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45548-AC8
The Interrelationship of Sequence Stratigraphy, Paleoclimatology, and Terrestrial Ichnology in Triassic Paleosol-Bearing Alluvial Successions, Moenkopi and Chinle Formations, Southwestern United States

Stacy C. Atchley, Baylor University

The proposed study involves sedimentological and stratigraphic characterization of a series of Late Triassic (Mid Norian) alluvial deposits within the Petrified Forest National Park (PFNP) in Arizona, USA.  The Sonsela Member of the Chinle Formation is composed of a cyclic succession of alternating alluvial deposits and bounding discontinuities upon which paleosols are developed.  The study area exposes the lower third of the Sonsela Member and consists of approximately 1km of alluvial deposits that are well-exposed within the badlands of the Blue Mesa overlook area of the PFNP.   The size and well-exposed nature of this outcrop allow detailed study of the reservoir scale distribution of alluvial architectural elements and associated paleosols. 

Progress to Date

            Field data for this study were collected in a 5 week period in the months of June and July, 2008 in the Petrified Forest National Park, Arizona, USA.  The study area was divided into 5 sub-areas and high-resolution panoramic photos were taken of each section.  These panoramic photos were used to record the lateral distribution of pedofacies, alluvial architectural elements and their bounding discontinuities. 

            A series of one-dimensional measured sections were recorded in order to document thickness of sedimentary units, grain size, sedimentary structures, alluvial architectural elements, and paleosol locations at all accessible locations in each of the sub-areas.   Fluvial Aggradation Cycles (FACs) were documented within each of the measured sections.  FACs are defined as a succession of alluvial deposits that fine upwards and have a paleosol weathered into their upper boundary and/or are overlain by a succession of coarser alluvial deposits.   Samples of non-pedogenically-altered alluvial sandstone deposits were collected for petrographic studies.  Each paleosol within the measured sections was described based on standard guidelines and were numbered according to their location within the study area and their correlative pedofacies (eg., S2C5P1 is within Sonsela Area 2, stratigraphically within pedofacies 5, and is associated with the first measured section of that area).  Paleosol descriptions include horizonation, structure, thickness, carbonate development, trace fossils, boundary conditions, root traces, redoximorphic features, Munsell color and grain size.  Samples of each horizon within each paleosol profile were collected for analysis.  A total of eight measured sections and 60 paleosol profiles were described.

            Since the end of the Summer 2008 field season, measured sections have been digitized and panoramic photos have been digitally mapped to show the lateral and vertical distribution of pedofacies, alluvial architectural elements and their bounding discontinuities.  Paleosols have been grouped into twelve pedotypes based on their field descriptions.

Future Objectives

Objective 1

            Pedotypes will be compared by pedogenic features such as depth, texture, structure, root traces, carbonate accumulation, color and redoximorphic features in order to determine relative paleosol maturity and drainage.   Pedotypes will be analyzed for grain size and content using the Malvern Mastersizer 2000 Laser Granulometer and for bulk geochemical data using the Rigaku ZSX Primus II X-ray Flourescence machine at the Baylor University Department of Geology.  Samples will also be sent to an independent company for thin-section manufacturing and for δ¹⁸O and δ¹³C isotope analysis.  Data obtained will be used to re-evaluate and further classify pedotypes.  Paleoclimate information based on the physical properties of the paleosols, geochemical weathering proxies, depth to carbonate functions, δ¹⁸O temperature estimations and δ¹³C pCO₂ estimations will be compared with the evolution of fluvial depositional style to determine the relative influence of climate on depositional patterns.  Correlations of paleosol maturity with stratal thickness and attendant facies associations in both space and time across the study interval will be used to evaluate cyclic alluvial depositional patterns, changing accommodation space and their causal mechanism(s).   A conceptual model that relates paleosol characteristics and depositional style will be developed.  

Objective 2

            Sandstone samples will be processed for grain size analysis and will be sent to an independent company for manufacturing thin-sections.  Petrographic analysis, including point-counting of the thin-sections, will aid in determining sandstone composition, mineralogical maturity, cement type, sorting and porosity.  Combined with grain size, this information will be used to characterize sandstone reservoir quality within the outcrop.  Bulk density calculations of the soil samples will be used to calculate % porosity.  This information combined with grain size and micromorphological data from the paleosols will be used to assess porosity and permeability characteristics of the paleosols.  The porosity and permeability of the sandstones, soils, and architectural elements will be superimposed onto the high-resolution two-dimensional cross-sections created for the study area to characterize the spatial distribution of reservoir quality.  This record will be combined with information about changing alluvial style through time in order to develop a better understanding of the relationship between reservoir quality and evolving fluvial style and climate.

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