Abstract
Calcareous and siliceous microorganisms are common components of mudrocks, and can be important in terms of stratigraphy and environmental interpretation. In addition, such microorganisms can have a significant ‘after life’, through post-mortem alteration, and represent a potential source of additional information about the diagenetic and deformation history of the rock unit. Some examples of the latter are illustrated in this study from foraminifera within a Cretaceous black shale of Colombia. This includes foraminifera tests acting as understudied repositories of authigenic calcite cement, and of elements such as Ba, Zn, Fe and S through the formation of baryte, sphalerite and iron sulphides (pyrite, marcasite). Such repositories, within the body chambers of foraminiferal tests, can provide important windows into the diagenetic processes within mudstones. If calcite cement is not recognised or separated from biogenic calcite, the depositional calcite budget can be easily overestimated, skewing the application of mudrock classification schemes, and affecting environmental interpretation including that of productivity. The elements Ba, Zn and Fe (often in ratio with Al) are commonly utilised as geochemical proxies of environmental parameters (productivity, bottom water redox conditions, etc.). Therefore, the presence of significant amounts of baryte, sphalerite and pyrite-marcasite (within foraminifera) should be noted and their origins (source and timing) investigated based on their spatial relationships before making environmental deductions based on geochemical analysis alone. Additionally, commonly observed marginal shell damage of many of the observed foraminifera is reported. We interpret this damage, for the first time, as an indicator of lateral dissolution, brought about by horizontal foreshortening during orogenesis. This is also supported by the occurrence of microscale anastomosing horizontal to inclined baryte-filled fractures within the mudstone matrix.
Highlights
Calcareous and siliceous nano- and microorganisms such as coccolithophores, foraminifera, and various forms of spicule are common within many marine mudstones, where they can form a significant component of the rock fabric (Figure 1)
Such fossilised remains of marine microorganisms can be important in terms of stratigraphy, environmental interpretation, evidence of transportation and sediment reworking, substrate consistency, flow dynamics, rates of sedimentation and as indicators of increased periods of productivity and nutrient upwelling [1,2,3,4,5,6,7,8,9]
The current paper foraminifera that display a range of interesting post-mortem features, (i) authigenic mineral infills body chambers and (ii) unusual marginal damage of the tests
Summary
Calcareous and siliceous nano- and microorganisms such as coccolithophores, foraminifera, and various forms of spicule are common within many marine mudstones, where they can form a significant component of the rock fabric (Figure 1). With foraminifera (f ), foraminifera stringers (fs), faecal pellets (fp), sulphides and sulphates (ss), and pyritised sponge spicule (p).illustrates an example of Cretaceous Colombian mudstone that contains. The current paper foraminifera that display a range of interesting post-mortem features, (i) authigenic mineral infills body chambers and (ii) unusual marginal damage of the tests. Eagle Ford Formation (USA), from similar age and environment to the Colombian material, have foraminifera that indicated display range of interesting post-mortem features,. Marginal damage of foraminifera testssimilar appears notage to have beenenvironment previously reportedto elsewhere, including frommaterial, the Eagle
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