DIFFERENTIATING BETWEEN TIDAL CHANNELS, DISTRIBUTARY CHANNELS AND INCISED VALLEYS IN TIDAL DELTAS

Tidal and distributary channels comprise important facies within tide-dominated deltaic strata. Many also contain incised valleys, consisting of nested channel complexes. To distinguish these channelized elements in tide-dominated deltas and their relation to one another permits a more accurate environmental reconstruction of these deltas. It will also improve the current facies models for tide-dominated deltas. The Triassic Torrey Member of the Moenkopi Formation, interpreted as strata of a tide-dominated delta, preserves multiple examples of tidal channels, distributary channels, and incised valleys, which permits a detailed comparison between them. This can potentially be compared to examples from the stratigraphic record and modern tide-dominated deltaic settings.

Distributary channel deposits are distinguished by concave-up erosional bases, defined by rip-up clasts, and cm-thick coarser-grained sandstone lenses within erosional pockets. These channel fills have internal scour surfaces and contain bedsets comprised of foresets up to 1 m thick, sometimes separated by beds of climbing ripples. Tidal indicators are absent in this setting. Distributary channel deposits are typically 3-7 m thick and exposures range from tens to several hundred meters across. Channel abandonment is represented by deposition of intertidal deposits. Tidal channels, in contrast, exhibit flat-bottomed, erosional lower contacts, lacking lag. Internally, these channels are composed of 20-30 cm thick beds, containing wave-modified current ripples and wave ripples. Mega ripples and climbing ripples are absent. The lack of flaser bedding within these beds indicates deposition in the subtidal zone. Tidal channels are typically 2-8 m thick and exposures can be several hundred meters across. As in modern tide-dominated deltas, tidal channels occur lateral to the distributary channel complex, where they serve as conduits for tidal water exchange in tidal and subtidal flats. Incised valleys are composed of nested channels, exhibiting tidal indicators (flaser bedding, double mud drapes, reactivation surfaces). Distributary channels form the last stage of valley fill, where they can initially downcut into the valley fill. Tidal channels are rarely identified as components of the incised valley fills.