Sedimentary structures from the Carbondale Formation (Middle Pennsylvanian) of Northern Illinois / Charles W. Shabica --.

v.33:no.29(1978)

Conical sedimentary structures, trace fossils or not? Observations, experiments, and review

Conical sedimentary structures are widespread in the geological column. Those that are mediated by organisms (or organic matter) can be attributed to seven principal processes, which are investigated by experiment and/or field observations: (1) sand collapse into a cavity (decomposed body, open shaft, or gallery), (2) upward (escape) or downward locomotion by an organism through the sediment, (3) upward adjustment (equilibration), (4) casting of coelenterates' excavations, (5) organism-mediated soft-sediment deformation in heterolithic sediment, (6) biodeformational small and large excavations by organisms, and (7) fluid (gas or liquid) escape structures. Footprint loading may also be included. Criteria are given to distinguish between these processes. Dewatering pipes are best recognized by a zone of deformed and fluidized sediment at the base, and association with non-life (lethal) facies. Care must be used in assigning specimens to ichnotaxa, and it is generally necessary to slab, and also to carry out stratinomic investigation in the field.

Mangrove Forests and Sedimentary Processes on the South of Coast of Sao Paulo State (Brazil)

Mangrove structure and distribution is conditioned by geomorphic processes. This paper describes the response of mangroves to sedimentary processes at the Cananeia-Iguape Coastal System on the south coast of Sao Paulo State (Brazil), between latitudes 24 degrees 40`S and 25 degrees 20`S. Within six study areas 41 plots were established along 14 transects. Plot size varied according to stem density from 2mx2m to 20mx20m. Here mangroves are strongly coupled to sedimentary processes, forming discrete architectural elements within particular depositional environments or topographic settings. These sedimentary structures and progradation environments are colonized by Laguncularia racemosa, associated with the smooth cordgrass Spartina alterniflora. Rhizophora mangle occurs typically near creeklets where tidal flooding is more frequent. Where tidal influence is restricted Avicennia schaueriana becomes dominant. Erosive margins are dominated by A. schaueriana or R. mangle. Single linkage cluster analysis yields three groups (A, B and C), with high levels of similarity, providing support to the classification of the data into two broad landform categories: depositional and erosive. Group A includes plots with the least structural development (nominal stem diameter d(n) between 1.05 and 4.61cm). Group B is composed of stems of intermediate diameter (4.99 cm <= d(n) <= 5.63cm). Group C plots have the largest structural development (5.50 cm <= d(n) <= 11.10cm). The structure of mangroves (dominance and structural development) reflects responses to geomorphology and habitat change.

Holocenic proxies of sedimentary organic matter and the evolution of Lake Arari-Amazon Region

Four sediment cores were sampled from Lake Arari, located on Marajo Island at the mouth of the Amazon River. The island's vegetation cover is composed mainly of Amazon coastal forest, herbaceous and varzea vegetation. The integration of data on sedimentary structures, pollen, carbon and nitrogen isotope records, C/N ratios and radiocarbon ages allowed the identification of changes in vegetation and the sources of organic matter accumulated in the lake during the Holocene. The data indicate a relatively high flow energy, marine water influence and the presence of mangroves during the lagoon phase between 8990 and 8690 cal yr B.P. and 2310-2230 cal yr B.P. Between 2310 and 2230 cal yr B.P. and similar to 1000 cal yr B.P., the flow energy decreased and the mangroves were replaced by herbaceous vegetation following the decline in marine influence, likely due to the increase in freshwater river discharge. During the last 1000 years, Lake Arari was established in association with the expansion of herbaceous vegetation and the dominance of freshwater algae. (C) 2011 Elsevier BM. All rights reserved.

Pseudo-organic structures from the Precambrian Bass limestone in Arizona / [by] Matthew H. Nitecki --

v.23:no.1(1971)

Revisiting the Late Jurassic-Early Cretaceous of the NW South Iberian Basin: new ages and sedimentary environments

The study of the Upper Jurassic-Lower Cretaceous deposits (Higueruelas, Villar del Arzobispo and Aldea de Cortés Formations) of the South Iberian Basin (NW Valencia, Spain) reveals new stratigraphic and sedimentological data, which have significant implications on the stratigraphic framework, depositional environments and age of these units. The Higueruelas Fm was deposited in a mid-inner carbonate platform where oncolitic bars migrated by the action of storms and where oncoid production progressively decreased towards the uppermost part of the unit. The overlying Villar del Arzobispo Fm has been traditionally interpreted as an inner platform-lagoon evolving into a tidal-flat. Here it is interpreted as an inner-carbonate platform affected by storms, where oolitic shoals protected a lagoon, which had siliciclastic inputs from the continent. The Aldea de Cortés Fm has been previously interpreted as a lagoon surrounded by tidal-flats and fluvial-deltaic plains. Here it is reinterpreted as a coastal wetland where siliciclastic muddy deposits interacted with shallow fresh to marine water bodies, aeolian dunes and continental siliciclastic inputs. The contact between the Higueruelas and Villar del Arzobispo Fms, classically defined as gradual, is also interpreted here as rapid. More importantly, the contact between the Villar del Arzobispo and Aldea de Cortés Fms, previously considered as unconformable, is here interpreted as gradual. The presence of Alveosepta in the Villar del Arzobispo Fm suggests that at least part of this unit is Kimmeridgian, unlike the previously assigned Late Tithonian-Middle Berriasian age. Consequently, the underlying Higueruelas Fm, previously considered Tithonian, should not be younger than Kimmeridgian. Accordingly, sedimentation of the Aldea de Cortés Fm, previously considered Valangian-Hauterivian, probably started during the Tithonian and it may be considered part of the regressive trend of the Late Jurassic-Early Cretaceous cycle. This is consistent with the dinosaur faunas, typically Jurassic, described in the Villar del Arzobispo and Aldea de Cortés Fms.

Late Quaternary drainage dynamics in northern Brazil based on the study of a large paleochannel from southwestern Marajó Island

Marajó Island shows an abundance of paleochannels easily mapped in its eastern portion, where vegetation consists mostly of savannas. SRTM data make possible to recognize paleochannels also in western Marajó, even considering the dense forest cover. A well preserved paleodrainage network from the adjacency of the town of Breves (southwestern Marajó Island) was investigated in this work combining remote sensing and sedimentological studies. The palimpsest drainage system consists of a large meander connected to narrower tributaries. Sedimentological studies revealed mostly sharp-based, fining upward sands for the channelized features, and interbedded muds and sands for floodplain areas. The sedimentary structures and facies successions are in perfect agreement with deposition in channelized and floodplain environments, as suggested by remote sensing mapping. The present study shows that this paleodrainage was abandoned during Late Pleistocene, slightly earlier than the Holocene paleochannel systems from the east part of the island. Integration of previous studies with the data available herein supports a tectonic origin, related to the opening of the Pará River along fault lineaments. This would explain the disappearance of large, north to northeastward migrating channel systems in southwestern Marajó Island, which were replaced by the much narrower, south to southeastward flowing modern channels.

Marine influence in the Barreiras Formation, State of Alagoas, northeastern Brazil

Tidal processes were important for deposition of the Barreiras Formation located in northern Brazil, while correlatable deposits in northeastern Brazil have been traditionally related to continental environments. Facies analysis in southern Alagoas revealed that the Barreiras Formation consists of cross-stratified conglomerates and sandstones (facies Sx and Cgx), compound cross-stratified sandstones (facies Cx), and heterolithic beddings (facies H). A significant portion of these deposits occurs within channel morphologies displaying fining and thinning upward successions. An abundance of sedimentary features is comparable to those from the northern Brazilian counterpart. These include: tidal bundles; herringbone cross-stratification; heterolithic beddings with sandstone and mudstone beds in sharp contacts; and ichnofossils mostly consisting of Ophiomorpha nodosa, Skolithos and Planolites. Altogether, these features point to a marginal marine depositional setting dominated by tidal processes, which are related to an estuarine system, an interpretation also provided for the Barreiras Formation in northern Brazil. The widespread occurrence of deposits with unambiguous evidence of tidal processes in the Barreiras Formation of northern Brazil, and now in the State of Alagoas, leads to argue that the early/middle Miocene worldwide marine transgression might have left a much more widespread sedimentary record along the Brazilian coast than currently regarded.

SDAR: a package for plotting and analyzing stratigraphy data in R

Stratigraphic Columns (SC) are the most useful and common ways to represent the eld descriptions (e.g., grain size, thickness of rock packages, and fossil and lithological components) of rock sequences and well logs. In these representations the width of SC vary according to the grain size (i.e., the wider the strata, the coarser the rocks (Miall 1990; Tucker 2011)), and the thickness of each layer is represented at the vertical axis of the diagram. Typically these representations are drawn 'manually' using vector graphic editors (e.g., Adobe Illustrator®, CorelDRAW®, Inskape). Nowadays there are various software which automatically plot SCs, but there are not versatile open-source tools and it is very di cult to both store and analyse stratigraphic information. This document presents Stratigraphic Data Analysis in R (SDAR), an analytical package1 designed for both plotting and facilitate the analysis of Stratigraphic Data in R (R Core Team 2014). SDAR, uses simple stratigraphic data and takes advantage of the exible plotting tools available in R to produce detailed SCs. The main bene ts of SDAR are: (i) used to generate accurate and complete SC plot including multiple features (e.g., sedimentary structures, samples, fossil content, color, structural data, contacts between beds), (ii) developed in a free software environment for statistical computing and graphics, (iii) run on a wide variety of platforms (i.e., UNIX, Windows, and MacOS), (iv) both plotting and analysing functions can be executed directly on R's command-line interface (CLI), consequently this feature enables users to integrate SDAR's functions with several others add-on packages available for R from The Comprehensive R Archive Network (CRAN).

Early Carboniferous age of the Versoyen ophiolites and consequences: non-existence of a "Valais ocean'' (Lower Penninic, western Alps)

Ophiolites occur at several places in the Lower Penninic of the W and Central Alps. They are generally ascribed to oceanic crust of a so-called ``Valais ocean'' of Cretaceous age which plays a fundamental role in many models of Alpine paleogeography and geodynamics. The type locality and only observational base for the definition of a ``Valais ocean'' in the W Alps is the Versoyen ophiolitic complex, on the French-Italian boundary W of the Petit St-Bernard col. The idea of a "Valais ocean'' is based on two propositions that are since 40 years the basis for most reconstructions of the Lower Penninic: (1) The Versoyen forms the (overturned) stratigraphic base of the Cretaceous-Tertiary Valais-Tarentaise series; and (2) it has a Cretaceous age. We present new field and isotopic data that severely challenge both propositions. (1) The base of the Versoyen ophiolite is a thrust. It overlies a wildflysch with blocks of Versoyen rocks, named the Mechandeur Formation. This ``supra-Tarentaise'' wildflysch has been confused with an (overturned) stratigraphic transition from the Versoyen to the Valais-Tarentaise series. Thus the contact Versoyen/Tarentaise is not stratigraphic but tectonic, and the Versoyen ophiolite has no link with the Valais basin. This thrust corresponds to an inverse metamorphic discontinuity and to an abrupt change in tectonic style. (2) The contact of the Versoyen complex with the overlying Triassic-Jurassic Petit St-Bernard (PSB) series is stratigraphic (and not tectonic as admitted by all authors since 50 years). Several types of sedimentary structures polarize it and show that the PSB series is younger than the Versoyen. Consequently the Versoyen ophiolitic complex is Paleozoic and forms the basement of the PSB Mesozoic sediments. They both belong to a single tectonic unit, named the Versoyen-Petit St-Bernard nappe. (3) Ion microprobe U-Pb isotopic data on zircons from the main gabbroic intrusion in the Versoyen complex give a crystallization age of 337.0 +/- 4.1 Ma (Visean, Early Carboniferous). These zircons show typical oscillatory zoning and no overgrowth or corrosion. and are interpreted to date the Versoyen magmatism. These U-Pb data are in excellent agreement with our field observations and confirm the Paleozoic age of the Versoyen ophiolite. The existence of a ``Valais ocean'' of Cretaceous age in the W Alps becomes very improbable. The eclogite facies metamorphism of the Versoyen-Petit St-Bernard nappe results from an Alpine intra-continental subduction, guided by a Paleozoic oceanic suture. This is an example of the lone term influence of inherited deep-seated structures on a Much younger orogeny. This might well be a major cause of of the inherent complexity of the Alps.

Anachronistic facies from a drowned Lower Triassic carbonate platform: Lower member of the Alwa Formation (Ba'id Exotic), Oman Mountains

- The lower member of the Alwa Formation (Lower Olenekian), found within the Ba'id Exotic in the Oman Mountains (Sultanate of Oman), consists of ammonoid-bearing, pelagic limestones that were deposited on an isolated, drowned carbonate platform on the Neotethyan Gondwana margin. The strata contain a variety of unusual carbonate textures and features, including thrombolites, Frutexites-bearing microbialites that contain synsedimentary cements, matrix-free breccias surrounded by isopachous calcite cement, and fissures and cavities filled with large botryoidal cements. Thrombolites are found throughout the study interval, and occur as 0.5-1.0 m thick lenses or beds that contain laterally laterally-linked stromatactis cavities. The Frutexites-bearing microbialites occur less frequently, and also form lenses or beds, up to 30 cm thick; the microbialites may be laminated, and often developed on hardgrounds. In addition, the Frutexites-bearing microbialites also contain synsedimentary calcite cement crusts and botryoids (typically <1 cm thick) that harbour layers or pockets of what appear to be bacterial sheaths and coccoids, and are indicative of biologically mediated precipitation of the cement bodies. Slumping following lithification led to fracturing of the limestone and the precipitation of large, botryoidal aragonite cements in fissures that cut across the primary fabric. Environmental conditions, specifically palaeoxygenation and the degree of calcium carbonate supersaturation, likely controlled whether the thrombolites (high level of calcium carbonate supersaturation associated with vertical mixing of water masses and dysoxic conditions) or Frutexites-bearing microbialites (low level of calcium carbonate supersaturation associated with anoxic conditions and deposition below a stable chemocline) formed. The results of this study point to continued environmental stress in the region during the Early Triassic that likely contributed to the uneven recovery from the Permian-Triassic mass extinction.

A High-Resolution 2D/3D Seismic Study of a Thrust Fault Zone in Lake Geneva, Switzerland

P130 A HIGH-RESOLUTION 2D/3D SEISMIC STUDY OF A THRUST FAULT ZONE IN LAKE GENEVA SWITZERLAND M. SCHEIDHAUER M. BERES D. DUPUY and F. MARILLIER Institute of Geophysics University of Lausanne 1015 Lausanne, Switzerland Summary A high-resolution three-dimensional (3D) seismic reflection survey has been conducted in Lake Geneva near the city of Lausanne Switzerland where the faulted molasse basement (Tertiary sandstones) is overlain by complex Quaternary sedimentary structures. Using a single 48-channel streamer an area of 1200 m x 600 m was surveyed in 10 days. With a 5-m shot spacing and a receiver spacing of 2.5 m in the inline direction and 7.5 m in the crossline direction, a 12-fold data coverage was achieved. A maximum penetration depth of ~150 m was achieved with a 15 cu. in. water gun operated at 140 bars. The multi-channel data allow the determination of an accurate velocity field for 3D processing, and they show particularly clean images of the fault zone and the overlying sediments in horizontal and vertical sections. In order to compare different sources, inline 55 was repeated with a 30/30 and a 15/15 cu. in. double-chamber air gun (Mini GI) operated at 100 and 80 bars, respectively. A maximum penetration depth of ~450 m was achieved with this source.