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EARLY PROTEROZOIC ( 1 . 9 GA ) THROMBOLITES OF THE ROCKNEST FORMATION , NORTHWEST TERRITORIES , CANADA

Thrombolitic fabrics of the Rocknest Formation, Northwest Territories, Canada (Hoffman, 1975; Grotzinger and Hoffman, 1983; Grotzinger, 1986a, 1989b) were disputed by Kennard and James (1986) and Kennard (1989) and classified as cryptomicrobial boundstones. Under close re-examination, these rocks reveal mesoscopic and microscopic fabrics which are indeed thrombolitic when considered in the context of the recent tripartate microbialite classification scheme developed by Kennard and James (1986) and Kennard (1989). Thrombolitic fabrics from the Rocknest Formation demonstrate conclusively that, in cases of pervasive dolomitization, palimpsest fabrics can continue to provide evidence for the depositional environment of microbial communities. These thrombolites are significantly different from younger Proterozoic thrombolites (Aitken and Narbonne, 1989) and their Phanerozoic counterpoints (Kennard and James, 1986; Kennard, 1989), containing far less detrital sediment and greater amounts of marine cements, which suggests rethinking of models pertaining to the origin of thrombolites, as well as how microbial communities interact with their environment in the formation of characteristic diagenetic fabrics. Additionally, Rocknest thrombolites indicate that development of diagnostic, clotted fabrics may result from microbially-induced "inorganic" calcification of microbial communities, rather than by in situ calcification of microbial sheaths (Kennard and James, 1986; Kennard, 1989). This suggests a stronger reliance on physical and chemical aspects of depositional environments in the formation of thrombolitic fabrics than previously suggested. Thesis Supervisor: John P. Grotzinger Title: Assistant Professor of Geology


Introduction
introduced the term thrombolite to classify "cryptalgal structures related to stromatolites but lacking lamination and characterized by a macroscopically clotted fabric". Controversy followed and various authors further defined these clotted fabrics as either being abiogenic, such as the tufas of many Proterozoic successions (Walter, 1987, personal communication to Kennard, 1989), or as the end result in disruption or obliteration of originally laminated structures (Hofmann, 1973). Indeed, the inherent "splotchy", "mottled", or "clotted" fabric could be the result of a number of degradative activities, such as oxidation of organic material, internal solution of laminated fabrics, and bioturbation. Gebelein (1974) suggested that a splotchy or clotted fabric could be obtained simply by degradation of once living organic material. For example, oxidation of dead algae comprising mammilate and pustular mats of Shark Bay, Western Australia, leaves irregular, often large, voids (Logan, 1976; Dravis, 1982), which could subsequently be filled with sediment, giving the mat a clotted appearance (Monty, 1976). Garrett (1970) and Awramik (1971) report similarities of external form between thrombolites and stromatolites and that intergradations frequently occur between the two.They proposed that thrombolitic textures were perhaps the result of bioturbation of stromatolites. This would explain not only the decline of stromatolites at the end of the Proterozoic, but also the apparent evolution of thrombolitic fabrics, for with metazoan radiation in the Phanerozoic, "bioturbated" stromatolites would certainly increase as pristine forms decreased. This argument was underscored by Walter and Heys (1985), who stated that thrombolites "owe their origin to and record the first macroscopic burrowing and possibly boring by animals".
More recent work, however, suggests that the appearance of calcareous -4microbes such as Girvanella, Renalcis, Epipyhton, and Nuia has been important in the development of thrombolitic fabrics (Kennard and James, 1986; Kennard, 1989). These works propose that a secular difference in the nature of the microbial community is responsible for clotted thrombolitic fabrics, rather than a disruption or modification of originally laminated fabrics. They interpret thrombolites to be characterized by a mesoscopically clotted fabric resulting from the penecontemporaneous growth and calcification of discrete colonies of coccoid-dominated microbial communities.
Their origin at the Precambrian-Cambrian boundary is attributed to the first appearance of calcareous cyanobacteria (Kennard and James, 1986; Kennard, 1989).
Recent work on thrombolitic facies and fabrics (Kennard and James, 1986;Kennard, 1989;Pratt, 1982a;and Pratt and James, 1982) has contributed to revived interest in microbial buildups and their origin (Noble, 1985;Burne and Moore, 1987). However, with the exception of Kennard's (1989) intensive study of Cambrian and Ordovician thrombolites, there has been no systematic analysis of thrombolite occurrences in the rock record.

5-
The most recent report of possible Proterozoic thrombolites (Aitken and Narbonne, 1989)  Microscopic analysis of 25 large thin-sections (4x7cm), both at medium magnifications (10-40x), and at higher magnifications (40-100x), assisted -6with evaluation of microstructure. However, the most useful analyses of fabric relationships were gained through examination of thin sections with a microfiche reader at magnifications of 25x and 50x. This unorthodox method provided a greatly increased field of view compared to that obtained with conventional microscopes.
It is important to note that the definition of thrombolite followed in this study is that of Aitken (1967), in contrast with that of Kennard and James (1986) and Kennard (1989). In this definition, genetic connotations of the term thrombolite are dismissed. There are no restrictions placed on the specific type of microbial community, its form of preservation, or the abilities of the community to adapt or conform to a particular environment of deposition. Instead, usage is preferred to be purely textural and descriptional.
Additionally, for simplification in description, I propose the term thromboid head, defined here as composed of thromboids, and which in turn are constituents of a thrombolite. Thromboid heads are described as the intermediate-sized clotted bodies, often centimeters in size, which are externally defined by sediment fill, but with very little internal sediment.
The Rocknest Formation is a dolomitic, cyclic shelf sequence flanked by a barrier reefal rim and shoal complex. Stratigraphic relationships indicate long term progradation and aggradation of the shelf, interrupted by several episodes of incipient drowning associated with glacio-eustatic sea level oscillations, followed by a final, terminal drowning associated with rapid subsidence during collision (Grotzinger, 1986b).
Stratigraphy of the Rocknest Formation consists of shallowing-upward cycles of slope, outer shelf, shoal, and inner shelf facies. The cyclic inner shelf sequence, or lagoonal sequence, occurs over most of the shelf region and is exposed in the eastern and central thrust sheets, the autochthon of Wopmay Orogen, and in the Peacock Hills area of the Kilohigok Basin (Grotzinger, 1986a). It consists of asymmetric, upward-shallowing cycles of subtidal, interstratified carbonates and siliciclastics that shoal into upper subtidal and intertidal stromatolitic dolomites. These cycles are classified according to cycle-base lithologies, which in turn reflect their paleogeographic position on the shelf (Grotzinger, 1986a).
Paleoenvironmental data shows that these facies shoaled from subtidal to intertidal zones, with thrombolitic fabrics forming in areas of greater wave agitation than linked and domal stromatolites. Wave energy was not of sufficient intensity to cause elongation of the heads (Grotzinger, 1986a).
Additionally, thrombolitic fabrics are easily distinguishable in the field, are regionally extensive, and thus provide strong guides for areal cycle correlation. A sample from these thrombolites, which reveals a "massive" and "clotted" fabric (Grotzinger, 1986a(Grotzinger, , 1989b, was collected from an outcrop at the north end of Kikerk Lake and forms the basis of this study. Other possible occurrences of thrombolites in the Rocknest Formation are found in shelf-edge reefal facies, where they interfinger with contemporaneous stromatolites (Grotzinger, 1989b). These thrombolites are not considered in this study.

Descriptions
Mesoscopic Structure The Rocknest sample studied consists of a large portion of a single mound measuring one meter in height and 2 meters in diameter. The sample measures 13cm x 21cm in cross-sectional area and displays no less than 5cm of synoptic relief. The fabric consists of five distinct elements: 1) a dark grey (pale grey weathering) dolomite, with a macroscopically clotted texture; 2) a dark grey (pale grey weathering) drusy dolomite with a laminated texture; 3) a medium to pale grey (orange weathering) micritic dolomite; 4) a white (orange weathering) blocky dolomite spar; and, 5) a clear (black in appearance within slabbed sections) resistive and dark weathering silica Commonly, however, micritic dolomite, which appears primarily as a void filling fabric, interrupts the accretionary growth of clotted regions. Above these horizons, clotted, accretionary growth continues along renucleation, or regrowth surfaces. The composite structure of the sample is shown through growth of two main clotted regions, which primarily accreted vertically, but, through a series of renucleation events, coalesce into a single body.
Synoptic relief during a particular stage of growth was determined by measuring the maximium vertical relief displayed by the basal surface of a single colony of renucleated thromboids.
These heads tend to accrete vertically rather than grow elongate, and are generally domal, lobed, or vaguely club shaped. The thromboid heads commonly coalesce to form an irregularly massive framework of clots and associated voids (Fig. 3). These clots commonly display pendate bases, though laminar bases often occur at renucleation surfaces. Silicification within the framework is selective, and generally selectively replaces thromboid material rather than isopachous cements. Silicification of material within the micritic void fillings is not observed. Silicified regions are primarily found scattered irregularly within the interior of the heads, but the highest concentrations of silica appear within or near the margins of the thrombolite heads. Diagenetic silica comprises perhaps 5% of the framework mass. In contrast to the silica, the blocky, dolomitic spar occurs only within micritic interframework sediments, is not associated with thromboids, and only rarely is observed in thromboid heads. Within thromboid heads, blocky spar dominates small, internal sediment pockets.
The spar comprises perhaps 10% of the interframework material.

Microscopic Structure
Many microstructural details of the thromboid material within thromboid heads and larger clots have been obscured due to pervasive dolomitization.
Thrombolitic material now consist of a mosaic of anhedral dolomite crystals.
However, detailed examination of discrete grain size changes and subtle textural changes reveal clues to the previous fabric.   (Fig. 4, Fig. 5). This form constitutes the dominant fabric within the thrombolite framework material, imposing a splotchy to mottled fabric on the thrombolite microstructure.
Interframework sediment fills are microscopically a mosaic of anhedral, microcrystalline dolomite. The micrite/thrombolite interface is frequently defined by a rim of isopachous drusy cement (see Fig. 4, Fig. 13). In laminated fills, laminae can be distinguished on the basis of a grain size -12change. Laminae commonly warp downward, suggesting compaction, or become displaced, when in close proximity to blocky spar.
Dolomite spar is found exclusively within the micrite-filled voids and displays a bladed to dentate rim that grows inward to large, blocky, euhedral crystals (Fig. 6, Fig. 7). The bladed fringe is composed of crystals with a low length-to-width ratio (commonly 7 5 .:125k), suggesting dolomitization of primary high-magnesian calcite. Large, euhedral internal crystals range in size from 0.25-1.5mm. Individual dolomite rhombs occur discontinuously at the interface between the blocky spar and the crystalline, bladed fringe. Rarely, twin planes within the dolomite spar display a distinct curvature and exhibit the sweeping extinction indicative of saddle dolomite (Fig. 8). Blocky spar is additionally found filling tiny cracks that run through the entire sample.
Solution seams are found both within the micrite in interframework voids, and within the thromboid material itself. The solution seams are quite pervasive within the micritic interframework matrix of the thrombolite, but rarely extend for more than the width of the filled void. Only rarely do solution seams incorporate thromboid material. In these cases, solution seams most commonly occur at interfaces between blocky spar and thromboid material (Fig. 9). Morphological relationships observed between framework and interframework constituents suggest that little material has been removed from these regions. Solution seams also occur along the margins of micrite and thromboid material. These solution features occur primarily at the upper margin of the micrite (Fig.10), but only rarely are found along vertical or lower margins . Finally, solution seams occur discontinuously within the thromboid heads, predominantly along the interfaces between siliceous regions and thromboid material.
Siliceous regions within the thrombolite are composed of a mosaic of microcrystalline to cryptocrystalline silica, with occasional incomplete, -13bladed boundaries. Siliceous regions appear to nucleate preferentially within the interior of the thromboid heads, forming small, irregular patches.
Silica found at the margins of thromboid heads is in elongate patches, often following the contours of drusy cements. Microcrystalline regions commonly contain carbonate inclusions of undefined composition, which define two characteristic fabrics when viewed under plane light. The first is interpreted as parallel laminated, encrusting cements (Fig. 11) individual spheroids ranging from 50-10Op in size. This structure is seen predominantly in silica found internally within thromboids, and more rarely near the margins. In marginal areas, the spheroidal form is often found in close association with the laminated structure ( Fig. 12). At the margins of thromboids, drusy cements encrust regions of inclusion-defined spheroids.
Thromboid material in this case is often silicified, while encrusting cements are not, displaying the preference of silica for thrombolite material (Fig. 13).

Deposition and Early Cementation
Growth of the thrombolite was contemporaneous with the precipitation of encrusting cements. The end of most accretionary growth stages within the head appear to be marked by encrustationof the growth surface by drusy cements. Rapid cementation and preservation of the thrombolite fabric resulted from these precipitated, subaqueous crusts. The isopachous habit of the bladed cements, an elongate crystal fringe growing perpendicular to the substrate, is indicative of marine deposition, and is common in reefal subtidal to intertidal environments (Harris, et al., 1985;James, et al., 1976).

Rocknest Thrombolites
Investigations of the mesoscopic and microscopic structure of the Rocknest "clotted" microbialites indicate that they are not simply diagenetically altered laminated stromatolites (Kennard, 1989), but rather true thrombolites in the context of Kennard's classification (Kennard, 1989; Kennard and James, 1986). Kennard and James (1986) and Kennard (1989) considered the irregular fabric of the Rocknest thrombolites to be an early diagenetic, rather than a primary microbial fabric. However, closer study reveals that recrystallization during diagenesis did not severely alter the original fabrics. The silicification, as an early diagenetic replacement feature, helped to preserve the microscopic characteristics of the thromboid heads, both by preserving evidence of the encrusting cements and by enhancing the relict "mottled" or "splotchy" internal characteristics of the thromboid heads. The dolomitization, though pervasive, was discriminatory enough to preserve remnants of the original fabric. There is little doubt as to the mesoscopic or microscopic distinction between interframework and framework components, and palimpsest microscopic fabrics remain.
Both mesoscopic and microscopic structures within the thrombolite give indications as to its environmental setting and growth regime. Thromboid heads, composed of lobate thromboid bodies and synsedimentary marine cement, with little internal micritic sediment, indicate that the microbial community that formed the thrombolite was non-trapping and binding, and formed primarily through accretionary growth and marine cementation.  Gebelein (1974) in that the fabrics were not a result of voids left by the degradation of laminated mats. It is true that a vaguely unlaminated fabric can occur under these conditions, but these are documented as supratidal occurences (Logan, 1976;Shinn, 1968), while all evidence in the Rocknest thrombolites suggests formation in an active subtidal to intertidal environment.

Comparison with Younger Thrombolites
While evidence argues that the Rocknest thrombolites are definitely not disrupted stromatolitic fabrics, they also are quite different from younger, documented thrombolites. They contain, proportionally, a far greater ratio of framework to interframework components than do Kennard's Cambrian and Ordovician thrombolites (1989), and also a much greater volume of framework-building marine cements. However, in general form, the major constituents and fabrics are quite similar, even though preservation does not allow for microfossil characterizations. Lacking microfossils, it is impossible to accurately determine the microbial composition of the thromboids. However, two questions have yet to be resolved in the study of microbial structures. It remains to be determined whether distinct microbial forms are necessary to produce distinct microbial fabrics, and to what extent depositional environment is responsible for the growth habit of microbial communities and the resulting fabrics.
-19-Both Aitken (1967) and Kennard (1989) indicate the minor presence of filamentous cyanobacteria within thromboids, yet it has also been documented that unlaminated microbial growths are primarily the product of coccoid cyanobacteria (Gebelein, 1974). The lack of lamination, and the general impression of spherical-to-lobate-to-cellular forms of the Rocknest thrombolite microfabrics suggest that a coccoid-dominated community generated the clotted fabric. This feature parallels conclusions of Kennard and James (1986) concerning the original microbial make-up of the community in Cambro-Ordovician thrombolites. Kennard and James (1986) and Kennard (1989), however, interpreted the thrombolitic microbial fabrics to have been formed through the in situ calcification of the microbial community. The thrombolitic fabrics from the Rocknest Formation provide no evidence of in situ calcification. This is of little surprise, considering it has been noted (Riding, 1982) that no cyanobacteria is an obligate calcifier. Kennard (1989) and Kennard and James (1986) suggest that the calcification of cyanobacteria evolved at the base of the Cambrian due to pressures from the explosion of metazoan life. However, there is an increasing amount of evidence that the ability for in situ calcification may have evolved in the late Proterozoic (John P. Grotzinger, personal communication, 1990). Therefore, this change to in situ calcification in the late Proterozoic/early Phanerozoic most likely occurred as a reaction to some stimulus other than the evolution of metazoans, for example, a reaction to changes in the chemistry of seawater. Therefore, at a time such as the early Proterozoic, where calcification is not stimulated by outside forces microbial communities may not calcify, but instead become cemented in a different manner, either by early cementation of trapped and bound sediments, or by the precipitation of encrusting marine cements. Gebelein (1974) states that coccoid algae, which primarily form non-  Grotzinger, 1990, personal communication). At shallower water depths in these same cycles, inorganically precipitated tufas are found (Grotzinger, 1986a). These factors indicate an initial seawater chemistry favorable to the precipitation of marine cements. A rate of precipitation of marine cements higher than that favored by seawater chemistry could have been induced through the extraction of CO 2 from seawater during photosynthesis. This could result in encrustation and microbial recolonization at a rate greater than sedimentation rates, and thus produce fabrics such as those observed in the Rocknest thrombolites.  Distribution of thrombolite material (white) and micrite filled voids (black).

Conclusions
This "reversal" of color schemes (cf. Pratt and James, 1982;Webb, 1987 Thromboid microstructure, displaying lobate to spheroidal crystalline bodies encrusted by isopachous cements. Ovoid body of cement at right shows the intrusion of a thromboid from the third dimension. In the following illustrations, "T" denotes thrombolitic material, "M" denotes micrite, "V" denotes void fillings of blocky dolomitic spar, and "C" denotes isopachous encrustations of drusy cements. All photos taken under plane light unless otherwise stated. Scale bar is 0.5mm. Void filled with dolomitic spar displaying a bladed to dentate rim with the low length-to width ratio and scalanohedral terminations indicative of primary deposition as high-magnesium calcite. Scale bar is 0.5mm. Closer view of the bladed to dentate rim of a blocky spar void filling. Note the pendate bottom of upper thromboid. Scale bar is 0.5mm. Distribution of dolomitic void fillings in relation to thrombolite material and micritic sediments. Stylolitic seams form at the contacts between thrombolite material and spar-filled voids, perhaps due to pressure solution, or to the textural difference at the interface. Scale bar is 1mm.
-47- Diagenetic silica along the margin of a thromboid head. Dotted lines indicate extension of the encrusting cements into non-silicified thrombolite material.
Note that spheroidal configuration defined by carbonate inclusions occur internally within the cement-bounded thromboid head.
Scale bar is 1mm.

Figure 13
Inclusion-defined, spheroidal form of diagenetic silica. This form is found primarily within the thromboids and is oftern encrusted by isopachous, drusy cements. Preferential silicification of thromboid material over precipitated cements is observed. Scale bar is 0.5mm. -55-