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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 69 (1985)

Issue: 5. (May)

First Page: 863

Last Page: 863

Title: High-Resolution Stratigraphic Correlations and Geochemical Analyses, Cretaceous Niobrara Formation, Northwestern Denver-Julesburg Basin: ABSTRACT

Author(s): T. E. Rodriguez, L. M. Pratt

Article Type: Meeting abstract

Abstract:

The middle Santonian-lower Campanian part of the Smoky Hill Member of the Niobrara Formation represents a fourth-order regressive-transgressive cyclothem. Studies of this interval have improved our understanding of the influence of depositional environments and structural setting on distributions of organic matter in epicontinental marine strata. Geochemical analyses of fresh quarried sections at Lyons and LaPorte, Colorado, show that, in general, Corg (organic carbon) levels are highest between mid-regression and mid-transgression. Rhythmic fluctuations of Ccarb (carbonate carbon) and Corg correspond to limestone-marlstone bedding couplets at a scale of 15-20 cm (6-8 in.). Pronounced lateral variations between Lyons and LaPorte exist in C SUB>org, HI (pyrolytic hydrogen index), sediment accumulation rates, and Tmax (temperature of maximum pyrolytic yield). Comparisons of geochemical averages at Lyons (Corg = 2.5%, Ccarb = 9%, HI = 100, Tmax = 445°C) and LaPorte (Corg = 4%, Ccarb = 9.5%, HI = 450, Tmax = 422°C) indicate an elevated thermal maturity at Lyons and depositional conditions more favorable for preservation of marine organic matter at LaPorte. In both sections, Corg and Ccarb show strong negative correlations, possibly reflecting cyclic climatic controls on the development of bedding couplets. High-resolution stratigraphic correlations of 100,000-year or smaller intervals between Boulder and Owl Canyo , Colorado, based on widespread bentonites and bedding couplets, reveal a paleostructural high near Lyons. Shallow-water conditions and increased turbulence over this high are reflected in sediment accumulation rates only 60% of those at LaPorte. Increased amounts and hydrogen richness of organic matter at LaPorte may reflect a deeper water, more quiescent depositional setting.

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