Print this pageImpact of Hyperpicnal Flows in Offshore Benthos: A Case of Depauperated Marine Cruziana Suite
Depauperated Cruziana suites are well known in brackish-water marginal marine environments. They show a significant falling in ichnodiversity when compared to the archetypical marine Cruziana suites, which is a response to stressing environmental conditions, caused by frequent salinity fluctuations and oxygen depleting in restricted areas. Episodic events can also cause temporary salinity reduction or oxygen depleting in shallow marine settings, impacting the estonohaline benthic fauna and reducing significantly its diversity. However, the effects of these episodes on shallow marine benthos are not usually recorded. The time involved between each episodic event and the reestablishment of normal marine conditions is short, re-opening the colonization window to the resident endofauna that reworks completely the substrate, destroying previous bioturbation. The trace fossil assemblage preserved in Santonian mudstone layers of Santos Basin, southeastern Brazil, is composed dominantly by abundant ichnofabrics representative of opportunistic ichnogenera (Chondrites, Helminthopsis, Planolites, Palaeophycus, Phycosiphon, Rhizocorallium, Thalassinoides, Teichichnus) and by few and discrete ichnofabrics of stenohaline ichnogenera (Asterosoma, Helminthoida, Rosselia, Zoophycos), all of them usually present in Cruziana Icnofacies. The dominance of shales, the high density of bioturbation, which frequently homogenizes the substrates, and the presence of burrows made by stenohaline k-strategists suggest deposition in offshore settings. However, a more diverse stenohaline ichnofauna would be expected in Cretaceous offshore deposits. The moderate to low ichnodiversity observed and the dominance of ichnofabrics related with opportunistic ichnogenera are clear ichnological signatures of brackish-water settings. Otherwise, the abundance of ichnofabrics related with Chondrites, Helminthopsis, Phycosiphon, and Zoophycos (even discrete) suggests periods of oxygen depletion in the bottom. Phytodetritus are common along the whole succession, being more abundant towards the top, where marginal marine and freshwater deposits are preserved. Thus, the significant reduction in k-strategist diversity might be related to environmental chemical stress caused by a high amount of freshwater and phytodetritus input into offshore settings, probably by intense hyperpicnal flows. The strong salinity fluctuations at the sea bottom and the obruption deposits caused by dense gravitational flows impacted significantly on the resident stenohaline benthos, resulting in the developing of a depauperated Cruziana suite in offshore settings.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil