Facies and paleoenvironmental setting of Thrombolite-Stromatolite Reefs, Terminal Proterozoic Nama Group (ca. 550-543 Ma), central and southern Namibia
Facies and paleoenvironmental setting of Thrombolite-Stromatolite Reefs, Terminal Proterozoic Nama Group (ca. 550-543 Ma), central and southern Namibia
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发表时间：
2000
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通讯作者：
J. Grotzinger
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作者：
J. Grotzinger
血栓岩和叠层石礁产于纳米比亚中部和南部的元古宙末纳马群的几个地层水平。生物礁是纳马群内几个碳酸盐台地的组成部分，包括纳马盆地北方的Kuibis台地（Zaris次盆地）和纳马盆地南部的Huns台地（Witputs次盆地）。礁体由凝块岩（凝块内部结构）和叠层石（层状内部结构）组成，形成横向连续的生物层、孤立的斑块礁和孤立的尖峰礁，其规模从一米到几公里宽。在大多数情况下，生物礁在地层学上作为组成库比斯和匈奴台地的层序的海侵体系域内的一个完整相出现。这表明，一个制度的增加住宿需要形成发育良好的珊瑚礁，虽然珊瑚礁偶尔发生在高位系统域设置。在一个给定的TST内，随着时间的推移，增加住宿的制度有利于从片状生物基质几何形状过渡到更孤立的补丁和尖峰生物礁几何形状。类似地，增加容纳空间，如剖面向下沉积倾向，显示出类似的过渡，从更多的片状几何形状在上倾位置更孤立的几何形状在下倾位置。礁相包括血栓状穹丘、柱状体和土丘，内部凝块结构发育良好，此外还有叠层石状穹丘、柱状体和土丘，内部纹层发育较好。在可容纳空间相对较低的条件下，在较高流速和较大泥沙流入的条件下，上倾位置的叠层石较好地发育。血栓岩在可容空间较大、沉积物流入量较小的条件下较发育。这两种类型的微生物岩都与第一种钙化后生动物密切相关，这些后生动物可能附着在生态复杂的垫层所形成的粗糙地形中的隐蔽凹陷中，或者生活在这些凹陷中。在元古代末期出现的血栓质结构与Feldman和McKenzie（1998）的假设一致，即高等藻类和后生动物对蓝藻垫的定殖是产生凝块结构的重要过程。纳马血栓岩的织物保存完好，并显示出血栓性中基质被纤维状海洋方解石过度生长的证据，解释为前文石。随后是微晶灰岩充填体的侵位，原生白云石作为等厚边缘胶结物沉淀，随后是块状方解石晶石对剩余孔隙的闭塞。
Thrombolite and stromatolite reefs occur at several stratigraphic levels within the terminal Proterozoic Nama Group of central and southern Namibia. The reefs form integral parts of several carbonate platforms within the Nama Group, including the Kuibis platform of the northern Nama Basin (Zaris subbasin) and Huns platform (Witputs subbasin) of the southern Nama Basin. The reefs are composed of both thrombolites (clotted internal texture) and stromatolites (laminated internal texture) that form laterally continuous biostromes, isolated patch reefs and isolated pinnacle reefs ranging in scale from a meter to several kilometers in width. In the majority of cases, the reefs occur stratigraphically as an integral facies within the Transgressive System Tracts of sequences making up the Kuibis and Huns platforms. This suggests that a regime of increasing accommodation was required to form well developed reefs, though reefs occur sporadically in Highstand Systems Tract settings. Within a given TST, a regime of increasing accommodation in time favors the transition from sheet-like biostromal geometries to more isolated patch and pinnacle biohermal geometries. Similarly, increasing accommodation space, such as a transect down depositional dip, shows a similar transition from more sheet-like geometries in updip positions to more isolated geometries in downdip positions. Reefal facies consist of thrombolitic domes, columns and mounds with well developed internal clotted textures, in addition to stromatolitic domes, columns and mounds, with crudely to moderately well developed internal lamination. Stromatolites are better developed in conditions of relatively low accommodation, updip locations under conditions of higher current velocities and greater sediment influx. Thrombolites are better developed in conditions of relatively high accommodation and low sedi ment influx. Both types of microbialites are intimately associated with the first calcifying metazoan organisms which may have attached themselves or otherwise lived within sheltered depressions within the rough topography created by ecologically complex mats. The appearance of thrombolitic textures during terminal Proterozoic time is consistent with the hypothesis of Feldman and McKenzie (1998) that colonization of cyanobacterial mats by higher algae and metazoans was an important process in generating clotted textures. Fabrics in the Nama thrombolites are well preserved and show evidence of thrombolitic mesoclots being overgrown by fibrous marine calcite, interpreted as former aragonite. This was followed by emplacement of geopetal micrite fills, precipitation of primary dolomite as an isopachous rim cement, followed by occlusion of remaining porosity by blocky calcite spar.