Near‐Surface K‐Feldspar Precipitation in Cretaceous Sandstones from the Potiguar Basin, Northeastern Brazil

The Palaeogene Maíz Gordo Formation is one of the main lacustrine events recorded in northwestern Argentina. It consists of sandstone, mudstone, and limestone beds 200 m thick, deposited in a brackish-alkaline lake and braided alluvial systems. The Maíz Gordo Lake evolved mainly as a closed system, with brief periods as an open one. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to study samples from seven sites, corresponding respectively to proximal, intermediate, and transitional positions of the fluvial environment and marginal and inner-lake environment, focusing on the clay mineralogy and analcime formation. The basinward zonation of diagenetic minerals identified in the Maíz Gordo Lake was: mordenite → analcime → K-feldspar. Although not a typical zonation of saline-alkaline lakes, it does indicate an increase in salinity and alkalinity towards the centre.In proximal fluvial settings, smectite predominates at the base of the sequence, with scarce kaolinite. Towards the top, a striking increase in kaolinite content suggests a change from a relatively arid climate with alternating humid and dry seasons, towards a warm and humid climate. Kaolinite content clearly decreases in a basinward direction. Such a variation is attributable to changes in hydro-geochemistry, denoting the progressive influence of the brackish and alkaline lake water on interstitial pores. SEM images of intermediate fluvial samples reveal authigenesis of illite at the expense of kaolinite booklets.In littoral and inner-lake settings the clay fraction is composed of muscovite, sometimes with subordinate smectite. Analcime occurs in variable amounts in all sedimentary facies, in rock pores or filling veins. It forms subhedral square to hexagonal, or anhedral rounded crystals, denoting that they coarsened at low to moderate degrees of supersaturation. Although the mordenite identified in a fluvial level would have been the precursor of analcime in the Maíz Gordo Basin, no textural evidence of analcime formation through replacement of mordenite or other precursor zeolite was found. Hence it is more probable that analcime formation took place by direct authigenic precipitation or through the reaction between interstitial brines and clay minerals or plagioclase.

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“…8h). These properties are similar to the authigenic K-feldspar described by Maraschin et al (2004) and Warnock and van de Kamp (1999).…”

Section: Lacustrine Environment Mineralogy and Texturessupporting

confidence: 79%

Clay mineral assemblages and analcime formation in a Palaeogene fluvial–lacustrine sequence (Maíz Gordo Formation Palaeogen) from northwestern Argentina

Campo

Papa

Jiménez-Millán

et al. 2007

Sedimentary Geology

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“…16 In sedimentary rocks, REE are mostly present in the clay fraction. A study on K-feldspar formation within the Açu sandstones showed that these minerals of a predominant kaolinitic type were infiltrated mechanically during dissolution-precipitation episodes 32 and could be incorporated during aggregation of asphaltenes in heavier, biodegraded or residual oils. Clays and their associated REE have already been identified in other crude oils.…”

Section: Association Of Elements To Water and Mineralsmentioning

confidence: 99%

Trace element distributions in biodegraded crude oils and fractions from the Potiguar Basin, Brazil

Duyck¹,

Miekeley²,

Fonseca³

et al. 2008

J. Braz. Chem. Soc.

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Perfis de distribuição de elementos traços foram determinados em óleos crus de reservatórios biodegradados situados ao longo de um caminho de migração secundaria (Bacia de Potiguar, Brasil). As amostras foram fracionadas em asfaltenos, parafinas, aromáticos e polares e resina para determinação de 22 elementos traços por ICP-MS. Esses se concentram nas frações de resina e asfaltenos, exceto V e Ni também encontrados na fração de aromáticos e polares. V, Co, Ni e Mo foram influenciados por processos de biodegradação e acumulam-se nas amostras mais afetadas e de maior distância de migração. Variações semelhantes da razão Ni/V nos aromáticos e polares e dos índices de homohopano e gamacerano nas parafinas evidenciaram também a influencia da biodegradação. As distribuições elementares foram similares para os óleos e seus asfaltenos, sugerindo que esses últimos, mais refratários que o óleo original, possam ser usados como uma ferramenta geoquímica inorgânica robusta de impressão digital para caracterização de óleos.Trace element distribution patterns were determined in crude oils from onshore biodegraded reservoirs along a secondary migration trend (Potiguar Basin, Brazil). Samples were fractionated into asphaltenes, paraffins, aromatics and polars and resins for the determination of 22 trace elements by ICP-MS. Most of them are strongly enriched in the resin and asphaltene fractions, but V and Ni are also associated with the aromatic and polar fractions. V, Co, Ni and Mo were influenced by biodegrading processes and accumulate in the most affected and migrated samples. Variations of the Ni/V ratio in aromatics and polars, matching those of homohopane and gammacerane indexes in paraffin, evidenced also the influence of biodegradation. Very similar elemental distribution patterns in oil and corresponding asphaltenes were observed, suggesting that the latter fraction, more refractory than the original oil, can be used as a robust inorganic geochemical fingerprint for oil characterization.

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“…The overgrowths are optically discontinuous (epitaxial) with the detrital grains, untwinned and range in width from around 0.036 mm and length up to 0.46 mm. Their thickness and continuity around the detrital feldspar grains suggest early precipitation before compaction (Maraschin et al ., , ). There are mechanically infiltrated clays visible in between grains and overgrowths infiltrated into fluvial deposits subjected to episodic flooding under a semiarid climate (Walker et al ., ; Moraes and De Ros, ) (Fig.…”

Section: Resultsmentioning

confidence: 99%

K–Ar dating of authigenic minerals in siliciclastic sequences: an example from the south Sanfranciscana Basin (Western Minas Gerais, Brazil)

et al. 2014

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K–Ar dating was applied on authigenic potassic minerals which are abundant in sandstones from the south of the Sanfranciscana Basin, Western Minas Gerais State, central Brazil. The Quintinos Member fluvial sandstones (Três Barras Formation, Areado Group) contain significant amounts of authigenic K‐feldspar as microcrystals of adularia and sanidine habits. The ages of these microcrystals cluster into three groups: 106.1 ± 2.2, 89.9 ± 1.9 and 88.8 ± 1.8 Ma (from Albian to Coniacian). The older age of 106.1 ± 2.2 Ma was obtained from the coarse fraction analysed (10–20 µm) that can contain a mixture of detrital potassic minerals (K‐feldspar, muscovite, biotite and illite) and different authigenesis of K‐feldspar (overgrowths and microcrystals). Thus, only the younger ages were interpreted as precipitation of K‐feldspar microcrystals during the Late Cretaceous into the Quintinos Member sandstones. Moreover, these ages can document the formation of microcrystals within a few million years after deposition of the sandstones. The ages of authigenic illite from the Capacete Formation epiclastic sandstones (Mata da Corda Group) range from 88.5 ± 1.9 to 71.5 ± 1.9 Ma (Coniacian–Campanian). These results suggest the timing of the illitization event in these sandstones as well as a synchrony with K‐feldspar authigenesis in the Quintinos Member sandstones. These results are well constrained and are in agreement with stratigraphic, biostratigraphic and radiometric ages previously reported for the Sanfranciscana Basin. Copyright © 2014 John Wiley & Sons, Ltd.

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Near‐Surface K‐Feldspar Precipitation in Cretaceous Sandstones from the Potiguar Basin, Northeastern Brazil

Cited by 21 publications

References 23 publications

Clay mineral assemblages and analcime formation in a Palaeogene fluvial–lacustrine sequence (Maíz Gordo Formation Palaeogen) from northwestern Argentina

Clay mineral assemblages and analcime formation in a Palaeogene fluvial–lacustrine sequence (Maíz Gordo Formation Palaeogen) from northwestern Argentina

Trace element distributions in biodegraded crude oils and fractions from the Potiguar Basin, Brazil

K–Ar dating of authigenic minerals in siliciclastic sequences: an example from the south Sanfranciscana Basin (Western Minas Gerais, Brazil)

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