Chapter 21 The Tambien Group, Northern Ethiopia (Tigre)

The Tambien Group of northern Ethiopia (Tigre), with probable correlatives in Eritrea, is a 2–3-km-thick siliciclastic–carbonate succession that was deposited in an intra-oceanic arc platform setting within the southern Arabian–Nubian Shield (ANS) area (southern extension of the Nakfa Terrane) of the Mozambique Ocean. Its deposition occurred prior to ocean closure between converging fragments of East and West Gondwana and concomitant structural emergence of the East African Orogen (EAO). The Tambien Group is well exposed and best studied in the Mai Kenetal and Negash synclinoria, where litho- and chemostratigraphy (including δ¹³C_carb, ⁸⁷Sr/⁸⁶Sr) provide the basis for a composite reference section. Two glaciogenic intervals have been suggested from exposures within the Didikama and Matheos Formation in the Negash Synclinorium. No reliable palaeomagnetic data exist to constrain the palaeolatitude of Tambien Group deposition and the southern ANS, but palaeogeographic reconstructions and evaporite pseudomorphs in lower carbonate units (Didikama Formation) imply low to intermediate latitudes (&lt;45°). Integration of available geochronological information (regional magmatism and detrital zircon) suggests <em>c.</em> 775–660 Ma as a plausible window constraining deposition of the prospective glacial intervals. The Tambien Group appears to preserve a coherent chemostratigraphic framework that can be effectively subdivided according to shifts in δ¹³C_carb polarity [polarity intervals A (+), B (–), C (+), D (–)]. Slates underlying and interstratified with polarity interval A carbonate preserve evidence of extreme chemical weathering that lessened prior to deposition of polarity interval B carbonate. Tambien Group carbonate units have sedimentological characteristics consistent with both shallow and deeper marine depositional settings. The lower prospective glacial interval lacks diagnostic sedimentological evidence of synglacial deposition, but is overlain by negative δ¹³C carbonate (polarity interval B) with sedimentological characteristics consistent with well-documented cap-carbonate successions. The upper prospective glacial interval in the Negash Synclinorium (Matheos Diamictite) best exhibits characteristics consistent with glaciogenic deposition (matrix-supported polymictic clasts, possible dropstones, possible bullet-nosed and striated clasts). In contrast to pericratonic rift margin settings that are common for Cryogenian glaciogenic deposits, palaeogeographic reconstructions for the 775–660 Ma timeframe place northern Ethiopia within an intra-oceanic setting that was likely far removed from cratonic hinterlands. More work on Tambien Group sedimentology, geochronology and palaeogeography is required to better evaluate the extent and timing of glacial conditions associated with the prospective glaciogenic intervals.

埃塞俄比亚北部提格雷地区的坦比恩群（Tambien Group）在厄立特里亚境内可能存在对应对比地层，其为一套厚度2~3千米的陆源碎屑-碳酸盐岩沉积序列，形成于莫桑比克洋域内阿拉伯-努比亚地盾（Arabian–Nubian Shield, ANS）南部区域——即纳克法地体（Nakfa Terrane）的南部延伸带——的洋内弧台地环境中。该群的沉积作用发生于东、西冈瓦纳聚合板块间洋盆闭合之前，且伴随东非造山带（East African Orogen, EAO）的同步构造隆升。坦比恩群在迈凯内塔勒（Mai Kenetal）与内加什复向斜（Negash synclinoria）中出露最为完整且研究程度最高，其岩石地层学与化学地层学（包含δ¹³C_carb、⁸⁷Sr/⁸⁶Sr数据）为构建综合参考剖面提供了核心依据。在内加什复向斜的迪迪卡马（Didikama）组与马塞奥斯（Matheos）组露头中，已被识别出两期冰成沉积间隔。目前尚无可靠古地磁数据可限定坦比恩群沉积时期以及阿拉伯-努比亚地盾南部的古纬度，但古地理重建结果与下伏碳酸盐岩地层（迪迪卡马组）中的蒸发岩假晶均指示，其沉积古纬度处于低至中等纬度范围（<45°）。整合现有年代学资料（区域岩浆活动与碎屑锆石数据），认为约775~660 Ma是约束该套潜在冰成沉积间隔沉积时限的合理区间。坦比恩群保留了一套连续完整的化学地层格架，可依据δ¹³C_carb的极性偏移划分为四个极性区间（极性区间A（正偏）、B（负偏）、C（正偏）、D（负偏））。覆于极性区间A碳酸盐岩之下且与其互层的板岩，记录了极端化学风化作用的证据，该风化作用在极性区间B碳酸盐岩沉积之前已显著减弱。坦比恩群的碳酸盐岩地层具备与浅海及深海沉积环境均匹配的沉积学特征。第一套潜在冰成间隔缺乏同沉积冰川作用的标志性沉积学证据，但其上方覆有负δ¹³C碳酸盐岩（极性区间B），该碳酸盐岩的沉积学特征与已有充分记载的盖帽碳酸盐岩序列一致。内加什复向斜中的第二套潜在冰成间隔（马塞奥斯杂砾岩（Matheos Diamictite））最能体现冰成沉积的典型特征：基质支撑的多组分碎屑岩、疑似坠石、以及可能的钝头状与擦痕砾石。与常见于克拉通周缘裂谷边缘环境的成冰纪（Cryogenian）冰成沉积不同，775~660 Ma时期的古地理重建结果显示，埃塞俄比亚北部处于远离克拉通腹地的洋内环境中。未来仍需针对坦比恩群的沉积学、年代学与古地理学开展更多研究，以更精准地评估这套潜在冰成沉积间隔对应的冰川作用范围与发生时间。