Sven Egenhoff | Geology Reports

USGS2018

The complexity of mudstone diagenesis - some insight from the Tøyen Formation, Lower to Middle Ordovician, southern Sweden

The Lower to Middle Ordovician Tøyen Shale in southern Sweden, a biostratigraphically well-dated siliciclastic mudstone unit, shows 18 distinct authigenic cements that include sulfides, carbonates, silicates, clays, and phosphates. Marcasite, sphalerite, galena, and six texturally distinct types of pyrite characterize the sulfides whereas only one type of dolomite and three different generations of calcite are observed in this unit. Quartz, phosphate, and organic matter occur as only one generation each. Authigenic clay minerals are represented by chlorite and kaolinite. The paragenetic sequence of cements is subdivided into the two pre-burial carbonates, succeeded by ten relatively early burial cements, and six late burial cements, the kaolinite being the latest of them all and potentially being of Cretaceous age. Based on textural relationships, the paragenetic sequence of alterations started with dolomite precipitation followed by calcite, and then five different generations of pyrite. All eleven other phases post-date these initial seven cements in the Tøyen Shale.

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USGS2015

Sedimentology of SPICE (Steptoean positive carbon isotope excursion): A high-resolution trace fossil and microfabric analysis of the middle to late Cambrian Alum Shale Formation, southern Sweden

The Cambrian Alum Shale Formation in the Andrarum-3 core from Scania, southern Sweden, consists of black siliciclastic mudstone with minor carbonate intercalations. Four facies comprise three siliciclastic mudstones and one fine-grained carbonate. The facies reflect deposition along a transect from deep ramp to basin on a Cambrian shelf. The three mudstone facies contain abundant clay clasts and laterally variable siltstone laminae. Bed-load transport processes seem to have dominated deposition on this deep shelf. These sedimentary rocks record mainly event deposition, and only relatively few, thin laminae probably resulted from suspension settling. The Alum Shale Formation deep shelf did not show a bioturbation gradient, but fecal strings are common and Planolites burrows are rare in all mudstone facies. Evidence for biotic colonization indicates that this mudstone environment was not persistently anoxic, but rather was most likely intermittently dysoxic. The Alum Shale Formation in the Andrarum-3 core shows an overall decrease of grain size, preserved energy indicators, and carbonate content upsection interpreted to reflect a deepening upward. The succession can also be divided into four small-scale fining-upward cycles that represent deepening, and four overlying coarsening-upward cycles that represent upward shallowing.

Book chapter↗