Abstract

The East Berlin Formation is a 200-m fluvial and lacustrine sequence of red and gray sandstone and mudstone that accumulated in a tropical rift valley during Early Jurassic time. The hematite pigment that colors the red beds is authigenic, produced by four post-depositional processes. (1) Brown and yellow-brown limonite that stained the surfaces of the detrital particles of sand and mud converted to hematite by aging. (2) In the sandstone, pervasive intrastratal solution of Fe-silicate grains, especially pyroxene, amphibole, epidote, chlorite, and biotite provided iron for precipitation of hematite, or a red ferric oxide precursor that then aged to hematite. These two processes were volumetrically the most important in generating hematite pigment. (3) In all the red beds, abundant magnetite grains were oxidized to hematite; ilmenite grains were oxidized to hematite-rutile. (4) In the sandstones, replacement of Fe-silicate grains by dolomite cement yielded additional iron for hematite cement. Conversion of limonite stains on clay particles to hematite by aging was the major source of hematite in flood-plain grayish red mudstone, a darker red (lower value and less yellow hue) than the interbedded stream channel sandstone of pale red and pale yellowish brown colors. Intrastratal solution of Fe-silicate grains in the fluvial and lacustrine sandstones produced an average of about 3 percent by volume of hematite cement. Pyroxene, amphibole, epidote, chlorite, and biotite grains were consistently protected from post-depositional solution in the impermeable dolomite concretions and grayish red mudstone of the flood-plain deposits and also in lacustrine gray mudstone. The limonite surface stains on detrital particles in the lakes were removed in the reducing, organic-rich bottom water so that the impermeable lacustrine gray mudstone and black shale are not reddened by aging of limonite to hematite nor by intrastratal solution.

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