RICARDO JAHNERT - Academia.edu (original) (raw)

Papers by RICARDO JAHNERT

eSpace (Curtin University), 2014

The southern New England Orogen (NEO) in eastern Australia is characterized by tight curvatures (... more The southern New England Orogen (NEO) in eastern Australia is characterized by tight curvatures (oroclines), but the exact geometry of the oroclines and their kinematic evolution are controversial. Here we present new data on the anisotropy of magnetic susceptibility (AMS), which provide a petrofabric proxy for the finite strain associated with the oroclines. We focus on a series of preoroclinal Devonian-Carboniferous fore-arc basin rocks, which are aligned parallel to the oroclinal structure, and by examining structural domains, we test whether or not the magnetic fabric is consistent with the strain axes. AMS data show a first-order consistency with the shape of the oroclines, characterized, in most of structural domains, by subparallelism between magnetic lineations, "structural axis" and bedding. With the exception of the Gresford and west Hastings domains, our results are relatively consistent with the existence of the Manning and Nambucca (Hastings) Oroclines. Reconstruction of magnetic lineations to a prerotation (i.e., prelate Carboniferous) stage, considering available paleomagnetic results, yields a consistent and rather rectilinear NE-SW predeformation fore-arc basin. This supports the validity of AMS as a strain proxy in complex orogens, such as the NEO. In the Hastings Block, magnetic lineations are suborthogonal to bedding, possibly indicating a different deformational history with respect to the rest of the NEO. 1. Introduction The formation of curved orogenic belts (oroclines) and associated vertical-axis block rotations is commonly attributed to overriding-plate deformation in convergent margins [Rosenbaum and Lister, 2004; Cifelli et al., 2007; Capitanio et al., 2011a; Rosenbaum, 2014]. Oroclines have traditionally been defined as originally near-linear orogenic belts, which were subsequently subjected to bending [Carey, 1955]. Numerous studies have addressed the possible mechanisms associated with oroclinal bending [

American Association of Petroleum Geologists eBooks, 2000

Geobiology, 2014

The present study investigated the influence of abiotic conditions on microbial mat communities f... more The present study investigated the influence of abiotic conditions on microbial mat communities from Shark Bay, a World Heritage area well known for a diverse range of extant mats presenting structural similarities with ancient stromatolites. The distributions and stable carbon isotopic values of lipid biomarkers [aliphatic hydrocarbons and polar lipid fatty acids (PLFAs)] and bulk carbon and nitrogen isotope values of biomass were analysed in four different types of mats along a tidal flat gradient to characterize the microbial communities and systematically investigate the relationship of the above parameters with water depth. Cyanobacteria were dominant in all mats, as demonstrated by the presence of diagnostic hydrocarbons (e.g. n-C17 and n-C17:1). Several subtle but important differences in lipid composition across the littoral gradient were, however, evident. For instance, the shallower mats contained a higher diatom contribution, concordant with previous mat studies from othe...

Sedimentary Geology, 2012

ABSTRACT The Holocene hypersaline carbonate system in Shark Bay, Australia is represented by micr... more ABSTRACT The Holocene hypersaline carbonate system in Shark Bay, Australia is represented by microbial deposits (stromatolites and thrombolites) and a supratidal coquina beach ridge system which overlies the Pleistocene Bibra Formation and is prograding seaward, since 4500 years ago, over supratidal–intertidal microbial deposits as a consequence of Late Holocene sea level fall, and the high volume of bivalve shells available. The extent, internal architecture and ridge constructional types of coquina deposits in this World Heritage area have not been previously investigated in detail by cores tied with geophysical images. Here we document external and internal architecture of the Holocene coquina system in southeast Hamelin Pool showing three depositional units which comprise the ridge system construction: (1) tabular layers complex, (2) convex-up ridges and (3) washover deposits. Bivalve skeletons represent almost the total amount of bioclasts with symmetrical semi-circular formats and valve sizes 5–9 mm. Based on Ground Penetrating Radar (GPR) images the number of prograding layers present in each transect line varies from 252 to 433, which when compared with the measured 14C ages, shows lateral growth of the system at average rates from 10 years per layer on proximal older sequences to 30 years per layer in the younger and thicker packages which display more gentle inclination and sub-horizontal arrangement. Divergent orientation of tabular layers relates to changes in energy of currents and waves and involves time gaps and erosion. After earliest crest construction the system experienced a decrease in sea level (3600–3000 14C years BP) that left behind a sequence of parallel tabular layers with dip angles (> 10° to < 20°) which accreted seaward as the first major prograding event. The accretionary tabular layers progressively lost height till near 3000 years BP, representing continuous relative SL and storm energy fall, when the first major swale was constructed. Between 2000 and 3000 years BP a series of events constructed 4 large convex-up crests separated by swales overlying tabular layers. Marked changes in layer arrangement occurred since 1000 years ago when subsequent to erosional events the coastal system rearranged and is characterized by layers with low dip angle (< 10°). The Shark Bay ridge system represents the results of waves, surges and swash during storm activity and subsequent reworking history of eroded material in a context of continuous sea level fall. Alternating beach ridges and swales represent events of storm intensity variation during falling late Holocene SL. This progradational model of coquina ridge construction at Shark Bay is a useful analog for ancient coquina reservoirs deposited in lacustrine systems, since the hydrodynamic setting and semi-closed environment affected by storms is similar to some ancient examples such as the coquina reservoirs in the Campos Basin, Brazil.

If you wish to contact a Curtin researcher associated with this document, you may obtain an email... more If you wish to contact a Curtin researcher associated with this document, you may obtain an email address from

6th International Congress of the Brazilian Geophysical Society

Journal of South American Earth Sciences

International Conference and Exhibition, Melbourne, Australia 13-16 September 2015, 2015

Revista Brasileira …, 2008

O processo de dolomitizacão que atingiu as rochas carbonáticas do Grupo Açungui, de idade neoprot... more O processo de dolomitizacão que atingiu as rochas carbonáticas do Grupo Açungui, de idade neoproterozóica, aflorantes na Mina Rio Bonito (PR) ocorreu em dois episódios distintos, sendo do tipo substituição e preenchimento. No primeiro caso, a rocha calcítica pretérita teve a ...

Revista Brasileira de …, 2009

... José Manoel dos Reis Neto1, Nilo Siguehiko Matsuda2, Almério Barros França2 & Ricardo... more ... José Manoel dos Reis Neto1, Nilo Siguehiko Matsuda2, Almério Barros França2 & Ricardo Jahnert2 Resumo O processo de dolomitizacão que atingiu as rochas carbonáticas do Grupo Açungui ... No primeiro caso, a rocha calcítica pretérita teve a matriz substituída por dolomita ...

Revista Brasileira de Geociências

Resumo O processo de dolomitizacão que atingiu as rochas carbonáticas do Grupo Açungui, de idade ... more Resumo O processo de dolomitizacão que atingiu as rochas carbonáticas do Grupo Açungui, de idade neoproterozóica, aflorantes na Mina Rio Bonito (PR) ocorreu em dois episódios distintos, sendo do tipo substituição e preenchimento. No primeiro caso, a rocha calcítica pretérita teve a matriz substituída por dolomita em cristais com granulação fina à média, distribuição unimodal, secundariamente polimodal, em indivíduos com aspecto turvo. A dolomitizacão do tipo preenchimento, responsável pela precipitação do material micrítico (Dp1), de composição dolomítica, rico em Fe, originou-se a partir de fluido em desequilíbrio químico com o fluido responsável pela dolomitização anterior. O cimento dolomítico Dp2 está constituído por dolomita do tipo em sela, com indivíduos não-planares, granulação grossa e forte extinção ondulante. Este tipo de dolomita ocorre, ainda, no preenchimento de fraturas, tanto na brecha, quanto no mármore dolomítico, com geração posterior à do tipo anterior. O processo de dedolomitização ocorreu na brecha dolomítica, em condições superficiais, com a precipitação de calcita tardia como preenchimento de cavidades em associação à dolomita da geração Dp2. Quartzo completa a última fase de preenchimento das cavidades.

Sedimentary Geology, Jul 1, 2016

The Araripe Basin, located in northeastern Brazil, originated during the Gondwana continental bre... more The Araripe Basin, located in northeastern Brazil, originated during the Gondwana continental break-up responsible for the opening of the South Atlantic during the Early Cretaceous. In the Araripe Basin, the post-rift Aptian sequence corresponds to the Santana Group, which is composed, in upward succession, of mostly clastic continental and rare carbonate layers of the Barbalha, Crato, Ipubi and Romualdo Formations. The laminated limestones of the Crato Formation were deposited in a lacustrine environment preceding the deposition of the Ipubi Formation evaporites. They are age-equivalent to the limestones of the pre-salt interval of the east coast of Brazil, which contains large petroleum reserves. The excellent preservation of its macrofossils has made the Crato Formation known worldwide as a Fossil Lagerstätte. The limestones are macroscopically homogeneous, and their deposition has been previously attributed to chemical precipitation. Although the carbonate laminites are macroscopically undifferentiated, mineralogical variations, microscopic texture and distinctive biotic aspects supported the characterization of four microfacies: planar laminated, crustiform, nodular and rhythmic. The microfacies analysis indicated a strong and pervasive biological activity in the Crato limestone morphogenesis. Organominerals precipitated by the metabolic action of cyanobacteria and/or sulfate-reducing bacteria and methanogenic-oxidizing archea are represented by calcite and pyrite. Calcified coccoid and filaments are common, furthermore, the presence of calcified biofilms composed of exopolymeric substances (EPS) is ubiquitous. The presence of amorphous organic matter (AOM) and gypsum, particularly in the rhythmic microfacies, indicates anoxic/dysoxic conditions and stressful environments during periods of drought and low lake levels which favored the development and preservation of microbial biofilms. Phytoclasts and miospores when present in the succession indicate an extrabasinal contribution during wetter periods, although the environment remained of very low energy. The evidence of microbial influence in the formation of the laminated limestones of the Crato Formation is of great importance for understanding the excellent preservation of the unit's fossils and for modeling the evolution of the Aptian carbonate sequences in Brazil.

The Holocene hypersaline carbonate system in Shark Bay, Australia is represented by microbial dep... more The Holocene hypersaline carbonate system in Shark Bay, Australia is represented by microbial deposits
(stromatolites and thrombolites) and a supratidal coquina beach ridge system which overlies the Pleistocene
Bibra Formation and is prograding seaward, since 4500 years ago, over supratidal–intertidal microbial deposits
as a consequence of Late Holocene sea level fall, and the high volume of bivalve shells available. The
extent, internal architecture and ridge constructional types of coquina deposits in this World Heritage area
have not been previously investigated in detail by cores tied with geophysical images. Here we document
external and internal architecture of the Holocene coquina system in southeast Hamelin Pool showing
three depositional units which comprise the ridge system construction: (1) tabular layers complex,
(2) convex-up ridges and (3) washover deposits. Bivalve skeletons represent almost the total amount of
bioclasts with symmetrical semi-circular formats and valve sizes 5–9 mm. Based on Ground Penetrating
Radar (GPR) images the number of prograding layers present in each transect line varies from 252 to 433,
which when compared with the measured 14C ages, shows lateral growth of the system at average rates
from 10 years per layer on proximal older sequences to 30 years per layer in the younger and thicker packages
which display more gentle inclination and sub-horizontal arrangement. Divergent orientation of tabular
layers relates to changes in energy of currents and waves and involves time gaps and erosion. After earliest
crest construction the system experienced a decrease in sea level (3600–3000 14C years BP) that left behind
a sequence of parallel tabular layers with dip angles (>10° to b20°) which accreted seaward as the first major
prograding event. The accretionary tabular layers progressively lost height till near 3000 years BP,
representing continuous relative SL and storm energy fall, when the first major swale was constructed. Between
2000 and 3000 years BP a series of events constructed 4 large convex-up crests separated by swales
overlying tabular layers. Marked changes in layer arrangement occurred since 1000 years ago when subsequent
to erosional events the coastal system rearranged and is characterized by layers with low dip angle
(b10°). The Shark Bay ridge system represents the results of waves, surges and swash during storm activity
and subsequent reworking history of eroded material in a context of continuous sea level fall. Alternating
beach ridges and swales represent events of storm intensity variation during falling late Holocene SL. This
progradational model of coquina ridge construction at Shark Bay is a useful analog for ancient coquina reservoirs
deposited in lacustrine systems, since the hydrodynamic setting and semi-closed environment affected
by storms is similar to some ancient examples such as the coquina reservoirs in the Campos Basin, Brazil.
© 2012 Elsevier B.V. All rights reserved.