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Re-Evaluating Depositional Models for Shelf Shales
 

Janok P. Bhattacharya¹ and James A. MacEachern²
 

¹Geosciences Department, University of Houston, 4800 Calhoun Rd., Houston, Texas  77204-5007 

²Department of Earth Sciences, Simon Fraser University, Burnaby, British Columbia, V5A 1S6, Canada

  

EXTENDED ABSTRACT

 

Despite the assumption that the bulk of marine “shelf” mud is deposited by gradual fallout from suspension in quiet water, modern muddy shelves and their associated rivers show that they are dominated by hyperpycnal fluid mud.  This has not been widely applied to the interpretation of ancient sedimentary shale successions.  We analyze several ancient Cretaceous prodelta shelf systems and their associated river deposits.  Paleodischarge estimates of trunk rivers show that they fall within the predicted limits of rivers that are capable of generating hyperpycnal plumes.  The associated prodeltaic mudstones match modern hyperpycnite facies models, and suggest a correspondingly hyperpycnal character.  Physical sedimentary structures include diffusely stratified beds that show both normal and inverse grading (Fig. 1), indicating sustained flows that waxed and waned.  They also display low intensities of bioturbation (Fig. 2), which reflect the high physical and chemical stresses of hyperpycnal environments.  Hyperpycnal conditions are ameliorated by the fact that these rivers were relatively small, dirty systems that drained an active orogenic belt during humid temperate to subtropical “greenhouse” conditions.  During sustained periods of flooding, such as during monsoons, the initial river flood may lower salinities within the inshore area, effectively “prepping” the area and allowing subsequent floods to become hyperpycnal much more easily.  Although shelf slopes were too low to allow long-run-out hyperpycnal flows, the storm-dominated nature of the seaway likely allowed fluid mud to be transported for significant distances across and along the paleo-shelf.  Prodelta hyperpycnites form leaner, gas-prone source rocks, prone to the generation of overpressure, versus more slowly deposited, organic-rich, anoxic laminites and condensed-section shales.

 

Reference CITED

 

Bhattacharya, J. P., and J. A. MacEachern, 2009, Hyperpycnal rivers and prodeltaic shelves in the Cretaceous seaway of North America:  Journal of Sedimentary Research, v. 79.

 

 

Bhattacharya, J. P., and J. A. MacEachern, 2009, Re-evaluating depositional models for shelf shales:  Gulf Coast Association of Geological Societies Transactions, v. 59, p. 81-83.

 

Figure 1.  Core photograph of diffusely bedded prodelta mudstones and siltstones, with no bioturbation.  Note the inverse grading at 7, 8, and 12 cm (2.8, 3.2, and 4.7 in, respectively).  Scale is 3 cm (1.2 in).  Core sample is from the Cretaceous Dunvegan Formation, Canada (from Bhattacharya and MacEachern, in press, reproduced with permission of the Society of Economic Paleontologists and Mineralogists [Society for Sedimentary Geology]).

Figure 2.  Core photo of prodelta facies of the Cretaceous Dunvegan Formation, showing both inverse and normally graded siltstones.  Wispy mud-streaks at 4.5 cm (1.8 in) are identified as Phycosiphon(Ph).  Note that these lie in the clayey tops of thick graded siltstone beds, and may reflect colonization of the bed top after deposition.  Lateral disruption of sandy and silty laminae may represent “mantle and swirl” (ms) structures, recording the activity of sediment-swimming organisms in the rather soupy substrate.   Small flame structures in upper units also indicate soft-sediment deformation.  Unlabeled Planolites (P) occurs on the right side of the photo, and marked on the litholog.

 

 

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AAPG Search and Discover Article #90093 © 2009 GCAGS 59th Annual Meeting, Shreveport, Louisiana