Bartley J K, Kah L C, Frank T D, Lyons T W
Geology Department, Gustavus Adolphus College, St. Peter, MN, USA.
Geobiology. 2015 Jan;13(1):15-32. doi: 10.1111/gbi.12114. Epub 2014 Oct 29.
Offshore facies of the Mesoproterozoic Sulky Formation, Dismal Lakes Group, arctic Canada, preserve microbialites with unusual morphology. These microbialites grew in water depths greater than several tens of meters and correlate with high-relief conical stromatolites of the more proximal September Lake reef complex. The gross morphology of these microbial facies consists of ridge-like vertical supports draped by concave-upward, subhorizontal elements, resulting in tent-shaped cuspate microbialites with substantial primary void space. Morphological and petrographic analyses suggest a model wherein penecontemporaneous upward growth of ridge elements and development of subhorizontal draping elements initially resulted in a buoyantly supported, unlithified microbial form. Lithification began via precipitation within organic elements during microbialite growth. Mineralization either stabilized or facilitated collapse of initially neutrally buoyant microbialite forms. Microbial structures and breccias were then further stabilized by precipitation of marine herringbone cement. During late-stage diagenesis, remaining void space was occluded by ferroan dolomite cement. Cuspate microbialites are most similar to those found in offshore facies of Neoarchean carbonate platforms and to unlithified, buoyantly supported microbial mats in modern ice-covered Antarctic lakes. We suggest that such unusual microbialite morphologies are a product of the interaction between motile and non-motile communities under nutrient-limiting conditions, followed by early lithification, which served to preserve the resultant microbial form. The presence of marine herringbone cement, commonly associated with high dissolved inorganic carbon (DIC), low O2 conditions, also suggests growth in association with reducing environments at or near the seafloor or in conjunction with a geochemical interface. Predominance of coniform stromatolite forms in the Proterozoic--across a variety of depositional environments--may thus reflect a combination of heterogeneous nutrient distribution, potentially driven by variable redox conditions, and an elevated carbonate saturation state, which permits preservation of these unusual microbialite forms.
加拿大北极地区迪兹莫尔湖群中元古代苏尔基组的近海相保存了形态异常的微生物岩。这些微生物岩生长在水深超过几十米的地方，与更近端的九月湖礁复合体中高耸的锥形叠层石相关。这些微生物相的总体形态由脊状垂直支撑体组成，上面覆盖着向上凹的近水平单元，形成了帐篷状尖顶微生物岩，具有大量原生孔隙空间。形态学和岩石学分析提出了一个模型，其中脊状单元的准同期向上生长和近水平覆盖单元的发育最初导致了一种浮力支撑的、未石化的微生物形态。微生物岩生长过程中，通过有机单元内的沉淀开始石化作用。矿化作用要么稳定了最初中性浮力的微生物岩形态，要么促进了其坍塌。然后，海洋人字形胶结物的沉淀进一步稳定了微生物结构和角砾岩。在成岩作用后期，剩余的孔隙空间被铁白云石胶结物充填。尖顶微生物岩与新太古代碳酸盐台地近海相中发现的微生物岩以及现代冰封南极湖泊中未石化的、浮力支撑的微生物垫最为相似。我们认为，这种异常的微生物岩形态是营养限制条件下活动群落和非活动群落相互作用的产物，随后是早期石化作用，这有助于保存由此产生的微生物形态。通常与高溶解无机碳（DIC）、低氧条件相关的海洋人字形胶结物的存在，也表明其生长与海底或地球化学界面处或附近的还原环境有关。元古代各种沉积环境中锥形叠层石形态的主导地位，可能反映了由可变氧化还原条件潜在驱动的非均匀营养分布，以及较高的碳酸盐饱和状态，这使得这些异常的微生物岩形态得以保存。
