Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Updip Migrating Bedforms in the Inclined Heterolithic Strata as a Compelling Evidence of a Sinuous, Meandering Tidal-Fluvial Channel: Comparisons Between Ancient (Upper Cretaceous Neslen Formation, USA) and Modern (Macrotidal Estuary, Gomso Bay, Korea) Analogues
(1) Faculty of Earth Systems and Environmental Sciences, Chonnam National University, Gwangju, Korea, Republic of.
(2) Jackson School of Geosciences, University of Texas at Austin, Austin, TX.
Updip migrating bedforms (UMB) are common in the up to 10-m-thick inclined heterolithic stratification (IHS) of the Upper Cretaceous Neslen tidal-fluvial channel deposits in the Book Cliffs of Utah, USA. The IHS consists of alternation of rippled sandstone beds and siltstone beds with thin sandstone lenses, which dips at varying angles from 2 to 5o. Bedforms are mostly current ripples and occasionally dunes that migrate obliquely to the updip direction of the master bedding of the IHS. Rhythmic interlaminations of sandstone and mudstone coupled with rare and diminutive trace fossil assemblages are suggestive of tidal-fluvial environments under the influence of brackish water. Changing dip angles of the IHS may result from the lateral migration of point bars in a tidal-fluvial channel. Observations from modern estuarine macrotidal environment (Gomso Bay, Korea) suggest that counterintuitive relations of dip angles between bedforms and masterbedding of IHS are due to the mutually evasive tidal current pattern within a channel. At the tight meander bend, unidirectional bedforms tend to be better preserved away from the bend apex. With the influence of tidal asymmetry, bedforms generated by dominant currents occupy the subordinate-currentward part of the meander bend, while those resulted from subordinate currents prevail in the dominant-currentward part of the meander bend. This trend becomes less obvious near the channel base where dominant-current bedforms are common. The dominant-currentward translation of point bar leads to the selective preservation of the dominant currentward part of meander bend, resulting in the UMB on the dominant-currentward dipping IHS. The creation of UMB on the IHS by unidirectional currents in conjunction with backflow is unlikely, because the backflow of the current is weaker than the ensuing current around the meander bend of tidal-fluvial channels, and that the timing of effective flow separation does not account for the elevation of UMB. The UMB within IHS can be regarded as a compelling evidence of a sinuous, meandering tidal-fluvial channel. The development of UMB on the IHS seems to be governed by various factors such as (1) channel sinuosity, (2) meander-bend migration pattern, (3) tidal asymmetry, (4) relative position within tidal frame, and (5) relative position within proximal-distal trend. Present study highlights the significance of UMB within IHS in the reconstruction of tidal-fluvial channel architecture.