High Life on the Seafloor during an Ocean Anoxic Event — The Sedimentology of Spice Middle to Late Cambrian Alum Shale, Sweden
The Alum Shale in Scandinavia, one of the most prominent Early Paleozoic source rocks in Northern Europe, consists of mostly black organic-rich mudstones and some intercalated carbonate beds. The unit has been previously interpreted as being deposited on a deep anoxic shelf because it contains high TOC contents (<17 wt%). δ13C and δ34S curves from Alum cores (work of others) shows that part of the succession correlates to the SPICE (Steptoean Positive Carbon Isotope Excursion) event, a time of interpreted worldwide anoxic conditions in Middle to Upper Cambrian strata. This study, using cores and outcrops in southern Sweden and Norway, aims at examining the mudstone microfacies over the entire SPICE stratigraphic interval to test how this postulated Cambrian anoxic event is reflected in sediments deposited on the deep shelf.
Alum mudstones are dominantly clay-rich but locally contain abundant silt-size quartz and carbonate grains, commonly arranged in distinct laminae that vary laterally in thickness. Locally, clay rip-up clasts are common, and mud ripples have been observed. Throughout the succession, the mudstones are highly bioturbated with ubiquitous vertical burrows and very common horizontal burrows.
The ichnofossil distribution follows a gradient with greater diversity in more proximal settings and decreasing diversity on more distal parts of the shelf. This distribution pattern is observed throughout the Alum in the study area. The trace fossil diversity gradient points to a downslope decrease in the oxygen content of bottom or near bottom seawater. However, the constant presence of benthic activity also indicates that the seafloor was most likely never persistently anoxic during Alum deposition. The presence of silt laminae and mudstone ripples further suggests that the deep shelf, even during SPICE times, was occasionally supplied with sediment and possibly oxygen-rich waters from the shallow shelf. Overall, the facies did not change throughout the stratigraphic interval that contains the SPICE event. Therefore, the deep shelf remained largely oxic to dysoxic, which calls into question the existence of any persistent anoxia during Alum deposition. Nevertheless, large quantities of organic matter could have been trapped in “marine snow” (organo-clay floccules) during the SPICE interval, which could provide a mechanism by which the observed changes in geochemical values could have occurred without requiring worldwide anoxic oceans.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California