Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 251, part 1
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Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 251, part 1
Prev Page--Methods and Growth Form ||
Next Page--Paleoecology
Within the algal bafflestone, three different lithologies are associated with the in situ erect algal thalli. They are (1) the sediment upon which the colonizing individuals grew (substrate), (2) sediment that filled the upright thalli and upon which subsequent individuals grew (cup-filling sediment), and (3) sediment that smothered or halted growth because sedimentation rates exceeded the rate of upward algal growth (smothering sediment). Figure 5 illustrates the occurrence within, and relationships between, these three lithologies in the algal bafflestone. More than one colonization event occurred at the study site, so the three recognized lithologies are repeated vertically.
Figure 5--Lithologies associated with in situ Calcipatera communities. (a) Wackestone (smothering sediment) overlying wackestone (cup-filling sediment). The cup-shaped algal thalli occur along the bottom edge of the photo (scale bar = 1 cm). (b) Wackestone-packstone (substrate) below in situ algal thalli (scale bar = 1 cm). (c) Thin section at the contact between the cup-filling sediment (lower half of photo) and the smothering sediment above (scale bar = 1 mm). (d) Cup-filling sediment. Note the peloidal appearance (scale bar = 1 mm). (e) Thin section of the substrate lithofacies (scale bar = 1 mm). (f) Shamovella (arrow) in the substrate lithofacies.
Based on the grain size and the data given in tables 1 and 2, the incipient substrate (fig. 5b and 5e) is a coarse-grained wackestone-packstone (53.4% skeletal grains, 46.6% carbonate mud). The carbonate mud matrix has been recrystallized to microspar. Of the three lithologies associated with the in situ algae, the insoluble content is highest in this one (6.4%, see table 2). Quartz or similar insoluble minerals were not encountered in the point count of constituents of the substrate lithology; however, suggesting that these insolubles are probably clay minerals seems reasonable, although the mineralogy was not determined.
Table 1--Percentage of total rock constituents (from point-count method of Chayes, 1949).
| Rock Constituent | Substrate | Cup-filling Sediment |
Smothering Sediment |
|
|---|---|---|---|---|
| Calcipatera fragments | 0.0 | 6.1 | 1.4 | |
| Clasts of Calcipatera fragments and cup-filling sediment | 7.3 | 0.0 | 0.0 | |
| Shamovella | 7.5 | 1.6a | 2.9 | |
| Mobile textulariine foraminifers | 0.5 | 1.0a | 0.0 | |
| Encrusting foraminifers (Tuberitina) | 0.0 | 0.07a | 0.1 | |
| Bryozoan (fenestrate, ramose) | 19.5 | 0.1a | 1.3 | |
| Echinoderms | 11.2 | 0.5a | 0.9 | |
| Echinoid spines | 0.8 | 0.0 | 0.0 | |
| Impunctate brachiopods | ||||
 | Fragments | 0.30 | 0.07a | 0.6 |
| Whole, articulated | 0.1 | 0.2a | 0.2 |
| Whole, disarticulated | 0.07 | 0.0 | 0.0 |
| Pseudopunctate brachiopods | 0.07 | 0.0 | 0.0 | |
| Trilobites | 1.1 | 0.0 | 0.0 | |
| Ostracodes | 0.2 | 0.3a | 0.2 | |
| Bivalves | 0.2 | 0.0 | 0.0 | |
| Unidentifiable skeletal grains | 4.6 | 1.0a | 5.4 | |
| Hematite? crystals | 0.0 | 0.0 | 0.1 | |
| Peloids | 0.0 | 1.3 | 0.0 | |
| Sparite | 0.0 | 2.8 | 0.0 | |
| Carbonate mud | 46.6 | 85.0 | 86.7 | |
| TOTAL | 100.0 | 100.0 | 99.8 | |
| aMedium-sized carbonate grains | ||||
Table 2--Substrate and sediment constituents from point-count data (see table 1).
| Substrate | Cup-filling Sediment |
Smothering Sediment |
||
|---|---|---|---|---|
| Soluble Components | ||||
| Skeletal grains | 53.4% | 10.9% | 13.2% |
| Carbonate mud | 46.6% | 85.0% | 86.7% |
| Sparite | 0.0% | 2.8% | 0.0% |
| Peloids | 0.0% | 1.3% | 0.0% |
| Insoluble Componentsa | 6.4% | 1.7% | 5.2% | |
| aInsoluble components represents percent not dissolved by 10% HCl. | ||||
The most abundant skeletal grains in the substrate lithofacies are bryozoan and echinoderm fragments (19.5% and 12.0%, respectively, of the total rock constituents; see table 1). Shamovella, now considered a senior synonym of Tubiphytes (Riding, 1993), is currently considered to be a combination of a non-preserved soft-bodied organism and a cyanobacterial envelope as stated by Senowbari-Daryan and Flugel, 1993 (see Wahlman, 2002 for review). Shamovella composes 7.5% of the substrate constituents and occurs encrusting and binding other skeletal grains (fig. 5f). Nearly as abundant as Shamovella are clasts of Calcipatera fragments and the cup-filling lithology (7.3%, table 1). Carbonate mud, the most abundant rock constituent in the substrate lithofacies, fills small pores in the wackestone-packstone. However, carbonate mud is much less abundant in the substrate lithofacies than in the other two lithologies associated with the in situ algae (table 1).
The substrate colonized by Calcipatera was composed of skeletal grains, and in some cases, clasts of other lithologies, that were probably bound together into a relatively stable surface. A significant factor in the stabilization of this grain-supported substrate may have been the partial binding of skeletal grains by microbial activity; however, peloids, often created by microbial activity (Chafetz, 1986), were not observed in this lithology (see table 1). There is evidence that Shamovella encrusted some of the hard skeletal fragments. Sediment binding, primarily by non-preserved microbial activity such as encrustations and cementation (Chafetz, 1986), probably stabilized the substrate. To some extent Shamovella may have helped stabilize the otherwise loose substrate and provided a more suitable surface for Calcipatera colonization because it occurs encrusting Calcipatera fragments. Although phylloid algae appear to have been opportunistic (Wahlman, 2001), some sediment binding may have been beneficial. Conversely, Shamovella may not have inhabited the area until Calcipatera had colonized the substrate. From our studies, it is impossible to determine who colonized this substrate first: microbes, Shamovella, or Calcipatera.
At this study site, Calcipatera apparently grew on both coarse carbonate sand and carbonate mud substrates and fits Konishi's (1961) paleoecologic interpretation of Anchicodium (Calcipateria?), as summarized by Crowley (1969, p. 42); similar substrates have been reported for the coralline alga Archaeolithophyllum by Wray (1964).
The lithology filling the thalli of Calcipatera (cup-filling sediment) (fig. 5a and 5d) is a wackestone dominated by recrystallized carbonate mud (85.0%, see table 2), with skeletal grains making up 10.9% (6.1% of these skeletal grains are fragments of Calcipatera, see table 1). The insoluble content is low (1.7%, table 2). Additionally, peloids and sparry calcite are present-two constituents that are absent in the other two lithologies (see tables 1 and 2). Excluding Calcipatera fragments, skeletal grains (4.8%, table 1) are mostly medium-sized. The matrix has a clotted or peloidal appearance (fig. 5c and 5d) that varies from loosely packed peloids to a more densely packed, homogenous carbonate mud. These peloids are similar to pellets in that they are rounded or elliptical aggregates that range from 0.003 to 0.15 mm in diameter and are devoid of internal structure. The poorly defined boundaries of these peloids have irregular, indistinct outlines that appear as aggregates of carbonate mud supported by a matrix of re-crystallized carbonate mud. Chafetz (1986) has described similar peloids as the product of microbial cementation (see also Wahlman, 1988, 2002). The density of packing is greatest at the bottom of the thalli cup, becoming loosely packed near the top. Loose packing also was observed under algal fragments or other skeletal fragments that had fallen into the cup.
The carbonate mudstone, now re-crystallized, within the erect Calcipatera cups was probably the result of baffling by the gregarious Calcipatera thalli. Carbonate mud (less than 4 microns) settled and accumulated within the cup-shaped thalli as energy decreased. Fragments of Calcipatera within the cups (fig. 4) were probably broken from the erect thalli during turbulent episodes. Increased packing caused by compaction destroyed the outline of the peloids, suggesting that these peloids result from recrystallization or clotting during sedimentation or diagenesis, rather than being fecal in origin. Crowley (1969) observed similar textures in the Wyandotte Limestone (Pennsylvanian) where areas beneath supporting algal fragments had been sheltered from compaction. Geopetal pelletal sediment was reported within these sheltered voids, on the upper surfaces of broad fragments of phylloid algae, and within erect conical-vase forms of phylloid algae by Cross and Klosterman (1981). Sparry calcite (sparite of tables 1 and 2) filled the shelter cavities created by algal and other skeletal fragments and also occurs as replacement or recrystallization of skeletal grains. Terrigenous silt and clay-sized particles accumulated under the erect thalli and in some voids, suggesting these particles sifted into empty spaces during times of increased sedimentation.
The lithology immediately overlying the in situ Calcipatera community (smothering sediment) (fig. 5a and 5c) is a wackestone composed primarily of recrystallized carbonate mud (86.7%, tables 1 and 2). The percentage of skeletal grains in this lithology (13.2%) is slightly higher than in the cup-filling lithology, and the insoluble content (5.2%) approaches that found in the substrate lithology (6.4%). Unidentified skeletal grains (5.4%), Shamovella (2.9%), and Calcipatera fragments (1.4%) are the dominant skeletal components.
The contact between the smothering and cup-filling sediments is frequently a sharp, irregular (but nonerosional) surface, which is easily recognized (fig. 5a and 5c). As noted above, there are two, and sometimes three, separate intervals of in situ Calcipatera communities within the bafflestone at this locality. This suggests that growth of the in situ Calcipatera thalli was interrupted from time to time by sediment influx. The lithology overlying each in situ Calcipatera event is referred to as the smothering sediment because there is no evidence of in situ Calcipatera growth within this lithology. This lithology suggests that either a sudden influx of sediment terminated Calcipatera growth, or that the rate of sedimentation exceeded the rate of upward algal growth. The major differences between the two lithologies are the low percentage of skeletal grains and very high percentage of carbonate mud in the smothering sediment. The sudden influx of this smothering sediment was probably the result of storm deposition or shifts in the depositional environment.
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