The Petrology and Petrography of Sediments from the Sigsbee Blanket, Yucatan Shelf, Mexico

Abstract

Abstract : The Sigsbee blanket is a lithologically distinct unit of the Holocene to Recent sediment mantle which covers the outer shelf and continental slope provinces of the Yucatan Shelf. A detailed petrographic study revealed that the unit is composed dominantly of planktonic lutite with varying percentages of calcareous pellets, ooids lithic fragments, non-skeletal aggregates, algal fragments, tests of benthonic foraminifera and fragments of shells and skeletons of mollusks, coral, bryozoans and echinoids. The landward boundary of the unit is a gradation with the adjacent skeletal calcarenites that occur on the inner shelf; the seaward boundary may extend as far as the Sigsbee Deep. The lower contact grades from an unconformity on the upper-outer shelf and outer shelf terrace, to a mild disconformity on the outer shelf margin and finally to a conformable contact near the shelf-slope break. The upper boundary of the unit is the sediment water interface. Terraces mark the outer shelf between -450 feet to -300 feet and between -170 feet and -210 feet. In the depth zone between the 170 foot and 300 foot isobaths the substratum is calcite-cemented limestones (Wisconsin in age). The Sigsbee blanket was deposited under the conditions of transgression in post-glacial time and the effect of the transgression is reflected by constituent and facies variation. The most striking variation is in the relative percentages of planktonic tests and shells, calcareous pellets and ooids.