Tidal Channel
to Bayhead Delta Couplets: Key to 4th Order Proximal Sequence Identification
and Correlation,
Gomez,
Fourth-order stratigraphic sequences have
been identified as the fundamental building blocks in the Upper Iles Formation,
using stacking patterns and facies associations from outcrops and well log
data. These sequences have been correlated from proximal to distal reaches,
along a ca. 150 km regional transect extending from southwest
The fourth-order cycles are some 20-35 m
thick and in their proximal-medial reaches are often composed of a
transgressive tidal channel association (T1) an intervening transgressive muddy
zone (T2), and overlying regressive bayhead-delta facies association (R1).
Facies association T1 is recognized in
the well logs by upward-fining, serrated or blocky gamma ray patterns, passing
upwards into muddier facies (T2). In the outcrop T1 is 7-13 m thick, composed
of up to 3 m-thick stacked fine-grained sandstone channels. These channels were
filled with 2-D dunes that exhibit signs of tidal influence. The base of such
channels cuts down into bayhead delta deposits, sometimes cannibalizing them
completely. Facies T2 is usually 7-10 m thick and not well preserved.
Facies Association R1 shows a coarsening
-upward, serrated log pattern. In outcrop it is composed of 8-10 m thick,
medium-grained sandstones, showing large-scale, downstream-accreting surfaces
and some bi-directional dunes. Each of the sand beds is few tens of centimeters
thick and interbedded with organic-rich, millimeter-thick mudstone layers.
Our ability to recognize these fourth-order
sequences in well logs, understand their internal configuration/architecture in
the field, and correlate them along sub-regional transects should aid our
understanding of the variability of these as reservoirs.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California