Early marine cementation in two ooid-rich facies (Lower Jurassic Trento Platform, Southern Alps): Integrated fluorescence and cathodoluminescence analysis

Porosity and permeability represent the most important petrophysical properties influencing the possibility of storing mineral, hydrocarbon or water resources. In sedimentary rocks, sediment composition and depositional processes control primary porosity and permeability, which may be later modified (either increasing or decreasing) significantly during diagenetic stages. As diagenetic processes occur from near-surface to deep burial settings, paragenetic sequences in carbonates result in complex histories, due to superimposed processes (e.g., cementation, dissolution, dolomitization). A particular contribution to the diagenetic evolution of porosity and permeability properties is given by type and abundance of early marine cements, which may prevent mechanical and chemical compaction during burial. Here we present petrographic data on the diagenesis of two ooidal limestones, covering the entire shallow-water Trento Platform (Early Jurassic, Southern Alps): the Loppio Oolitic Limestone and the San Vigilio Oolite. Whilst the former comprises essentially fine and poorly to well-sorted ooidal grainstone, the latter includes well-sorted crinoidal and coated grains grainstone, associated with variably sorted ooidal grainstone, showing frequently fitted fabrics and mechanical and chemical compaction features (e.g., ooid breakage, concavo-convex to sutured grain contacts and stylolites). An integrated petrographic approach has been carried out using transmitted light (TL), cathodoluminescence (CL) and fluorescence (FL), to identify and characterize different generations of cements. Grainstone from the Loppio Oolitic Limestone are commonly characterized by very thin (5-10μm) isopachous fibrous early marine cement fringing grains, non-fluorescent and non-luminescent, predating precipitation of bladed prismatic cement overgrowths, dull green to slightly reddish in FL and dull brown in CL. Integrating TL-CLFL petrography proved essential for distinguishing between fibrous and bladed prismatic cements. Porosity is completely occluded by later equant calcite, dull to patchy bright green in FL and dull brown to patchy bright yellow in CL. Grainstone of the San Vigilio Oolite rarely display thin (10-15μm) isopachous fibrous early marine cement, non-fluorescent and non-luminescent, which is present in poorly compacted samples. Fittedfabric grainstone are instead characterized by discontinuous scalenohedral and syntaxial calcite (dull green in FL and zoned dull red to bright yellow in CL). The results indicate that integrated TL-CL-FL analysis is fundamental to resolve thin diagenetic phases and to provide insights into early marine cementation, which in this case is more efficient in the Loppio Oolitic Limestone. The role of early marine cements is double: reducing primary porosity and permeability during early diagenesis and decreasing compaction of partlycemented sediments during burial.