Evidence of a "Mid-Carnian" transgression in the western Southern Alps (Lombardy, Italy): stratigraphic and paleogeographic implications
Evidence of a "Mid-Carnian" transgression in the western Southern Alps (Lombardy, Italy): stratigraphic and paleogeographic implications
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DOI：
10.13130/2039-4942/5460
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发表时间：
2002-03
影响因子：
2.3
通讯作者：
F. Berra;F. Jadoul
中科院分区：
文献类型：
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作者：
F. Berra;F. Jadoul
在贝尔加马斯阿尔卑斯山北部，记录了在Breno碳酸盐台地和S.Giovanni Bianco FM的半干旱海岸碳酸盐岩-Sabkha相之间发育的“中-卡尼期”海侵序列。这一事件的特点是存在两个地层标志：a)深灰色页岩和粉砂岩(“黑色泥页岩”)，以前被认为是Gorno-Low S.Giovanni Bianco FMS的北部闭合，但重新解释为Lozio页岩沉积体系的西部尖灭。早卡尼期Lozio页岩首先沉积在Valle di Scalve-Lozio海槽中，后来覆盖碳酸盐台地(Breno FM)。B)贝加马斯山北部的潮下石灰岩化石、泥灰岩和洞穴状泥灰岩(“生物碎屑层”)。页岩和粉砂岩的扩张代表了伦巴第最后一次卡尼期层序的第一个海侵阶段，在“中卡尼期”(朱利安亚阶段)区域碳酸盐台地危机(Valcamonica Breno FM.的顶部)之后。“生物碎屑层”记录了在整个贝加马斯山识别和对比的正常、开阔的海相代表的MFS。Lozio页岩“黑泥岩”和Gorno-San Giovanni Bianco FMS的岩石学和化学特征不同。指示不同的源区：前者的特征是来自变质侵入区(向北和向西)的碎屑，而后者的特征是盛行的火山碎屑物质。气候变化(从干旱到相对潮湿的条件)可以用来解释南阿尔卑斯山西部“中卡尼期”碳酸盐台地的危机和细粒陆源物质的区域扩散。
A "mid-Carnian" transgressive succession, developed between the Breno carbonate platform and the semiarid coastal carbonates-sabkhas facies of the S. Giovanni Bianco Fm., is recorded in the northern Bergamasc Alps. This episode is characterized by the presence of two stratigraphic markers: a) Dark grey shales and siltstones ("Black Pelites"), considered previously as the northern closure of the Gorno-Lower S. Giovanni Bianco Fms., but re-interpreted as the western pinch-out of the Lozio Shale depositional system. The Early Carnian Lozio Shale was deposited first in the Valle di Scalve-Lozio trough and later covered the carbonate platform (Breno Fm.). b) Fossiliferous, open subtidal limestones, marls and burrowed marly limestones ("Bioclastic Horizon") of the northern Bergamasc Alps. The spreading of shales and siltstones represents the first transgressive stage of the last Carnian sequence in Lombardy, after the "mid- Carnian" (Julian substage) regional carbonate platform crisis (top of the Valcamonica Breno Fm.). The "Bioclastic Horizon" records the mfs represented by normal, open marine facies, identified and correlated throughout the Bergamasc Alps. Different petrographic and chemical characters between the Lozio Shale - "Black Pelites" and the Gorno-San Giovanni Bianco Fms. suggest different source areas: the former units are characterized by clasts derived from a metamorphic-intrusive area (placed northward and westward), whereas the latter units are characterized by prevailing volcaniclastic material. A climatic change (from arid to relatively humid conditions) may be invoked to explain the crisis of the "mid-Carnian" carbonate platforms in the western Southern Alps and the regional spreading of fine-grained terrigenous material.