Avulsion of terminal distributary channels in modern and ancient delta deposits

By

Cornel Olariu and Janok P. Bhattacharya

Geosciences, University of Texas at Dallas, P.O. 830688, Richardson, TX, 75083

Terminal distributary channels are relatively small features within delta front deposits, formed by multiple bifurcation of a trunk channel.

In modern deltas (Atchafalaya, Lena, Volga) numerous coeval terminal distributary channels are formed. Two main phases can be distinguished in evolution of terminal distributary channels:

1 initial distributary channel forms a mouth bar;

2 mouth bar growth causes the channel to split and may initiate an avulsion.

Usually high occurrence of avulsion results from the high sedimentation rates in front of the distributaries. Mouth bar growth in front of distributary change the channel hydraulic geometry by extension of terminal distributary channel and by increase of friction.

Alternation of mouth bar growth/ avulsion phases occurs in a quasi-cyclic manner, resulting in an increase in the number of terminal distributary channels. Such an evolution has been described in the growth of the Atchafalaya delta.

In ancient delta front deposits we propose the name   “terminal distributary channels”, to describe high-order channels contained within delta front deposits and presumed to be at the terminus of the delta system. A strike cliff face exposes terminal distributary channels of the Cretaceous Panther Tongue delta front deposits in north–central Utah. Terminal distributary channels appear as shallow and narrow channels filled with structureless or trough-cross stratified fine sandstone. A distinguishing characteristic is intimate alternation of terminal distributary channels with mouth bar deposits. Panther Tongue terminal distributary channels die out over a distance of a few hundred meters, on the opposite cliff faces and pass into distal bar deposits.

AAPG Search and Discovery Article #90010©2003 AAPG Southwest Section Meeting, Fort Worth, Texas, March 1-4, 2003