Anton F.-J. Wroblewski1,
Ron J. Steel1
(1) University of Wyoming, Laramie, WY
Abstract: High frequency sequence boundaries in a tectonically active foreland basin
Maastrichtian and Danian marine, brackish water, and coastal plain strata of the Ferris (ca. 66-63 Ma) and Hanna (ca. 61-55 Ma) formations in south-central Wyoming's Hanna and Carbon basins contain high frequency (fourth and fifth-order) sequence boundaries (SBs) located within incised valley-fill (VF) complexes. Sequence boundaries are recognizeable on the basis of: 1) widespread fluvial truncation of underlying estuarine and/or tidal flat and distal subtidal bar deposits, 2) basinward shifts of facies, 3) demonstrable biostratigraphic hiatus, and 4) great lateral extent (tens of kms.) of erosion surfaces.
Middle Ferris and lower Hanna SBs are dateable on the basis of mammalian fossils found in the fluvial and estuarine deposits bordering the unconformities. In the lower Ferris (ca. 66-65 Ma,) SBs within the 40-60 m-thick VF cannot be dated (owing to a lack of chronologically significant fossils) but their gross similarity to those in the middle Ferris suggests that they represent <500 k.y. In the middle Ferris (ca. 65-64 Ma), the SBs within the 10-80 m-thick VFs represent <300 k.y. on the basis of mammalian paleofaunas. In the lower Hanna (61-58 Ma), at least three SBs are present within 15-70 m-thick VFs that represent 1-3 m.y. of accumulation punctuated by episodes of re-incision.
Local Laramide tectonism in the uplifts surrounding the Hanna and Carbon basins was probably the driving mechanism for base level fall and valley incision/sequence boundary development. The high frequency of the SB development reflects the combined effect of episodic uplift of Laramide structures, catastrophic increases in fluvial sediment supply (?dam bursts), reduced rates of subsidence and base level fall in the basins. Subsequent acceleration of subsidence and/or eustatic sea level rise then caused marine and brackish water influence to seek preferentially into the back-filling valleys.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana