Hyperpycnal Flow Within Low Gradient River Deltas and Implications for Both Sand and Mud Transport Onto the Shelf: Brazos River, Northern Gulf of Mexico*

Selected Figures

	Location map for Brazos River and its mouth (after Rodriguez et al., 2000).
	Image of Brazos River mouth and environs.
	Brazos River plume.
	Turbidity and salinity vs. depth.
	Shore normal cross-section, showing grain-size distribution with depth.
	Bottom dissolved oxygen concentrations from various legs of NOAA SEAMP cruise, 7 June – 3 July, 2007. Hypoxic plume emanating from vicinity of Brazos River mouth is indicated (figure courtesy of Gulf of Mexico Hypoxia Watch- NOAA:NCDDC).

Hyperpycnal River Plumes

They normally are asociated with:

• Muddy, small to medium size rivers.
• High gradient rivers.
• Moderate to highly dipping and narrow shelves.
• Proximity to submarine canyon.

Brazos River

Brazos River is:

• Low gradient-mainly flows along coastal plains.
• Coastal plain contains easily eroded muddy sediments.
• Wide (125 km), gently dripping shelf.
• No proximal submarine canyon.
• Ranked as a moderately dirty river capable of hyperpycnal flow every 100 years or less (Mulder and Syvitski, 1995), based on average suspended sediment concentration at peak discharge.
• 118,000 km²
• 39 metric tons per square kilometer sediment yield.
• 11th longest river in the US.
• Only river on the Texas Coast that consistently drains in the the Gulf of Mexico.

Conclusions

• On July 12, 2007, during the peak discharge of Brazos River, we observed both a buoyant surface (hypopycnal) plume of suspended sediment and a highly turbid bottom layer (potentially even fluid mud), derived from the effluent of the Brazos River and extending over 5 km from the mouth of the river onto the shelf.
• Was this a hyperpycnal plume or a wave supported boundary layer? It was probably a combination of both; there were 1.5-2-m waves in 8-15 m of water.
• The shore-parallel transect revealed that the only place where a bottom turbid layer was observed was within 2 km of the axis of the plume.
• Bottom water salinities were low, further demonstrating the linkage with the river effluent.
• Very low angle dip of the shelf likely limited the distance offshore the flow extended.
• River discharge data show comparable floods occur every 1-5 years, suggesting hyperpycnal flow is a very common occurrence (Mulder and Syvitski, 1995, suggested (100 years).
• 210Pb and grain-size profiles suggest multiple pulses of sediment discharge.
• 7-12-07 cruise sampled the first big pulse.
• 10-13-07 cruise sampled the preserved evidence of subsequent pulses.
• Cores reveal sand was transported 5 km offshore.
• Preserved layers in the cores suggest that previous, larger floods transported much thicker sand layers at core locations and likely transported the sand offshore of our study site.
• Hypoxia on Brazos shelf appears to be to result from water column stratification due to high river discharge.
• When hyperpycnal flow occurs, nutrient and organic rich plume is injected into bottom waters and maintained by water column stratification, enhancing hypoxia if occurring in summer.
• These linkages need to be further investigated.

Possible Application to Petroleum Geology

Brazos River provides an ideal natural laboratory to investigate:

• Role of hyperpycnal flow and shelf processes on sediment dispersal of both sand as well as mud on a low-gradient, wide, passive-margin shelf comparable to many depositional system relevant to petroleum geology systems.
• Relationship between hyperpycnal flow and the formation of hypoxia - broad implications in terms of formation of source rocks proximal to reservoir systems on shelf settings.

References

Kineke, G.C., K.J. Woolfe, S.A. Kuehl, J.D. Milliman, T.M. Dellapenna, and R.G. Purdon, 2000, Sediment export from the Sepik River, Papua New Guinea; evidence for a divergent sediment plume: Continental Shelf Research, v. 20/16, p. 2239-2266.

Mulder, T., and J.P.M. Syvitski, 1995, Turbidity currents generated at river mouths during exceptional discharges to the world oceans: Journal of Geology, v. 103/3, p. 285-299.

Parsons, J.D., C.T. Friedrichs, P.A. Traykovski, D. Mohrig, J. Imran, et al., 2007, The mechanics of marine sediment gravity flows: Continental Margin Sedimentation; from Sediment Transport to Sequence Stratigraphy, Special Publication International Association of Sedimentologists, v. 37, p. 275-337.

Rodriguez, A.B., M.D. Hamilton, and J.B. Anderson, 2000, Facies and evolution of the modern Brazos Delta, Texas; wave versus flood influence: Journal of Sedimentary Research, v. 70/2, p. 283-295.

Acknowledgments

Appreciation is expressed to Carmen Fraticelli, Damian O’Grady, and ExxonMobil Upstream Research Company.