Abstract In the Central Unit of the South Riffian Ridges, the Toarcian succession is represented by two thick terrigenous detrital formations, Sned and Lmachkha formations, each one is divided into two members. The lower member of the Sned Formation, often rich in belemnite rostra and ammonites of the Tenuicostatum Zone, consists of grey marlstones, with rare marly limestone intercalations. The upper member consists of silty marlstones, with frequent thin limestone beds and contains rare ammonites of the Serpentinus Zone. Its upper part is locally rich in terebratulids. The lower member of the Lmachkha Formation consists of a silty marlstone and silty limestone alternation, and is the richest in benthic macrofauna, composed of rhynchonellids, gastropods, common bivalves, and rare ammonites of the Bifrons Zone. Seven taxa of gastropods and six taxa of bivalves were recorded from this member and described for the first time from the study area. The upper member of the Lmachkha Formation consists of silty marlstone and silty limestone alternation, and contains rare bivalves and rare ammonites of late Toarcian age. The lithofacies and pelagic macrofauna of the lower member of the Sned Formation characterise a relative deep marine environment unfavourable to benthic macrofauna. The second member indicates a gradual decrease in sea-level allowing from its upper part the recurrence of benthic macrofauna, especially terebratulids. The lithofacies and macrofauna of the lower member of the Lmachkha Formation indicate a shallow terrigenous shelf environment. The second member, containing coral reefs in the western and southern peripheries of the Central Unit, characterises a still shallow marine environment. This significant but gradual decrease in sea-level of the Central Unit from the latest early Toarcian to late Toarcian is likely related to the acceleration of sea-floor spreading in the Central Atlantic Ocean.

中文翻译：

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南里菲安海脊中央单元（摩洛哥北部）的托尔克系列的地层、古环境和软体动物（腹足类动物和双壳类动物）

摘要 在南里菲安海脊中央单元，托阿尔阶序列以两个厚的陆源碎屑地层为代表，Sned 和Lmachkha 地层，每个地层分为两个成员。Sned 组的下段通常富含 Tenuicostatum 带的箭闪石和菊石，由灰色泥灰岩组成，夹杂稀有的泥灰岩灰岩。上段由粉砂质泥灰岩组成，经常有薄薄的石灰岩层，含有蛇纹石带的稀有菊石。它的上部在当地富含 terebratulis。Lmachkha 组下段由粉砂质泥灰岩和粉砂质石灰岩交替组成，是底栖大型动物群中最丰富的，由双壳类动物、腹足类动物、普通双壳类动物和 Bifrons 带的稀有菊石组成。该成员记录了七个腹足动物分类群和六个双壳类分类群，并首次从研究区进行了描述。Lmachkha组上段由粉砂质泥灰岩和粉砂质灰岩交替组成，含有罕见的双壳类和晚托阿尔世时期的罕见菊石。Sned 组下部岩相和远洋大型动物群的特征是相对较深的海洋环境，不利于底栖大型动物群。第二个成员表明海平面逐渐下降，允许从其上部重新出现底栖大型动物，特别是 terebratulids。Lmachkha 组下段的岩相和大型动物群表明为浅陆陆架环境。第二个成员，包含中央单元西部和南部周边的珊瑚礁，描绘了一个仍然浅海的海洋环境。从最近的托阿尔世晚期到托阿尔世晚期，中央单元的海平面显着但逐渐下降，这可能与中大西洋海底扩张的加速有关。