--> --> Organically enriched rocks have been causally attributed to transgressive seas largely because of a misunderstanding of the condensed section in sequence stratigraphy

AAPG Rocky Mountain Section Meeting

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Organically enriched rocks have been causally attributed to transgressive seas largely because of a misunderstanding of the condensed section in sequence stratigraphy

Abstract

We are now aware that organically enriched rocks are formed during OAEs by large scale volcaniclastic deposition. The Cretaceous GOM Shelf Margin was overwhelmed by clastic deposition four times during OAEs and interpreted as evidence for sea level rise (Phelps 2012). In the GOM, “Fifteen separate ashes {from the Yellowstone Plateau from 0.6 to 12 mya – DMP} have been documented. The paleo-ages of these ash beds are correlative with maximum flooding surfaces on sea-level curves.” (Hanan et al, 1988). Geoscience has been interpreting altered ash deposition as evidence for sea level rise. Howell et al. (2018) returned to a “birthplace” of sequence stratigraphy and could not find evidence of sea level fall. “These observations indicate that a key aspect of the sequence stratigraphic model is not applicable in outcrops widely considered to be one of the type areas for sequence stratigraphic teaching and research.” Sercombe (2009) observed high gamma ray ‘hot’ Eocene shales in southern deepwater GOM wells. They were deposited in water depths greater than 20,000’ sstvd. “This forces the questioning of the association of ‘hot’ shales and maximum flooding surfaces.” The condensed section and its association with the Maximum Flooding Surface has been misinterpreted. The description of a condensed section (see Loutit et al.,1988) is actually one of a volcanic ash fall event. (1) ash fall preserves a diverse fossil assemblage, (2) ash fall extends from offshore to onshore, (3) hiatuses associated with ash fall are documented in the Greenhorn Cyclothem, (4) hardgrounds are formed during the alteration of ash, (4) the authigenic minerals siderite and phosphorite include Fe and P, elements that Cin-ty A. Lee (2018) determined to be leached from ash shortly after deposition, (5) iridium anomalies are observed from Eocene explosive volcanism and, (6) bentonites are found in condensed sections. Condensed sections are those times when ash fall episodically dominates other sedimentary processes, not times when other sedimentation is redirected by sea level change. The condensed section does not imply a sea level high stand and has been misunderstood since it was described.