Heavy mineral beach placers of the Ordovician Amdeh Formation (Member 4, Wadi Qahza, Saih Hatat, eastern Oman Mountains)

Where is the main source area?

Frank Mattern, Bernhard Pracejus, Lujaina Al Balushi

Research output: Contribution to journalArticle

3 Citations (Scopus)

Abstract

We studied an excellently exposed section of the slightly metamorphic siliciclastic Amdeh Formation. The section is near the base of the Amdeh Member 4 (Am4). Am4 is of Early to Mid-Ordovician age and 1677 m thick. Two lensoid placers were examined for their mineral composition (X-ray diffraction, transmitted and reflected light microscopy), geochemistry (X-ray fluorescence), in-situ gamma-ray emissions and the sedimentary structures of the host rocks. The placers were deposited in a beach environment as indicated by sedimentary structures in the associated quartzites, such as parallel lamination, parting lineations and sand volcanoes and as indicated by the compositional and textural maturity of the host rock and the placers. Placer deposition may have been favored by rising sea-levels. Overall, the dark placers contain the following heavy minerals: anatase (up to 35–40%) > ilmenite > zircon > hematite (a few %) > and apatite (<1%, may be present in trace amounts as indicated by low phosphorous contents in XRF analysis). The presence of monazite is indicated in our study by LREEs plus yttrium. Due to the high amounts of ilmenite and anatase, the placers’ titanium contents are high (up to 20.5 wt%). In both placers, the concentrations of the light rare earth elements (LREEs) lanthanum, cerium, praseodymium and neodymium plus yttrium (geochemically affine to HREEs) are noteworthy (between ∼3000 and 7000 ppm). The source rocks can be expected in metamorphic and acidic plutonic rocks in the basement of Oman, which may have shed detritus from exposed basin flanks, but other potential source areas lay to the north and south of the basin. A source area with pre-enriched quartz in sandstone to explain the massive quartz influx would also be possible. The coastal/shallow shelf facies of Am4 shows very little bathymetric variability despite its thickness, which requires rapid and continuous subsidence. We attribute basin subsidence to rifting, withdrawal (and subrosion?) of salt in the subsurface (Ara Formation) due to sediment loading and possibly to gravity-driven subsidence due to the presence of dense ophiolite units below the Ara Salt. The absence of the Amdeh Formation in the west (Jabal Akhdar Dome) may be explained by rift shoulder uplift. While the Amdeh Formation accumulated in the rift basin, the Jabal Akhdar region occupied parts of the northwestern rift shoulder.

Original languageEnglish
Pages (from-to)633-646
Number of pages14
JournalJournal of African Earth Sciences
Volume147
DOIs
Publication statusPublished - Nov 1 2018

Fingerprint

heavy mineral
Ordovician
beach
yttrium
subsidence
mountain
anatase
sedimentary structure
ilmenite
X-ray fluorescence
basin
host rock
rare earth element
quartz
salt
neodymium
cerium
lamination
plutonic rock
monazite

Keywords

  • Gamma-ray emission
  • LREEs
  • Rift basin
  • Rising sea-level
  • XRD
  • XRF

ASJC Scopus subject areas

  • Geology
  • Earth-Surface Processes

Cite this

@article{4559b119c61e418984e3b549b1bd78e5,
title = "Heavy mineral beach placers of the Ordovician Amdeh Formation (Member 4, Wadi Qahza, Saih Hatat, eastern Oman Mountains): Where is the main source area?",
abstract = "We studied an excellently exposed section of the slightly metamorphic siliciclastic Amdeh Formation. The section is near the base of the Amdeh Member 4 (Am4). Am4 is of Early to Mid-Ordovician age and 1677 m thick. Two lensoid placers were examined for their mineral composition (X-ray diffraction, transmitted and reflected light microscopy), geochemistry (X-ray fluorescence), in-situ gamma-ray emissions and the sedimentary structures of the host rocks. The placers were deposited in a beach environment as indicated by sedimentary structures in the associated quartzites, such as parallel lamination, parting lineations and sand volcanoes and as indicated by the compositional and textural maturity of the host rock and the placers. Placer deposition may have been favored by rising sea-levels. Overall, the dark placers contain the following heavy minerals: anatase (up to 35–40{\%}) > ilmenite > zircon > hematite (a few {\%}) > and apatite (<1{\%}, may be present in trace amounts as indicated by low phosphorous contents in XRF analysis). The presence of monazite is indicated in our study by LREEs plus yttrium. Due to the high amounts of ilmenite and anatase, the placers’ titanium contents are high (up to 20.5 wt{\%}). In both placers, the concentrations of the light rare earth elements (LREEs) lanthanum, cerium, praseodymium and neodymium plus yttrium (geochemically affine to HREEs) are noteworthy (between ∼3000 and 7000 ppm). The source rocks can be expected in metamorphic and acidic plutonic rocks in the basement of Oman, which may have shed detritus from exposed basin flanks, but other potential source areas lay to the north and south of the basin. A source area with pre-enriched quartz in sandstone to explain the massive quartz influx would also be possible. The coastal/shallow shelf facies of Am4 shows very little bathymetric variability despite its thickness, which requires rapid and continuous subsidence. We attribute basin subsidence to rifting, withdrawal (and subrosion?) of salt in the subsurface (Ara Formation) due to sediment loading and possibly to gravity-driven subsidence due to the presence of dense ophiolite units below the Ara Salt. The absence of the Amdeh Formation in the west (Jabal Akhdar Dome) may be explained by rift shoulder uplift. While the Amdeh Formation accumulated in the rift basin, the Jabal Akhdar region occupied parts of the northwestern rift shoulder.",
keywords = "Gamma-ray emission, LREEs, Rift basin, Rising sea-level, XRD, XRF",
author = "Frank Mattern and Bernhard Pracejus and {Al Balushi}, Lujaina",
year = "2018",
month = "11",
day = "1",
doi = "10.1016/j.jafrearsci.2018.06.031",
language = "English",
volume = "147",
pages = "633--646",
journal = "Journal of African Earth Sciences",
issn = "0899-5362",
publisher = "Elsevier Limited",

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TY - JOUR

T1 - Heavy mineral beach placers of the Ordovician Amdeh Formation (Member 4, Wadi Qahza, Saih Hatat, eastern Oman Mountains)

T2 - Where is the main source area?

AU - Mattern, Frank

AU - Pracejus, Bernhard

AU - Al Balushi, Lujaina

PY - 2018/11/1

Y1 - 2018/11/1

N2 - We studied an excellently exposed section of the slightly metamorphic siliciclastic Amdeh Formation. The section is near the base of the Amdeh Member 4 (Am4). Am4 is of Early to Mid-Ordovician age and 1677 m thick. Two lensoid placers were examined for their mineral composition (X-ray diffraction, transmitted and reflected light microscopy), geochemistry (X-ray fluorescence), in-situ gamma-ray emissions and the sedimentary structures of the host rocks. The placers were deposited in a beach environment as indicated by sedimentary structures in the associated quartzites, such as parallel lamination, parting lineations and sand volcanoes and as indicated by the compositional and textural maturity of the host rock and the placers. Placer deposition may have been favored by rising sea-levels. Overall, the dark placers contain the following heavy minerals: anatase (up to 35–40%) > ilmenite > zircon > hematite (a few %) > and apatite (<1%, may be present in trace amounts as indicated by low phosphorous contents in XRF analysis). The presence of monazite is indicated in our study by LREEs plus yttrium. Due to the high amounts of ilmenite and anatase, the placers’ titanium contents are high (up to 20.5 wt%). In both placers, the concentrations of the light rare earth elements (LREEs) lanthanum, cerium, praseodymium and neodymium plus yttrium (geochemically affine to HREEs) are noteworthy (between ∼3000 and 7000 ppm). The source rocks can be expected in metamorphic and acidic plutonic rocks in the basement of Oman, which may have shed detritus from exposed basin flanks, but other potential source areas lay to the north and south of the basin. A source area with pre-enriched quartz in sandstone to explain the massive quartz influx would also be possible. The coastal/shallow shelf facies of Am4 shows very little bathymetric variability despite its thickness, which requires rapid and continuous subsidence. We attribute basin subsidence to rifting, withdrawal (and subrosion?) of salt in the subsurface (Ara Formation) due to sediment loading and possibly to gravity-driven subsidence due to the presence of dense ophiolite units below the Ara Salt. The absence of the Amdeh Formation in the west (Jabal Akhdar Dome) may be explained by rift shoulder uplift. While the Amdeh Formation accumulated in the rift basin, the Jabal Akhdar region occupied parts of the northwestern rift shoulder.

AB - We studied an excellently exposed section of the slightly metamorphic siliciclastic Amdeh Formation. The section is near the base of the Amdeh Member 4 (Am4). Am4 is of Early to Mid-Ordovician age and 1677 m thick. Two lensoid placers were examined for their mineral composition (X-ray diffraction, transmitted and reflected light microscopy), geochemistry (X-ray fluorescence), in-situ gamma-ray emissions and the sedimentary structures of the host rocks. The placers were deposited in a beach environment as indicated by sedimentary structures in the associated quartzites, such as parallel lamination, parting lineations and sand volcanoes and as indicated by the compositional and textural maturity of the host rock and the placers. Placer deposition may have been favored by rising sea-levels. Overall, the dark placers contain the following heavy minerals: anatase (up to 35–40%) > ilmenite > zircon > hematite (a few %) > and apatite (<1%, may be present in trace amounts as indicated by low phosphorous contents in XRF analysis). The presence of monazite is indicated in our study by LREEs plus yttrium. Due to the high amounts of ilmenite and anatase, the placers’ titanium contents are high (up to 20.5 wt%). In both placers, the concentrations of the light rare earth elements (LREEs) lanthanum, cerium, praseodymium and neodymium plus yttrium (geochemically affine to HREEs) are noteworthy (between ∼3000 and 7000 ppm). The source rocks can be expected in metamorphic and acidic plutonic rocks in the basement of Oman, which may have shed detritus from exposed basin flanks, but other potential source areas lay to the north and south of the basin. A source area with pre-enriched quartz in sandstone to explain the massive quartz influx would also be possible. The coastal/shallow shelf facies of Am4 shows very little bathymetric variability despite its thickness, which requires rapid and continuous subsidence. We attribute basin subsidence to rifting, withdrawal (and subrosion?) of salt in the subsurface (Ara Formation) due to sediment loading and possibly to gravity-driven subsidence due to the presence of dense ophiolite units below the Ara Salt. The absence of the Amdeh Formation in the west (Jabal Akhdar Dome) may be explained by rift shoulder uplift. While the Amdeh Formation accumulated in the rift basin, the Jabal Akhdar region occupied parts of the northwestern rift shoulder.

KW - Gamma-ray emission

KW - LREEs

KW - Rift basin

KW - Rising sea-level

KW - XRD

KW - XRF

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U2 - 10.1016/j.jafrearsci.2018.06.031

DO - 10.1016/j.jafrearsci.2018.06.031

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VL - 147

SP - 633

EP - 646

JO - Journal of African Earth Sciences

JF - Journal of African Earth Sciences

SN - 0899-5362

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