Abstract The Calcarenite di Gravina is a poorly lithified, heterogeneous bio-lithoclastic Plio-Pleistocene limestone formed in a temperate shallow-water setting. Since prehistoric times it has been used for construction, as witnessed by the “Sassi”, the old city center of Matera (southern Italy). Here, the relatively distal, fine-grained bioclastic grain-packstone is characterized by large bioturbation structures of crustaceans. The aim of this work is to understand how bioturbation controls texture, porosity, permeability, and the cementation of the calcarenite. Crustaceans such as Callianassa are known to produce vertical burrows and sort grains. A bioturbated calcarenite level, outcropping in the Parco Scultura- Cava Paradiso in La Palomba (Matera northern area), has been selected as case study. Large rock samples including fossil traces and surrounding rocks have been analyzed. Optical microscope and SEM observations show a close connection between sediment sorting and cementation, and 3D analyses by means of CT scans indicate that the internal structure and the connected porosity of the samples is a function of the cementation. Moreover, the porosity network is reconstructed in 3D in order to derive the real connected porosity variations and their implications for the evolution in terms of diagenesis and alteration of the rocks. Indeed, a cementation gradient from the external lining of the burrows to the internal part of the trace fossil is observed. Cementation is less pronounced outside of the trace fossil. Cluster analysis of point-counting results allows to identify four sub-facies characterized by differences in composition, cementation and porosity. One of them is present in the interior of the burrows while the other two characterize the external lining (crust) and storage chamber of crustacean burrows. The last sub-facies occurs in the area surrounding the trace fossils and shows a weaker cementation. The presence of a cementation gradient inside the trace fossil is explained through a diagenetic model that considers the contribution of different factors. The geochemical environments allow the CaCO3 precipitation inside the burrows starting from the lining to the internal area thanks to the microbial actions, water circulation and relative oxygenation of fluids. Dissolution of aragonitic components probably represents the principal source of CaCO3. Variation in cementation and porosity are quite moderate, but capable of generating major differences in the resistance to weathering. From this study it emerges how the diagenetic processes lead to significant changes in the bioturbated fabric, making the burrows less permeable than the surrounding sediment. This is confirmed by the selective erosion that brings to light very well-preserved traces of Callianassidae even on building stones.

中文翻译：

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Calcarenite DI GRAVINA 地层中生物扰动沉积物的微观分析和 3D 重建（意大利南部）

摘要 Calcarenite di Gravina 是一种在温带浅水环境中形成的岩石化程度低、非均质的生物碎屑生物上新世-更新世石灰岩。自史前时代以来，它就被用于建筑，正如马泰拉（意大利南部）的老城中心“Sassi”所见证的那样。在这里，相对远端的细粒生物碎屑颗粒堆积石的特征是甲壳类动物的大型生物扰动结构。这项工作的目的是了解生物扰动如何控制方解石的质地、孔隙度、渗透率和胶结。众所周知，Callianassa 等甲壳类动物会产生垂直的洞穴并对谷物进行分类。已选择在 La Palomba（马泰拉北部地区）的 Parco Scultura-Cava Paradiso 出露的生物扰动方解石层作为案例研究。已经分析了包括化石痕迹和周围岩石在内的大型岩石样本。光学显微镜和扫描电镜观察表明沉积物分选与胶结作用密切相关，CT 扫描的 3D 分析表明样品的内部结构和连通孔隙度是胶结作用的函数。此外，孔隙度网络在 3D 中重建，以推导出真正相关的孔隙度变化及其对岩石成岩和蚀变演化的影响。事实上，观察到从洞穴外部衬里到痕迹化石内部的胶结梯度。在痕迹化石之外，胶结作用不太明显。点计数结果的聚类分析允许识别以成分差异为特征的四个亚相，胶结度和孔隙度。其中一个存在于洞穴内部，而另外两个则表征了甲壳类洞穴的外部衬里（地壳）和储藏室。最后一个亚相出现在遗迹化石周围区域，胶结作用较弱。痕迹化石内部胶结梯度的存在是通过考虑不同因素贡献的成岩模型来解释的。由于微生物作用、水循环和流体的相对氧化作用，地球化学环境允许洞穴内从衬砌到内部区域的 CaCO3 沉淀。文石组分的溶解可能是 CaCO3 的主要来源。胶结度和孔隙度的变化相当适中，但能够在耐候性方面产生重大差异。从这项研究中可以看出，成岩过程如何导致生物扰动结构发生显着变化，使洞穴的渗透性低于周围沉积物。选择性侵蚀证实了这一点，即使在建筑石头上也能发现保存完好的 Callianassidae 痕迹。