Mg-Clay Characterization of a Lacustrine Carbonate Deposit, Green River Formation (Eocene) Sanpete Valley, Utah, U.S.A.

Abstract

Lacustrine magnesian clays, carbonate, and volcanoclastic deposits of the Eocene Green River Formation crop out in Sanpete Valley, Utah. One 40m detailed stratigraphic section was measured at Spring City Cuesta with the aim of characterizing the fine-grained sediments and microbialite-bearing carbonates. Qualitative and quantitative mineralogical analyses were obtained using petrography, X-ray diffraction, scanning electron microscopy with an energy dispersive X-ray spectrometer and electron microprobe.

Six main facies were identified: argillaceous dolomitic limestone, laminite, lithoclast laminite, microbial carbonate, grainstone and tuff. All facies, except for the tuffs, consist mainly of calcite, dolomite, clay, minor quartz, and, rarely, feldspar. Mg-clays (stevensite, kerolite, and kerolite/stevensite mixed layered minerals -MLM) comprise most the clay fraction of all facies, except for the tuffs, where the Al-clays (Al-smectites, Al-micas, and MLM illite/smectite) dominate. All Mg-clays are considered authigenic minerals.

1) The argillaceous dolomitic limestone facies consists of fine-grained carbonate matrix containing 10-40% (Avg=20%) clay. Some carbonate crystals occur in the matrix. Low-energy depositional environment [LE]. 2) The laminite facies has laminated sheets of stevensite and MLM kerolite/stevensite, with clays compacting around carbonate rhombs and crystal aggregates; [LE]. 3) The lithoclast laminate facies has reworked intraclasts alternating with micrite and sheets of Mg-clays. Long LE periods with transient high-energy conditions [HE]. 4) The microbial carbonate facies has undulatory to columnar-layered stromatolites and closely spaced, minicolumnar stromatolites. The lamination consists of carbonate and Mg-clay couplets; [HE/LE]. 5) The grainstone facies consists of stevensite coated grains (<2mm) and lithoclasts (up to pebble size) with carbonate cement; [HE]. 6) Tuffs are mainly composed of alkali feldspars (<90%), biotite flakes and quartz.

The origin of Mg-clay assemblages is likely related to authigenic processes involving sedimentary neoformation. Stevensite and MLM kerolite/stevensite clays are related to direct precipitation from ionic or colloidal solutions under evaporative conditions, pH>9 and low Si/Mg. Stevensite and MLM kerolite/stevensite layers alternate vertically due to hydrochemical oscillations in the lake. No direct correlation between facies and the composition of their Mg-clays has yet to be found.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018