section ~20 m thick, within a unit of partly silici- DESCRIPTION OF NEW fied olive-brown–weathering dolostone with a STROMATOLITES The new stromatolite locality was discovered northerly strike and westerly dip. The samples il- by A. F. Trendall in 1984, and part of the out- lustrated in Figure 2 are in a 3.3-m-thick dolo- Macrostructure crop (but not the precise location) was subse- stone that overlies a 2-m-thick dolostone with quently examined by Grey (1984). She consid- large crystal fans pseudomorphic after aragonite, At the new locale, beds strike northerly, dip ered material from this locality to be abiogenic barite, or gypsum; they underlie several meters of moderately steeply west, and are cut by narrow, (enterolithic folding of evaporitic laminations). silicified stromatolites and massive black chert. very steeply dipping, westerly trending quartz The stromatolitic outcrop was visited again by Reference specimens are deposited at the Geo- veinlets, and minor fractures. Sections perpen- Hickman, Trendall, and Thorpe in 1990; photos logical Survey of Western Australia (GSWA), dicular to bedding display morphologically dis- and small samples were shown to Hofmann, catalogued under GSWA F50177 (GSWA sam- tinct pseudocolumnar laminated structures, com- who proposed that they be more fully studied, ple 138990). prising first-order conical forms, connected because of their strong similarity with Protero- The occurrence is at approximately the same laterally by planar to slightly wavy laminae (Fig. 2, zoic conically laminated structures, the bio- stratigraphic level, and is in a similar association A and B); all elements are stacked with high de- genicity of which is unquestioned. This site was of lithofacies, as the Strelley Pool Chert stroma- grees of inheritance. The cones are generally visited by Grey, Hickman, and Hofmann in tolites to the northwest, reported and discussed symmetrical and erect, 5–20 cm across at the 1997, during an examination of all reported by Lowe (1980, 1983, 1994). The environmental base, with as much as 15 cm of synoptic relief, stromatolite localities in the Archean rocks of setting for these rocks, a partially restricted, low- yielding apical angles that average 70°–80° the Pilbara region. From this investigation we energy shallow hypersaline basin, was discussed (Fig. 2, A, B, and D), but that can be as small as concluded that stromatolites are more wide- in detail by Lowe (1983). Lowe (1980, 1983) 30° (Fig. 2E). The apical portions are pointed spread in the Warrawoona Group than previ- characterized the stromatolites as unbranched, where the axis coincides with the present-day ously reported, but that preservation at many lo- laterally linked conical columnar stromatolites erosion surface, and more rounded in off-axial calities is generally poor. The Trendall locality with 2–6 cm of relief and as high as 0.6 m; apical sections. The first-order elements are modified described here is of particular significance be- angles average 70°–80° (locally as low as 30°), by stacked second-order, low-amplitude corru- cause of its exceptional preservation over a few and cross sections are circular to elliptical, with gate to sinusoidal lamination of centimeter wave- square meters; it exhibits morphologic attributes fine continuous laminae, and detrital sand grains length, the crests and troughs commonly forming consistent with biologic input. and laminae in the intermound portions; some oblique pseudocolumns on the cones, or vertical The structures occur on the southwest flank of preferred direction of elongation of the cones has to somewhat inclined pseudocolumns on hori- the North Pole dome (Fig. 1), in a stratigraphic been noted (Lowe, 1980, Fig. 3; 1983, Fig. 6C). zontal intercone elements. Their dimensions vary

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Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/111/8/1256/3383302/i0016-7606-111-8-1256.pdf by guest on 27 September 2021 Figure 2. Outcrop views of conically laminated stromatolites from Warrawoona Group, North Pole Dome. (A) Section perpendicular to steeply dipping bedding, showing two pseudocolumns composed of uniform conical laminae with obliquely stacked, low-amplitude second-order corru- gations, and horizontal intercone areas with more irregular laminae. West is toward top of view. Note the locally unequal vertical positions of same laminae (dashed markers) on either side of the cone on the right. (B) Vertical section showing rounded cones with obliquely stacked second- order corrugate lamination and satellitic branching pseudocolumns near the middle of the view. Scale is marked in decimeters. (C) Enlarged view of branching pseudocolumn in B; note progression from steep conical- to convex-shaped elements. (D) Top view of bedding surface, looking east; note close spacing and preferred orientation of elongate protuberances, and size differences between larger cones at top and smaller cones toward bottom of view. Black arrow at top points to steep cone illustrated in detail in E. Scale is marked in decimeters. (E) Close-up view of steep cone in D. Scale divisions in centimeters and millimeters. (F) Plan view of conically laminated pseudocolumns, showing preferred elongation in east-west direction (east at top). Scale is marked in decimeters.

1258 Geological Society of America Bulletin, August 1999

between pseudocolumns, but in individual pseudocolumns they remain consistent, although their dimensions may differ on opposite sides of conical elements (Fig. 2B). In rare places, the second-order convexities develop into satellitic, divergently branching turbinate columnar forms as much as 5 cm across and with several centimeters of synoptic relief (Fig. 2, B and C). Ax- ial zones of the conical forms—the loci of super- posed successive apices of the sharply inflexed laminae commonly found in Conophyton—are nonexistent, or exceedingly narrow and hardly distinguishable, because the pointed nature of the inflexion tends to form a line rather than a zone (Fig. 2A, right portion). In bedding-surface views, the conical elements have circular, elliptical to slightly lanceolate outlines with a preferred overall, present-day east- west elongation direction (Fig. 2, D and E), al- most orthogonal to the structural trend of bedding, indicating that the elongation is not at- tributable to tectonic deformation. The laminae in the conical and domal elements merge with es- sentially flat to slightly domed or somewhat un- dulating intermound laminae that are more irregular in thickness, and commonly present a pinching or lenticular appearance. The level of an intermound lamination may be at slightly different elevations on either side of a section of a conical element (Fig. 2A, right), implying the presence of a local slope around the cone at the locale illustrated, the slope being in the general direction of present-day north, at right angles to the direction of preferred orientation of the flattened cones. The intermound spacing of conical elements ranges from contiguous to open (nil to decimeter), and depends on cone size and on stratigraphic level in the unit. The surface in Fig- ure 2D displays noticeable size differences along a bedding plane.

Microstructure

The microstructure is secondary, but recrystal- lization and silicification processes have preserved a modified palimpsest primary banded lamination (Fig. 3A); dark and light layers differ in thickness, grain size, and mineralogy. The Figure 3. Microstructure of flank of conically laminated pseudocolumns in partly silicified dolostone is composed of variable proportions of dolostone viewed at different scales. (A) Low-magnification general view (in transmitted light) recrystallized dolomite and chert and/or micro- of banded microstructure of dark-colored dolomite and light-colored siliceous laminae. Rec- quartz (Fig. 3, B and D). In thin section, dark tangle outlines enlarged view in B. (B) Detail of light and dark laminae in transmitted light, dolomite laminae compose a mosaic of mostly showing irregular contacts, and floating quartz grains in carbonate. Rectangle outlines are en- brownish, polysynthetically twinned, equant an- µ larged area in C. (C) Enlarged view, under crossed nicols, showing constituents of light and dark hedral grains with dimensions of 100–600 m. laminae-dolomite with floating microquartz, and microquartz with floating carbonate. Rectan- Siliceous laminae are composed of a mosaic of gle outlines area magnified in D. (D) High-magnification view, under crossed nicols, of cloudy clear, mostly equant anhedral quartz crystals with µ dolomite grain at top, and quartz with carbonate inclusions underneath. dimensions in the range of 30–100 m. Where chert and dolomite are present in about equal proportions, contacts between siliceous and dolomitic laminae are typically very uneven;

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quartz commonly projects as convexities into the on Lowe’s reinterpretation of Archean stromato- conceive of basin-wide conditions that would embaying dolomite (Fig. 2B). Some dark lami- lites older than 3.2 Ga, and discussed the nonde- yield such a rich and unusual fold geometry. Us- nae, however, present a lower boundary that is formational, nonprecipitational origin of the ing the concept of balanced cross section, it is not somewhat more evenly developed than their top, somewhat older pseudocolumnar structures in possible to unfold the structures and derive a pre- a feature observed in some younger stromatolites the Towers Formation at North Pole, citing such sumed transported mass either from the top of the with preserved organic matter. The dolomite lam- features as relief and uneven lamination, several cones, nor from squeezing laterally from the in- inae contain variable small percentages of float- orders of laminar curvature, rippled clastic sedi- tercone areas, because no deficit of material can ing quartz crystals of a size comparable to those ment, draping and pinching-out against the struc- be identified at either place. Diapir-like folding is in the silicified layers, and quartz crystals in the tures, overgrown curved intraclasts, and micro- ruled out for lack of indication of an annular de- silicified layers contain inclusions of as much as laminae with 5–10 µm kerogenous clots. In his pression around the base, and, in any case, the 5% anhedral spheroidal to euhedral, rhombic in- reply to the Buick et al. (1995) comments, Lowe conical form is not one of the characteristics of clusions of carbonate <5 µm in size (Fig. 3D). (1995) countered that these structures are too diapirs. Nor are the Warrawoona features cone- Opaque matter makes up <1% of the rock, form- small to allow differentiation between biogenic in-cone structures, percussion-like features found ing tiny crystals (pyrite?), and minute reticulate and abiogenic structures, and that their three- associated with calcareous shale beds, compris- patterns that possibly represent remobilized or- dimensional nature is uncertain; he reiterated his ing densely aggregated right circular cones, with ganic matter along grain boundaries in the silici- opinion that the Strelley Pool conical forms prob- ribbed or striated dislocation surfaces, their fied layers. Some tabular, bedding-parallel pock- ably represent the best candidates for biologically apices directed down. The conclusion that the ets in dolomite are preserved as cryptocrystalline mediated forms. Inasmuch as Lowe (1995) and conical forms are deformational thus is unten- chert of 1–10 µm grain size, and possibly repre- Buick et al. (1995) did not contribute new data to able. An alternative explanation involving accre- sent silicified fenestrae. No microfossils were the discussion of Lowe’s Strelley Pool structures, tion is more suitable. seen. The stromatolitic microfabric is transected we here use our new evidence to revitalize the 3. The continuity of laminae across different by narrow quartz veinlets with elongated crystals, original interpretation that biologic activity may types of architecture, such as cones and adjacent the long axes of which are perpendicular to sub- have been involved in the formation of these par- stubby, centimeter-sized, convexly laminated perpendicular to the vein boundaries. ticular constructs. branching pseudocolumns (Fig. 2C), is difficult to For the following reasons, we prefer an inter- reconcile with strictly chemical precipitation, DISCUSSION pretation of the conically laminated pseudo- which would be expected to be isopachous. It is, columns that includes a biogenic component. however, well known in Proterozoic stromatolite We here address the nature and origin of the 1. The laminae in the conical pseudocolumns occurrences considered to be biogenic. Stromato- conical and branching pseudocolumnar Warra- are of distinctly greater uniformity than those in lites with rounded cones and obliquely accreting woona Group structures, and base our arguments the flattish intermound areas, suggesting (1) that domal laminae similar to those in Figure 2 are mainly on new morphologic evidence from the somewhat more variable environmental condi- widespread. For example, they are found in the Trendall locality. This restudy was prompted by tions prevailed in the intercone depressions, Neoarchean Transvaal Supergroup of South Africa the controversy engendered by the reinterpreta- (2) that different processes were active there, or (Bertrand-Sarfati and Eriksson, 1977, Fig. 15), the tion of what were considered to be the oldest bio- (3) that the relative contributions of the individual Mesoproterozoic Apache Group of Arizona genic stromatolites (Lowe, 1992, 1994, 1995; processes differed in the cone and intercone ar- (Bertrand-Sarfati and Awramik, 1992, Fig. 12, A Buick et al., 1995). Lowe (1980, 1983) originally eas. The wispy to lenticular nature of the inter- and B), and the Neoproterozoic Atar Group of interpreted laminated conical mounds in the cone laminae could represent rearrangement of Mauritania (Bertrand-Sarfati and Moussine- Strelley Pool Chert as biogenic, citing similarities loose detrital material due to water motion, par- Pouchkine, 1985, Fig. 10D). The wide variety of in gross morphology and internal alternating light ticularly if there was an excess of sediment that architecture demonstrated by such cones was il- and dark fine laminae characteristic of younger could not be stabilized. lustrated by Vlasov (1977). Somewhat similar stromatolites, particularly Conophyton. Although 2. The stacking of the second-order, low- steep conical forms with second-order corrugation an axial thickening of the laminae was observed, amplitude corrugate lamination on the slopes of referred to Conophyton also occur in a Messinian no axial zones per se were recognized; detrital the cones is directed obliquely upward, away (Neogene) carbonate complex associated with grains and laminae in the flat intermound areas from the axis of the conically laminated pseudo- evaporites in southeastern Spain (Feldmann and were ascribed to trapping and binding on mats, column, and the convexities become attenuated McKenzie, 1997, Fig. 13). The bifurcating struc- presumably cyanobacterial, in a subtidal to lower in the same direction, suggesting upward accre- ture in Figure 2C is explainable as a construction intertidal, evaporative setting. tion rather than downward-directed slumping or of steadily thickening laminae over a small initial Lowe (1992, 1994) reinterpreted his conically sideways compression. If folding were at the ori- convexity that rapidly grew into a steep-sided pro- laminated structures as being due to precipita- gin of the structures, one would have to explain tuberance that then bifurcated, before being buried tion, citing as evidence the restriction to an evap- how folding produced widely spaced cylinders by attenuating overlying laminae. orative setting, the presence of extremely fine and 10 cm wide and as much as 80 cm high (upward 4. That contiguous to widely spaced conical even lamination, the lack of crinkling and axial continuations of pseudocolumns in Fig. 2A), forms of centimeter to decimeter dimensions, al- zones, no evidence for soft surfaces, folded or with internal cones pointing only upward, situ- ways pointing stratigraphically upward, and contorted lamination, or mat fragments, and the ated between areas with more wispy flat lamina- stacked with high degrees of inheritance and pre- absence of gas-filled pockets or fenestrae. He ac- tions, and how the second-order corrugate lami- ferred azimuth of elongation, occur over tens of knowledged, however, that simple cones are not nation of the cones is produced (Fig. 2B). In square kilometers, but are restricted to thin dolo- known as inorganic evaporitic structures, and that addition, a different stress field or lamination stone or silicified carbonate units at specific the presence of a cover of biological material thickness would be necessary in an immediately stratigraphic levels, indicates that their formation cannot be ruled out. underlying bed that contains contiguous or is unlikely to be due to deformational processes Buick et al. (1995) subsequently commented closely spaced cones (Fig. 2D). It is difficult to such as those that could produce interference

1260 Geological Society of America Bulletin, August 1999

folds. Rather, they can be more rationally consid- younger rocks, is represented by taxa such as domal and branching stromatolites is generally ered to connote growth with distinctive morphol- Thyssagetes and Jacutophyton. attributed to environmental influences, such as ogy, resulting from accretion on a substrate under 5. Slopes on the cones are characteristically currents and phototropism. This may also apply basin-wide uniform conditions existing for lim- higher than 40°, attaining as much as 75° to the preferred orientation of long axes in coni- ited intervals of time. (Fig. 2E), and thus are far in excess of the angle of cally laminated stromatolites with elliptical or The question arises whether the accretion is to- repose of loose granular material; this connotes lanceolate cross sections, although such an inter- tally abiogenic. On the one hand, it has now been rhythmic accumulation of mineral matter on or in pretation does not account for the existence of shown that biofilms are involved in the formation a substrate of steep conical shape. Such accumu- multiple radial ribs and their preferred azimuthal of geyserites (Cady and Farmer, 1996), long lation thus cannot be due to simple mechanical orientation. In different Proterozoic occurrences, thought to be a good model for generating abio- sedimentation, but must reflect chemical and/or such alignment may follow the elongation direc- genic stromatolitic structures (Walter et al., 1976). biological action or binding. We are not aware of tion of the ellipse (e.g., Trompette 1969, Plate 3; On the other hand, Grotzinger and Rothman any literature on abiogenic precipitative structures Bertrand-Sarfati and Moussine-Pouchkine 1985, (1996) modeled abiogenic stromatolite morpho- with laterally linked conical forms of the type en- Fig. 100b); be arranged at some consistent angle genesis using a continuum partial-differential countered in the Warrawoona Group. The conical to it; be bifurcating; or be apparently random equation that expresses the individual contribution forms interpreted as abiogenic in the Paleopro- (H. Hofmann, personal observation). Tectonic to growth by (1) fallout of suspended sediment, terozoic Cowles Lake Formation (Grotzinger and deformation was adduced as an explanation for (2) diffusive downslope movement of settled sedi- Rothman, 1996) do not show the combination of the preferred alignment of the elliptical axes of ment and surface-tension effects in chemical pre- steep slopes and flat intercolumn areas, nor the Conophyton in Mauritania (Monod, in Trompette, cipitation, (3) surface-normal precipitation, and second-order convex-up curvatures. The accumu- 1969, p. 135), but Trompette (1969, p. 136) con- (4) random effects; compaction was not included. lation in steep conical shapes requires the pres- cluded that the flattening of the shape was syn- The model fits conically laminated structures with ence of a substantially cohesive substrate capable chronous with growth; Bertrand-Sarfati and growth surfaces that have self-affine fractal geom- of maintaining its morphologic integrity over time Moussine-Pouchkine (1985, p. 223, 227) subse- etry in the 1.9 Ga Cowles Lake Formation in to produce decimeter-sized laminated structures quently attributed a biological origin to the flat- northwestern Canada that are interpreted as purely over a wide area. Microbial mats, or at least slimy tening, excluding unidirectional currents as a abiogenic; analyses of convex-up stromatolites biofilms of organic material, together with ce- cause. Currents with bipolar orientation but of in the reef were not presented. Even though mentation, offer the most plausible explanation. different strengths may account for cones with an Grotzinger and Rothman’s (1996) results demon- Such conically laminated biogenic structures are asymmetrical position of the axis within the strate that the morphology of certain constructs well known from Phanerozoic and Cryptozoic oc- plane containing the long axes of the elliptical with inflexed laminae can be exclusively abiotic, currences, including modern ones, like the cono- cross sections. these authors conceded that microbes exist in vir- phytonid and thyssagetid stromatolites in Yellow- 7. The antiquity of life on this planet is widely tually all shallow-marine environments, and that stone National Park (Walter et al., 1976). Conical accepted, based on the evidence preserved in ap- their contributions to morphology may be masked stromatolites are characteristic features through- proximately coeval rocks in the form of authentic by abiotic factors. A separate term for biologic ac- out Cryptozoic time, being particularly abundant microfossils (including benthic assemblages), tivity does not appear in their equation, and for this in Neoarchean, Paleoproterozoic, and Mesoprot- kerogen, and the C-isotope fractionation record reason one might consider modifying the model to erozoic time, and their biogenicity, though rarely (e.g., Schopf, 1994; Strauss and Moore, 1992). accommodate stromatolites for which a significant demonstrable, is widely accepted by specialists Moreover, completely sterile surface environ- biological contribution can be demonstrated, such (Walter, 1996). Conical stromatolites containing ments are difficult to find on Earth; microbes today as pustular mats in intertidal zones. In order to sim- morphologically preserved microfossils are rare are known to live under very broad environmental ulate a biogenic stromatolite, one could add a fifth (e.g., see Schopf et al., 1984, p. 344), and even if conditions of temperature, pressure, salinity, pH, term to their equation 2, separately expressing bio- microfossils are preserved therein, it does not nec- and energy sources, and it is reasonable to suspect logical effects on lamina morphology. Possible ef- essarily follow that such biophoric structures must that this also applied during Archean time. There fects that come to mind are localized colony be biogenic, because the microfossils could be ad- thus is no imperative by which a biogenic contri- formation, the volume increase reflecting distinc- ventitious (Hofmann, 1973, p. 350). Until purely bution to the formation of the Warrawoona stro- tive growth patterns and dynamics of microbial chemical, conically laminated structures become matolites should be excluded. cell division (coccoid versus filamentous; three- known, it is preferable to use positive evidence dimensional versus linear reproduction), secretion over negative evidence; thus, we favor a biogenic CONCLUSIONS of extracelluar organic compounds, active micro- component in the interpretation of the Warra- bial motility, or behavioral responses to environ- woona conical structures, based on the similarity The Archean coniform stromatolites from the mental stimuli. Reproduction would tend to add, to with younger stromatolites. Warrawoona Group reported here are unlikely to a basically abiotic lamination, localized convex-up 6. Although phototaxis of filamentous mi- have formed by deformational processes. On the elements that progressively change shape as they crobes has been inferred as the mechanism re- basis of morphological evidence, such as illus- are stacked, just as observed in the convex laminae sponsible for the development of conical form in trated here, we conclude that accretion played the associated with the stromatolites at the Trendall the Yellowstone microbial cones (Walter et al., dominant role in shaping the structures. Using a locality (e.g., Fig. 1C) and, more elaborately, in 1976), and for Cryptozoic conical forms, no purely chemical process, it is difficult to account Jacutophyton reefs in general, or even bizarre widely accepted interpretation has yet been for the combination of steeply conical form mod- forms such as the J. sahariensis in Mauritania found for the formation of preferentially aligned ified by second-order corrugate lamination, coex- (Trompette, 1969; Bertrand-Sarfati and Moussine- elongated cones, such as those most spectacu- istence of cones with branching pseudocolumns, Pouchkine, 1985). The association of conical and larly developed in the Neoproterozoic carbonates and juxtaposition of uniform laminae in the con- convex-up laminae suggests that the Warrawoona of Mauritania (Trompette, 1969; Bertrand-Sarfati ically laminated parts with more uneven wispy, structures may be early representatives of what, in and Moussine-Pouchkine, 1985). Elongation in lenticular horizontal laminae in intercone areas.

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Formation, northern Cape Province, South Africa: Part 1: Nelson, D. R., 1997, Compilation of SHRIMP U-Pb geochronol- Their morphologic similarity to younger stroma- Systematic and diagnostic features: Palaeontologia ogy data, 1996: Western Australia Geological Survey tolites like Jacutophyton and Thyssagetes, africana, v. 20, p. 1–26. Record 1997/2, p. 142–149. viewed as the result of input from biologic activ- Bertrand-Sarfati, J., and Moussine-Pouchkine, A., 1985, Schopf, J. W., 1994, The oldest known records of life: Early Evolution and environmental condition of Conophyton- Archean stromatolites, microfossils, and organic matter, ity, indicates that they may have a similar origin, Jacutophyton associations in the Atar Dolomite (Upper in Bengtson, S., ed., Early life on Earth. Nobel Sympo- and may thus be of paleontological interest. We Proterozoic, Mauritania): Precambrian Research, v. 29, sium 84: New York, Columbia University Press, p. therefore regard the stromatolites at the Trendall p. 207–234. 193–206. Buick, R., Dunlop, J. S. R., and Groves, D. I., 1981, Stroma- Schopf, J. W., Zhu, W-Q., Xu, Z.-L., and Hsu, J., 1984, Prot- locale as constituting additional evidence, albeit tolite recognition in ancient rocks: An appraisal of ir- erozoic stromatolitic microbiotas of the 1400–1500 Ma- nonconclusive, for microbial activity in hyper- regularly laminated structures in an Early Archaean old Gaoyuzhuang Formation Near Jixian, northern China: chert-barite unit from North Pole, Western Australia: Precambrian Research, v. 24, p. 335–349. saline settings, supporting the opinion of Lowe Alcheringa, v. 5, p. 161–181. Strauss, H., and Moore, T. B., 1992, Abundances and isotopic (1983) that microbes were involved in the forma- Buick, R., Groves, D. I., and Dunlop, J. S. R., 1995, Abiologi- compositions of carbon and sulfur species in whole rock tion of conically laminated Warrawoona stroma- cal origin of described stromatolites older than 3.2 Ga: and kerogen samples, in Schopf, J. W., and Klein, C., eds., Comment: Geology, v. 23, p. 191. The Proterozoic biosphere: A multidisciplinary study: tolites. Our data provide encouragement in our Burne, R. V., and Moore, L. S., 1987, Benthic microbial com- Cambridge, Cambridge University Press, p. 709–798. search for even better preserved and more reveal- munities and microbialites: Baas Becking Geobiological Thorpe, R. I., Hickman, A. H., Davis, D. W., Mortensen, J. K., ing locales for study. Laboratory, Annual Report 1985, p. 10–12. and Trendall, A. F., 1992, U-Pb zircon geochronology of Cady, S. L., and Farmer, J., 1996, Fossilization processes in Archean felsic units in the Marble Bar region, Pilbara siliceous thermal springs: Trends in preservation along Craton: Precambrian Research, v. 56, p. 169–189. ACKNOWLEDGMENTS thermal gradients, in Bock, G. R., and Goode, J. A., eds., Trompette, R., 1969, Les stromatolites du “Précambrien Evolution of hydrothermal ecosystems on Earth (and supérieur” de l’Adrar de Mauritanie (Sahara occidental): Mars?): Ciba Foundation Symposium 202: New York, Sedimentology, v. 13, p. 123–154. Financial support from the Natural Sciences John Wiley & Sons p. 150–173. Vlasov, F. Y., 1977, Precambrian stromatolites from the Satka and Engineering Research Council of Canada Feldmann, M., and McKenzie, J. A., 1997, Messinian stroma- Suite of the Southern Ural, in Materialy po paleontologii tolite-thrombolite associations, Santa Pola, SE Spain: srednego paleozoya Urala i Sibiri. Sverdlovsk: Akademiya (grant A7484) for Hofmann, and logistical sup- An analogue for the Palaeozoic?: Sedimentology, v. 44, Nauk SSSR, Uralskii nauchnii tzentr; Trudy instituta ge- port from the Geological Survey of Western Aus- p. 893–914. ologii i geokhimii (in Russian), v. 126, p. 101–128. tralia as part of the Pilbara Craton Regional Geo- Grey, K., 1984, Abiogenic stromatoloids from the Warrawoona Walter, M. R., 1978, Recognition and significance of Archaean Group (Early Archaean), Shaw River, Marble Bar, stromatolites, in Glover, J. E., and Groves, D. I., eds. Ar- science Project, are gratefully acknowledged. We 1:250 000 sheet area: Geological Survey of Western Aus- chaean cherty metasediments: Their sedimentology, mi- thank Alec Trendall, who first drew our attention tralia Palaeontology Report 74/84, 11 p. cropalaeontology, biogeochemistry, and significance to Grotzinger, J. P., and Rothman, D. H., 1996, An abiotic model mineralization: University of Western Australia Special to the locality, M. Kachaami, for technical assis- for stromatolite genesis: Nature, v. 383, p. 423–425. Publication 2, p. 1–10. tance in the preparation of specimens, and C. W. Hickman, A. H., 1983, Geology of the Pilbara Block and its en- Walter, M. R., 1983, Archean stromatolites: Evidence of the Jefferson, J. Martignole, M. R. Walter, J. P. virons: Western Australian Geological Survey Bulletin Earth’s earliest benthos, in Schopf, J. W., ed., Earth’s ear- 127, 268 p. liest biosphere—Its origin and evolution. Princeton, New Grotzinger, J. W. Schopf, and an anonymous re- Hofmann, H. J., 1973, Stromatolites: Characteristics and util- Jersey, Princeton University Press, p. 187–213. viewer for critical comments. Grey and Hickman ity: Earth-Science Reviews, v. 9, p. 339–373. Walter, M. R., 1994, Stromatolites: The main geological source publish with permission of the Director of the Hofmann, H. J., 1999, Archean stromatolites as microbial of information on the evolution of the early benthos, in archives, in Riding, R., and Awramik, S. M., eds., Micro- Bengtson, S., ed., Early life on Earth. Nobel Symposium Geological Survey of Western Australia. This is bial sediments: Berlin, Springer-Verlag, in press. 84: New York, Columbia University Press, p. 270–286. Geological Survey of Canada Contribution Lowe, D. R., 1980, Stromatolites 3400-Myr old from the Ar- Walter, M. R., 1996, Old fossils could be fractal frauds: Nature, chaean of Western Australia: Nature, v. 284, p. 441–443. v. 383, p. 385–386. 1997274. Lowe, D. R., 1983, Restricted shallow-water sedimentation of Walter, M. R., Bauld, J., and Brock, T. D., 1976, Microbiol- Early Archean stromatolitic and evaporitic strata of the ogy and morphogenesis of columnar stromatolites Strelley Pool Chert, Pilbara block, Western Australia: Pre- (Conophyton, Vacerrilla) from hot springs in Yellow- REFERENCES CITED cambrian Research, v. 19, p. 239–283. stone Park, in Walter, M. R., ed., Stromatolites: Amster- Lowe, D. R., 1992, Probable non-biological origin of pre-3.2 dam, Elsevier, p. 273–310. Bertrand-Sarfati, J., and Awramik, S. M., 1992, Stromatolites of Ga-old “stromatolites” in the Barberton and Pilbara Walter, M. R., Buick, R., and Dunlop, J. S. R., 1980, Stromato- the Mescal Limestone (Apache Group, Middle Protero- greenstone belts: Geological Society of America Ab- lites 3400–3500 Myr old from the North Pole area, West- zoic, central Arizona): Taxonomy, biostratigraphy, and stracts with Programs, v. 24, no. 7, p. A137. ern Australia: Nature, v. 248, p. 443–445. paleoenvironments: Geological Society of America Bul- Lowe, D. R., 1994, Abiological origin of described stromato- letin, v. 104, p. 1138–1155. lites older than 3.2 Ga: Geology, v. 22, p. 387–390. MANUSCRIPT RECEIVED BY THE SOCIETY APRIL 17, 1998 Bertrand-Sarfati, J., and Eriksson, K. A., 1977, Columnar stro- Lowe, D. R., 1995, Abiological origin of described stromato- REVISED MANUSCRIPT RECEIVED JULY 20, 1998 matolites from the Early Proterozoic Schmidtsdrift lites older than 3.2 Ga: Reply: Geology, v. 23, p. 191–192. MANUSCRIPT ACCEPTED OCTOBER 14, 1998

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1262 Geological Society of America Bulletin, August 1999

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