No abstract

No abstract

No abstract

No abstract

The Triassic to Lowermost Jurassic mudrocks from continental redbeds of the Longi-Taormina Unit (Calabria-Peloritani Arc; southern Italy) have been mineralogically, chemically and petrographically analyzed, in order to reveal their complex history, which record an important phase of the geological evolution of the Mediterranean area.The Late Triassic corresponds with a low first-order sea level stand and a time of high continental emergence whereas the Early Jurassic was the time of the initiation of the first-order sea level rise in the mid-Mesozoic, generally marked by a transgressive trend.The mineralogical assemblage, dominated by the occurrence of illite and illite/smectite mixed layers, coupled to the CIA index and to the A-CN-K plot, indicates post-depositional K-enrichments.Palaeoweathering indices (CIW and PIA ratios) suggest that the source experienced intense weathering and that they likely record a recycling effect from their metasedimentary basement rocks. A recycling effect is also suggested by the distribution of Al 2 O 3 , TiO 2 , and Zr. In the Zr/Sc vs. the Th/Sc plot the redbeds are not clustered along the primary compositional trend but fall along a trend involving zircon addition and thus sediment recycling. Recycling could significantly affect the weathering indices which likely monitor a cumulative effect including a first cycle of weathering at the source rocks. Weathering occurred under hot, episodically humid climate with a prolonged dry season. Wet-humid conditions favored the formation of stream channels that eroded the soil profiles, whereas the dry season promoted the sedimentation. The climate alternation in the Early Jurassic favored recycling.An additional provenance terrane occurring in the basement, including metavolcanic rocks having mafic composition cannot be excluded, since the Eu anomaly is slightly higher than the PAAS value. Although the effects of recycling on REE distribution are uncertain, the Eu/Eu* should increase, as more feldspar is destroyed during weathering and diagenesis. This involves that the Eu/Eu* ratio could monitor a supply of low Eu/Eu* mafic detritus which compensate for the recycling effect by reducing Eu/Eu*. Eu released during the dissolution of feldspar could be retained by clay minerals contributing to minimize the recycling induced increase of the Eu-anomaly. This may involve that the recycling effect on the Euanomaly was minor and that the low Eu/Eu* mafic detritus supply was also minor. The subordinate importance of a mafic supply is confirmed also by other provenance proxies including the La-Th-Sc plot and the Cr/V and Y/Ni ratios.The proportions of illitic layers in I/S mixed layers coupled with the illite crystallinity values, expressed as Kübler Index (KI), suggesting an estimated temperature experienced by the Longi Units in the range of 100-150°C. Starting from this range the diagenetic/tectonic evolution should correspond to a lithostatic/tectonic loading of about 4-5 km.

No abstract

No abstract

Despite abundant data on volcaniclastic sand(stone), the compositional, spatial and temporal distribution of volcanic detritus within the sedimentary record is poorly documented. One of the most intricate tasks in optical analysis of sand(stone) containing volcanic particles is to distinguish grains derived by erosion of ancient volcanic rocks (i.e. palaeovolcanic, noncoeval grains) from grains generated by active volcanism (subaqueous and/or subaerial) during sedimentation (neovolcanic, coeval grains). Deep‐marine volcaniclastic sandstones of the Middle Topanga Group of southern California are interstratified with 3000‐m‐thick volcanic deposits (both subaqueous and subaerial lava and pyroclastic rocks, ranging from basalt, andesite to dacite). These rocks overlie quartzofeldspathic sandstones (petrofacies 1) of the Lower Topanga Group, derived from deep erosion of a Mesozoic magmatic arc. Changes in sandstone composition in the Middle Topanga Group provide an example of the influence of coeval volcanism on deep‐marine sedimentation. Volcaniclastic strata were deposited in deep‐marine portions of a turbidite complex (volcaniclastic apron) built onto a succession of intrabasinal lava flows and on the steep flanks of subaerially emplaced lava flows and pyroclastic rocks. The Middle Topanga Group sandstones are vertically organized into four distinctive petrofacies (2–5). Directly overlying basalt and basaltic‐andesite lava flows, petrofacies 2 is a pure volcanolithic sandstone, including vitric, microlitic and lathwork volcanic grains, and neovolcanic crystals (plagioclase, pyroxene and olivine). The abundance of quenched glass (palagonite) fragments suggests a subaqueous neovolcanic provenance, whereas sandstones including andesite and minor basalt grains suggest subaerial neovolcanic provenance. This petrofacies probably was deposited during syneruptive Periods, testifying to provenance from both intrabasinal and extrabasinal volcanic events. Deposited during intereruptive periods, impure volcanolithic petrofacies 3 includes both neovolcanic (85%) and older detritus derived from plutonic, metamorphic and palaeovolcanic rocks. During post‐eruptive periods, the overlying quartzofeldspathic petrofacies 4 and 5 testify to progressive decrease of neovolcanic detritus (48–14%) and increase of plutonic‐metamorphic and palaeovolcanic detritus. The Upper Topanga Group (Calabasas Formation), conformably overlying the Middle unit, has dominantly plutoniclastic sandstone (petrofacies 6). Neovolcanic detritus is drastically reduced (4%) whereas palaeovolcanic detritus is similar to percentages of the Lower Topanga Group (petrofacies 1). In general, the volcaniclastic contribution represents a well‐defined marker in the sedimentary record. Detailed compositional study of volcaniclastic strata and volcanic particles (including both compositional and textural attributes) provides important constraints on deciphering spatial (extrabasinal vs. intrabasinal) and temporal relationships between neovolcanic events (pre‐, syn‐...