Affect of Sedimentation on Stromatolite Reef Growth and Morphology, Ediacaran Omkyk Member (Nama Group), Namibia

Superbly preserved reefs of Ediacaran age in Namibia give clues to environmental controls on stromatolite-thrombolite growth morphology and nucleation. Digital mapping and Measured stratigraphic sections reveal parasequences featuring meter-scale alternation of shallow marine shale and stromatolitic-thrombolitic microbialites associated with other clastic carbonates (grainstones, mudstones). Stromatolite-thrombolite column width, spacing and height vary systematically with the type of sediment being deposited, with growth inhibited during shale deposition. Columns are generally wider and more closely spaced during carbonate sediment deposition and narrower and more widely spaced during shale deposition. While stromatolite growth should be sensitive to sediment type explicitly, we also interpret sediment type as a general proxy for changing environmental conditions (e.g. water depth, turbidity) that may directly affect reef growth. A simple rule-based numerical simulation of microbialite growth is formulated based on the field interpretations of sedimentological and topographic growth controls. The model cannot explain detailed morphologic attributes, but can recreate correlations between stromatolite column widths, column spacing and layer bed thickness as a function of sediment type. environmental conditions, such as water depth or turbidity, and sediment type may be a proxy for other variables affecting stromatolite growth. Sediment type is also the only variable for which evidence is directly preserved in the rock record. Stromatolites are expected to be directly sensitive to sedimentation rates and events because they grow by sediment accretion; too little sediment and the mat will not be preserved, whereas too much sediment may terminate growth because the active layer of microorganisms becomes buried (Grotzinger and Knoll, 1999). Recent studies of modern stromatolite growth have only considered the interactions between microorganisms and carbonate sand, not clay or siliciclastic sediment (MacIntyre et al., 2000; Reid et al., 2000). Local deposition of both clays and siliciclastic sediment would likely be detrimental to the microbial community, due to light blocking, inhibition of advection and diffusion of nutrients to the microorganisms, or a lack of early cementation preventing a hard or stable surface on which to develop. Stromatolites usually form in carbonate sediment because it lithifies early, giving them the necessary strength to maintain positive topography and to not get swept away by storms and currents (Grotzinger and Knoll, 1999). Finally, antecedent topography is likely to be a deterministic control on reef nucleation, again influenced by sediment type and sedimentation rate. Topographic highs will tend to get buried in less sediment, and will therefore be better locations for stromatolite growth. The importance of antecedent topography has been shown for coral reefs (Edwards and Brown, 1999; Ferro et al., 1999), and is expected for microbial reefs as well (Grotzinger, 1989; Grotzinger and Knoll, 1999). In general, coarser-grained sediments will tend to accumulate mostly in lows, where finer-grained sediments – particularly cohesive clays – will tend to stick to highs as well as accumulating in topographic lows. The goal of this paper is to evaluate the role of sediment type and relative sedimentation rate in controlling the growth and morphology of Ediacaran stromatolite-thrombolite reefs in the northern Nama basin, Namibia. The study involves digital mapping of reef geometry, reef-sediment relationships, and numerical simulation of these mapped geometries to better understand the controlling variables. Geologic and Stratigraphic Setting The reef examined in this study developed within a transgressive systems tract of a carbonate ramp system developed in a foreland basin of Ediacaran age (Figure 1; Adams et al., 2005; Grotzinger, 2000; Saylor et al., 1995). The reef occurs at the Zebra River farm, in the Omkyk Member of the Kuibis Subgroup (Nama Group). An ash bed constrains the Omkyk to be somewhat older than 548.8+/-1 Ma (Grotzinger et al., 1995). The stratigraphic architecture of the Omkyk Member at Zebra River was digitally mapped by Adams et al. (2005). The Omkyk Member is subdivisible into two sequences: Omkyk Sequence 1 (OS1) consisting primarily of shelf grainstones, and Omkyk Sequence 2 (OS2), which contains grainstones, shales, and thrombolite-stromatolite reefs and biostromes at multiple SOUTH AFRICAN JOURNAL OF GEOLOGY AFFECT OF SEDIMENTATION ON EDIACARAN STROMATOLITE REEF GROWTH AND MORPHOLOGY 88 Figure 1. Regional map of central and southern Namibia, showing the location of the field area within the Kuibis Subgroup of the Nama Group of sedimentary units. After Adams et al. (2005). Figure 2. Panoramic photograph of patch reefs in unit 1 and unit 2 of OS2, separated by a maximum flooding surface under the taluscovered slope. The unit 1 reef focused on in this work is in the lower left corner of the photograph, and the adjacent inter-reef sediment pocket of parasequences is in the lower center, with ledges that correspond to the tops of carbonate beds. Two people are circled for scale. stratigraphic layers. OS2 in turn is divided into five parasequence sets (units 1 to 5). Unit 1 forms the transgressive system tract and is a backstepping paraseqence set containing discrete patch reefs which are progressively onlapped and overlain by fine-grained sediments, dominated by shale. The reef studied here occurs in this lower unit (Adams et al., 2005). Unit 2 – the interval of maximum flooding – contains a smaller number of reefs, which managed to grow despite the maximum flux of shale at this time. Local variations in sediment type indicate higher frequency relative water depth fluctuations well expressed by meter-scale parasequences marked by alternating shale and carbonate. Consistent with the increase in shale deposition, a maximum flooding surface (representing the greatest water depth) occurs above the studied reef and separates OS2 unit 1 from unit 2, which in turn contains many well-developed patch reefs (Figure 2). These Ediacaran reefs at Zebra River offer excellent exposure of both the internal structure of reefs and the spatial relations between reefs. Adams et al. (2005) digitally mapped the field relations between reefs and explored quantitative spatial relations between reefs and depositional patterns within parasequence sets. In this complementary study we focus on internal depositional and growth structures within a single thrombolitestromatolite reef. We present quantitative measurements of reef parameters including stromatolite widths and time-equivalent interfingering clastic carbonates and terrigenous sediments. We show how parasequences formed of alternating shales and carbonates correlate with contemporaneous changes in reef growth, and we present a numerical simulation of stromatolite growth that suggests ways in which variable sediment type may cause the observed growth changes. Methods The geometric relations between stromatolites and associated clastic carbonates and terrigenous sediments were observed in the context of possible controls on stromatolite nucleation and their subsequent lateral and vertical growth. Measurements were then made of stromatolite column widths, the widths of sediment fill between stromatolite columns, the rotation of fractured stromatolite pieces, and reef layer thicknesses within a single stromatolite-thrombolite reef (Figure 3). Widths of stromatolite columns and sediment fill within the reef were primarily measured in the field, with some additional values later estimated from photographs. A stratigraphic section adjacent to the reef was measured at the centimeter scale in order to establish a reference for correlation of inter-reef strata into the reef itself. The superb exposures allowed four parasequences of alternating shale and carbonate to be defined and also traced into the reef core. An attempt was made to correct the measured thicknesses for the effects of compaction. For this approximation, we assumed carbonate sediment compaction to be negligible due to early lithification. We estimated shale compaction relative to carbonate from shale beds that contained carbonate nodules (assumed to lithify early with zero compaction), with changes in shale bedding thickness compacted around concretions giving an average amount of differential compaction. The mean value of compaction, given as strain (!l/lorig), is 0.63+-0.06 (1", 7 measurements). Compaction of reef layers was calculated from the fracturing and rotation of stromatolite columns (Figure 3). Because the stromatolites deform rigidly by rotating and breaking, the columns or column pieces can be used as strain markers to estimate vertical shortening of reef layers. In each reef layer, plunge angles of stromatolite fragments were measured in the field, with some additional angles measured from photographs. Plunges were measured systematically at equal vertical spacings, although segment lengths were not explicitly measured. The vertical rotated length ld of each segment is related to the unrotated length lu and plunge angle from horizontal q simply as ld=lu sin(#). The average vertical strain e in the layer is then given as $=%(1-ld)/n for n measurements, assuming lu =1. While this method of estimating the strain in each reef layer is only approximate, the measured strains are relatively small, and lateral variability in reef compaction appeared to be larger than the error that is caused by the strain measurement methodology. Uncompacted reef and inter-reef parasequence thicknesses were calculated using the respective reef layer and shale compaction estimates. J. JOHNSON AND J. P. GROTZINGER SOUTH AFRICAN JOURNAL OF GEOLOGY 89 Figure 3. Centre of unit 1 study reef, showing labeled reef parasequences that correspond to alternating carbonate and shale deposition of the inter-reef parasequences. Reef layers suggest variable compaction. Circled staff is 2 m in length. Layers are numbered corresponding to the parasequences and labeled “r” for reef, “s” reef growth equivalent to shale deposition, and “c” for growth equivalent to carbonate deposition. Note the fractured and rotated segments of stromatolite columns and sediment fill, used to constrain compaction of reef layers. Field observations of reef growth relationships Field observations suggest that shales tend to inhibit stromatolite nucleation and growth. Stromatolites were never observed to nucleate directly on clay-rich sediment. However, even thin carbonate beds allow stromatolite growth (Figure 4). The general progression of morphology and texture begins with nucleation on a horizontal surface formed by the tops of carbonate grainstone beds, and to a lesser degree carbonate mudstone beds. Crinkly, subhorizontal laminations develop on the carbonate sediment, and local highs gradually grow in amplitude until a distinct initial dome formed of crinkly laminations can be seen, with distinct sediment pockets adjacent to the dome. Domes then develop into well-formed columns with distinct intercolumn sediment fills. Tracing of bridging stromatolite laminae through these fills indicates that synoptic relief of the columns was typically small (cm scale). Shale appears to be more effective than carbonate sediment at smothering stromatolite columns, because a thin layer of shale is observed over the top of many individual columns and entire reef bodies. Shale also compacts more than carbonate sediment which could cause an observational bias that thin shale layers can effectively smother columns. In some places it appears that shale deposition effectively smothered the tops of lower stromatolites but not higher ones, suggesting that stromatolite growth could at least tolerate the environmental condition of high shale flux (Figure 4). Some reef growth must have occurred during shale deposition because upper and lower bounds of reefal growth increments correspond to shale deposition in the adjacent inter-reef depressions. During shale deposition, reef growth in some places reverted to more of a crinkly lamination fabric with individual stromatolite columns becoming less clear and distinct. Lateral progradation of reef margins occurred most often when reefal facies were able to downlap against carbonate beds; lateral expansion of reefs directly on shales was not observed. These episodes resulted in an interfingering pattern of growth in cross section. Lateral growth seems to occur by lateral extension of crinkly laminae, which then grew in amplitude to become stromatolites. Quantitative Correlations Figure 5 shows the stratigraphic column measured adjacent to the reef. The stratigraphy shows parasequences that alternate between dominantly carbonate and dominantly shale layers. We categorized beds as shale, carbonate mudstone, carbonate grainstone, or crinkly laminate (which encompasses all microbially-laminated beds including stromatolites). Parasequences 2 through 5 correspond to the main phase of reef growth exposed in outcrop. Parasequence 6 shale is continuous over the top of the reef outcrop marking the termination of reef growth; the reef has an overall narrowing (backstepping) trend from parasequences 2 through 5, consistent with its earlier interpretation as part of a transgressive system tract (Adams et al., 2005). No column widths or spacings are reported from parasequence 3 shale deposition because SOUTH AFRICAN JOURNAL OF GEOLOGY AFFECT OF SEDIMENTATION ON EDIACARAN STROMATOLITE REEF GROWTH AND MORPHOLOGY 90 Figure 4. Small stromatolite-thrombolite columns amongst alternating carbonate and shale layers. These are not part of the studied reef, but are in a laterally-equivalent stratigraphic interval. Columns nucleate on carbonate grainstone or mudstone layers, and are commonly covered by shale. Visible staff length ~60 cm. Figure 5. (a) Measured stratigraphic column thicknesses adjacent to the reef (Figure 2), with numbered parasequences of shale and carbonate deposition. Individual beds were categorized as shale, carbonate mudstone, carbonate grainstone, or microbially laminated carbonate, which included crinkly laminites and small stromatolite heads (e.g. Figure 4). These thicknesses are compared to measured reef thicknesses. (b) The same stratigraphic section in which shale has been decompacted, compared to uncompacted reef layer thicknesses. The decompacted inter-reef parasequences sum to be ~2.5 m thicker than the decompacted reef