Resultados obtenidos

The primary aim of this project was to determine the provenance and depositional environment of the Bar River Formation using detailed sedimentary lithofacies analysis, petrography, and geochemistry. In addition, this study attempted to determine the major factors controlling the quartz-rich composition of the Bar River Formation, which include tectonic setting, provenance, chemical weathering, recycling, reworking, and diagenesis.

Eight lithofacies were identified in the Bar River Formation near Flack Lake: (1) Quartz arenite breccia (QAB), (2) Planar and cross-bedded sandstone (PCBS), (3) Siltstone/mudstone (SM), (4) Overturned, cross-bedded sandstone (OCBS), (5) Tangential cross-bedded, pebbly granulestone (TCBPG), (6) Planar and cross-bedded sandstone with siltstone interbeds (PCBSMI), (7)Pebbly, cross-bedded, pink to purple sandstone (PCBPPS), and (8) Large-scale, cross-bedded sandstone (LSCBS). Lithofacies associations enabled identification of tidal channel, and subtidal to intertidal shoal deposits. These deposits, which overlie fine-grained intertidal deposits of the Gordon Lake Formation, are common along macrotidal coasts, embayments and estuaries characterized by tidal ranges exceeding 4 m. Sedimentary structures consistent with tidal activity, periodic emergence and wave action, combined with polymodal paleocurrent directions, and textural and mineralogical maturity, all point to deposition of the Bar River Formation along a tide-dominated, stable shelf characterized by high tidal ranges.

Structures interpreted by previous workers as synaeresis cracks are microbially induced sedimentary structures (MISS), referred to as sand cracks. Additional MISS identified include microbial sand and silt chips, mat chips, pyrite patches and iron-rich laminae. These features are consistent with microbial mat destruction and decay. This thesis is the first to report MISS in the Bar River Formation.

Point count data for the Bar River Formation samples reveals a quartzose sedimentary and/or felsic igneous source, based on the abundance of quartz and minor lithic fragments composed mainly of sedimentary rock types. The QFL plot reveals that the Bar River

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Formation sandstones were derived originally from an interior craton source, commensurate with erosion of the Archean granitoid and gneiss terranes exposed north of the study area. Paleocurrent analysis reveals a polymodal trend with the vector mean pointing to the SE. This result supports a SE-facing paleoslope, and points to derivation of detritus from a source located to the NW.

Major and trace element results reveal that the samples are enriched in SiO2 and also have relatively high Zr abundances. The localized presence of hematite cement in some sandstone indicates a very minor mafic component. The Bar River Formation samples plot primarily within the passive margin field on the tectonic discrimination plots. Deposition in this tectonic setting is supported by well rounded grains and well sorted, quartz-rich samples, which are typical of stable shelf deposits. However, samples from the weathering profiles of the Toorongo granite and Otago schist were plotted on the tectonic discrimination diagram, and the results revealed that weathered samples plot in different fields compared to fresh samples, which calls into question the validity of these plots for quartz-rich deposits.

Light REE-enriched patterns combined with a negative Eu anomaly determined for a majority of the Bar River samples are characteristic of a felsic source rock (Na-rich and/or K-rich granites). Comparison of the REE patterns with Average Archean Upper Crustal (AAUC) rocks reveals six distinct REE patterns. Groups A and B are comparable to AAUC rocks. Slight negative Eu anomalies displayed by Group B rocks indicate minor contributions from a K-rich granitic source. Group C rocks are LREE enriched and display flat HREE patterns with a strong negative Eu anomaly, consistent with a tonalitic source with minor contributions from K-rich granite. The LREE enriched and HREE depleted pattern of Group D samples is similar to that of K-rich granites or tonalites. Group E samples display LREE enriched and flat HREE patterns with slight to no negative Eu anomalies, which points to derivation from a Na-rich granitic source or a tonalitic source. Group F samples display LREE and HREE depletions, and mid-REE enrichments, similar to that observed for one of the tonalites of the Wabigoon province, and a basaltic source of the Archean bimodal suite. Zirconium enrichment within the samples is indicated on the plot of Th/Sc versus Zr/Sc, and suggests that the Bar River

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Formation is a product of multiple recycling episodes. This conclusion is supported by petrographic observations wherein the sandstones exhibit a high degree of textural and mineralogical maturity.

The Bar River Formation samples have undergone K-metasomatism, as indicated by their trends on the A-CN-K plot of weathering. Corrected CIA values indicate a highly weathered source rock.

Oxygen isotopic analysis of bulk samples and individual quartz pebbles reveals that the

δ18_{O values are consistent with derivation from an Archean felsic and/or quartzose} sedimentary source rock. Additionally, the whole-rock δ18O values show a positive correlation with quartz and a negative correlation with phyllosilicate. The positive correlation with quartz is attributed to increasing abundance of detrital quartz as a result of sediment recycling in a supracrustal environment. Higher δ18O values compared to those expected for the original igneous sources of the Archean granitoid terrain, indicate silica addition as overgrowths and cement. The negative correlation with phyllosilicate content indicates that the phyllosilicates were either derived from igneous/meta-igneous sources or that recrystallization due to metamorphism has reset original δ18O values of the phyllosilicates. The negative correlation also indicates that the phyllosilicates are not products of low temperature weathering/diagenesis.

The results from lithofacies analysis, petrography and geochemistry of the Bar River Formation near Flack Lake indicate that the composition of the deposits was controlled by a combination of tectonic setting, chemical weathering, recycling, depositional environment, and diagenesis.

The Bar River Formation and underlying Gordon Lake Formation are currently the subjects of an investigation concerning microbially induced sedimentary structures (Hill et al., in prep.). The results of the MISS study will help to further constrain the paleodepositional subenvironments of the macrotidal estuarine system. Future investigations should examine the contact relationship between the Bar River and Gordon Lake formations in the Cobalt and Killarney areas where the contact is not obscured by a diabase sill. A more regional study would shed light on the nature and extent of the

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Paleoproterozoic estuarine system, which is an uncommonly preserved environment in the ancient rock record.

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