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A description of the microfabric of marine mudstones from Jagüel (Maastrichtian Danian) and Roca formations (Danian) has been performed by scanning electron microscopy (SEM). The mudstones were studied in four sections located near Cipolletti, Allen and Roca localities in the Río Negro province (Cerro Azul, Cerro Tres Picos, Cañadón Cholino and Este Puesto López). In these sections, the mudstone´s age was refined by calcareous nannofossils analyses as is reported in Musso et al., (2009) and Del Río et al., (In press). SEM observations were investigated using a LEO-EVO 40 (15 kV). Prior to SEM examination, freshly fractured surfaces of representative samples were coated with Au (SPI). The mineralogical characterization of the bulk sample and clay fraction was determined through X ray diffraction. Grain size analyses were carried out by a laser-beam particles analyzer (Cilas 1180 L). Bulk mineral compositions are mainly clay minerals (> 50%) and calcite (15%-35%). Quartz, feldspars, cristobalite and zeolites are present in smaller amounts. Clay minerals are dominated by Illite/Smectite (R0, 80% Sm), containing detectable amounts of caolinite and illite. The carbonate content is mostly biogenic (calcareous nannofossils). In Roca Formation caolinite has not been recognized and a high amount of Illite/Smectite (I/S) is detected. According to X-ray and grain size analyses, these rocks are classified as Allo - chemic mudstones. Considering the texture, in hand specimens, the mudstones are massive to slightly laminated. However, bioturbation can be observed in samples from Jagüel Formation. At low magnification (1000 X, 3000 X), mudstone´s microfabric of Jagüel Formation shows a looser packing of individual clay particles randomly oriented. An important amount of calcareous nannofossils is observed and they are rounded by clay flakes with an edge-to-edge arrangement. At higher magnification (8000 X), clay flakes are arranged in edge-to- face or face-to-face contacts. These individual flakes are forming domains with randomly shaped voids. The morphology of the I/S is slightly crenulated to flaky and individual particle size is around 4 μm. In Roca Formation mudstones, a preferred particle orientation can be seen in a few samples (1000 X, 3000 X). The texture is closed and is primarily represented by I/S particles arranged in face-to-face contacts forming a shingle type fabric. At higher magnification (5000 X, 7000 X), chains of clay plates formed by domains of stepped face-to-face oriented flakes are common. The morphology of the I/S is smooth, slightly crenulated, occasionally flaky and the size of the particles is around 2 μm. SEM evidences in mudstones from Jagüel and Roca formations would suggest that two main processes were involved in the origin of the microfabric: physico-chemical and bio-organic (Bennet et al., 1991). In both formations, clay was deposited in waters with high electrolytes concentration and hence flocculated (electrochemical mechanism). Nevertheless, the particle orientation found in some samples from Roca Formation would indicate a more dispersed clay deposition. In Jagüel Formation, calcareous nannofossils abundance exerts a significant influence on microfabric causing particle reorientation around the cocoliths (bio-organic mechanism).et al., (2009) and Del Río et al., (In press). SEM observations were investigated using a LEO-EVO 40 (15 kV). Prior to SEM examination, freshly fractured surfaces of representative samples were coated with Au (SPI). The mineralogical characterization of the bulk sample and clay fraction was determined through X ray diffraction. Grain size analyses were carried out by a laser-beam particles analyzer (Cilas 1180 L). Bulk mineral compositions are mainly clay minerals (> 50%) and calcite (15%-35%). Quartz, feldspars, cristobalite and zeolites are present in smaller amounts. Clay minerals are dominated by Illite/Smectite (R0, 80% Sm), containing detectable amounts of caolinite and illite. The carbonate content is mostly biogenic (calcareous nannofossils). In Roca Formation caolinite has not been recognized and a high amount of Illite/Smectite (I/S) is detected. According to X-ray and grain size analyses, these rocks are classified as Allo - chemic mudstones. Considering the texture, in hand specimens, the mudstones are massive to slightly laminated. However, bioturbation can be observed in samples from Jagüel Formation. At low magnification (1000 X, 3000 X), mudstone´s microfabric of Jagüel Formation shows a looser packing of individual clay particles randomly oriented. An important amount of calcareous nannofossils is observed and they are rounded by clay flakes with an edge-to-edge arrangement. At higher magnification (8000 X), clay flakes are arranged in edge-to- face or face-to-face contacts. These individual flakes are forming domains with randomly shaped voids. The morphology of the I/S is slightly crenulated to flaky and individual particle size is around 4 μm. In Roca Formation mudstones, a preferred particle orientation can be seen in a few samples (1000 X, 3000 X). The texture is closed and is primarily represented by I/S particles arranged in face-to-face contacts forming a shingle type fabric. At higher magnification (5000 X, 7000 X), chains of clay plates formed by domains of stepped face-to-face oriented flakes are common. The morphology of the I/S is smooth, slightly crenulated, occasionally flaky and the size of the particles is around 2 μm. SEM evidences in mudstones from Jagüel and Roca formations would suggest that two main processes were involved in the origin of the microfabric: physico-chemical and bio-organic (Bennet et al., 1991). In both formations, clay was deposited in waters with high electrolytes concentration and hence flocculated (electrochemical mechanism). Nevertheless, the particle orientation found in some samples from Roca Formation would indicate a more dispersed clay deposition. In Jagüel Formation, calcareous nannofossils abundance exerts a significant influence on microfabric causing particle reorientation around the cocoliths (bio-organic mechanism).