Search

In this work different types of chert from the Oriñón Limestone Formation (upper Aptian-lower Albian) are studied. This formation outcrops in the eastern coast of Cantabria (Liendo-Castro Urdiales area) and includes outstanding and abundant cherty nodules, lenticular layers and crusts. The host rock is mainly a biocalcarenite (wackestone/packstone) of pellets, echinoids fragments, oysters, foraminifers and calcareous or calcified siliceous sponge spicules. The Oriñón Limestone Formation was deposited in a marine open-shelf environment and preserves a hardground of regional extent with particular chert crusts. The silica source is associated to the dissolution of siliceous sponge spicules or to their calcification. Most of the chert is constituted by mosaics of micro-cryptocrystalline quartz and calcedonite, and it is generated by the selective silicification of biogenic structures, mainly dwelling trace fossils (Thalassinoides isp.) because of the higher amount of organic matter and the higher porosity and permeability of the burrow infill. In the hardground, selective silicification affects body fossils such as belemnites, oysters and echinoids, and trace fossils (feeding burrows and borings) where in addition cherts is accumulated as an indeterminate crust. The silicification of the biogenic structures firstly occurred in form of opaline phases during the early diagenesis while the oxidation of the organic matter was active. Thus, Thalassinoides trace fossils affected by silicification preserve filaments and cocoids that might have had a microbial origin. Neoformation of dolomite and calcite occur only within the Thalassinoides trace fossils which indicates that diagenetic processes taking place within these burrows differed from those affecting the host rock and other biogenic structures. Dwelling trace fossils would have supposed a close micro-environment where the oxidation conditions changed from high to low rate.

In this work different types of chert from the Oriñón Limestone Formation (upper Aptian-lower Albian) are studied. This formation outcrops in the eastern coast of Cantabria (Liendo-Castro Urdiales area) and includes outstanding and abundant cherty nodules, lenticular layers and crusts. The host rock is mainly a biocalcarenite (wackestone/packstone) of pellets, echinoids fragments, oysters, foraminifers and calcareous or calcified siliceous sponge spicules. The Oriñón Limestone Formation was deposited in a marine open-shelf environment and preserves a hardground of regional extent with particular chert crusts. The silica source is associated to the dissolution of siliceous sponge spicules or to their calcification. Most of the chert is constituted by mosaics of micro-cryptocrystalline quartz and calcedonite, and it is generated by the selective silicification of biogenic structures, mainly dwelling trace fossils (Thalassinoides isp.) because of the higher amount of organic matter and the higher porosity and permeability of the burrow infill. In the hardground, selective silicification affects body fossils such as belemnites, oysters and echinoids, and trace fossils (feeding burrows and borings) where in addition cherts is accumulated as an indeterminate crust. The silicification of the biogenic structures firstly occurred in form of opaline phases during the early diagenesis while the oxidation of the organic matter was active. Thus, Thalassinoides trace fossils affected by silicification preserve filaments and cocoids that might have had a microbial origin. Neoformation of dolomite and calcite occur only within the Thalassinoides trace fossils which indicates that diagenetic processes taking place within these burrows differed from those affecting the host rock and other biogenic structures. Dwelling trace fossils would have supposed a close micro-environment where the oxidation conditions changed from high to low rate. El presente trabajo estudia los diferentes tipos de sílex que aparecen en la Formación Calizas de Oriñón (Aptiense superior-Albiense inferior), en un zona concreta de la costa oriental de Cantabria (Liendo-Castro Urdiales), debido a su gran abundancia y espectacularidad. Las rocas que los incluyen son principalmente biocalcarenitas (wackestones/packstones de pellets, fragmentos de equinodermos, ostreidos, foraminíferos y espículas calcáreas o silíceas calcificadas), siendo el ambiente de depósito marino de plataforma abierta, y existiendo un hardground regional, donde las silicificaciones son diferentes. La fuente de la sílice proviene de la calcitización o disolución de las espículas de esponjas silíceas. Se determina que la gran mayoría de sílex (mosaicos de cuarzo micro-microcriptalino y calcedonita principalmente), se forman por la silicificación selectiva de estructuras biogénicas, siendo las galerías de Thalassinoidespreferentemente silicificadas, debido a que en su relleno existe mayor proporción de materia orgánica y mayor porosidad y permeabilidad. En las calizas que soportan el hardground además de las silicificaciones selectivas sobre fósiles (rostros de belemnites, ostreidos y equínidos), la sílice se acumula de forma importante en huellas de alimentación y rellenos de perforaciones, dando lugar a su vez a costras sobre bioturbaciones y perforaciones que a pesar de su importancia no han podido ser clasificadas. La silicificación de todas las estructuras biogénicas, originó inicialmente fases opalinas y tuvo lugar durante la diagénesis temprana cuando la oxidación de la materia orgánica estaba todavía activa, conservándose en los Thalassinoides silicificados formas (filamentos y cocoides) que podrían ser consideradas microbianas. La transformación ópalo-cuarzo fue temprana, preservándose dichas formas. En las galerías silicificadas de Thalassinoides los procesos diagenéticos son diferentes a los de los sedimentos carbonáticos marinos que las incluyen, y al resto de estructuras biogénicas silicificadas, existiendo en estas galerías neoformación de dolomita y calcita, en un microambiente cerrado donde las condiciones cambian de oxidantes a reductoras.

Proposed depositional models for Miocene Amazon foreland basin strata (Pebas Formation, Peru) are controversial. Recent depositional models include lacustrine and tidally influenced, brackish-water embayment. This paper presents data that support tidally influenced, brackish-water deposition for at least part of Pebas time (10-14 Ma). Two parasequences are presented (Santa Julia and Tamshiyacu). Both crop out along the Amazon River in Upper Amazonia near Iquitos, Peru. At these locations, abundant evidence of brackish-water, tidally influenced deposition is documented, including marginal marine bioturbation, sedimentary couplets, semidiurnal couplets (preserved in burrows), and pinstripe lamination. The deposits are locally highly bioturbated. At both locations ichnogenera normally associated with marine to brackish-water depositional environments are common. Three normally marginal-marine ichnofabrics are reported: (1) a Chondrites-reburrowed, Planolites ichnofabric resident only in massive-appearing muds; (2) a Scolicia (Laminites), Thalassinoides, Ophiomorpha ichnofabric that is manifested as intensely bioturbated silty sands; and (3) a Thalassinoides-generated ichnofabric that is interpreted to have descended into consolidated substrates and is thus representative of the Glossifungites Ichnofacies. Several trace fossils contain laminated infills organized into distinct sedimentary couplets that are best interpreted as resulting from semidiurnal processes. Six conclusions are arrived at: (1) sedimentological and ichnological data consistently indicate that sediment accumulation dominantly occurred in sporadically dysaerobic, marine to brackish water, under a tidal influence; (2) sediment accumulation occurred in bay-margin environments that prograded into a shallow, quiescent bay; (3) a stratified water column is indicated by the ichnofauna; (4) low accommodation space, repetitive and rapid adjustments of relative sea level, shallow wave base, and a stratified water column combined to generate an atypical parasequence architecture; (5) previously published isotopic data are consistent with sediment accumulation in brackish to marine water; and (6) marine incursion into Amazonia occurred during the Middle Miocene.