Sedimentological Controls from Mud-Cored Growth Faults: Outcrop Analogs

Janok P. Bhattacharya¹, Russell K. Davies², Eric Blankenship¹, Michael Murphy¹, and Karen McGowen^3
1Geosciences, University of Houston, Houston, TX
²RDR, Rock Deformation Research USA Inc., McKinney, TX
³Conoco/Phillips, Houston, TX

Outcrop examples of small-throw, listric, growth faults, in Carboniferous and Cretaceous delta systems, show the relationship between sedimentological processes, depositional setting, and fault initiation. These mid-continent deltas formed on ramp margins within the Cretaceous Ferron sandstone in Utah and the Pennsylvanian Dobbs Valley sandstone in Mineral Wells, Texas.

Pre-growth strata consist of several-meter thick successions of heterolithic silty mudstones and sandstones. Normal and inverse grading, climbing ripple cross lamination, small-scale soft sediment deformation, and a lack of burrowing, suggest high sedimentation rates in a hyperpycnal-dominated prodelta setting. These deposits form a highly porous and easily fluidized mud substrate, as indicated by abundant diapirs, which forms the underlying platform for strain accommodation.

Growth strata consist of meters-thick medium-grained sandstone. In some examples, sandstones show abundant medium-to large-scale cross stratification interpreted as the deposits of dunes and mouth bars respectively. Climbing dune-scale cross strata suggest landward accretion at diffluence zones on the upstream ends of mouth bars. In other examples, growth strata comprise primarily upper-plane bed or “humpback” stratification, thought to represent upper flow regime conditions.

Faults locally initiate at sites of sand deposition, which are related to the vagaries of mouth bar growth, although over time faults initiate progressively seaward as the deltas prograde. In these ramp-type margins, strain accommodation depth relates to parasequence thickness (usually < 10m) resulting in highly listric faults with a few meters of throw. In shelf-edge deltas, where strain accommodation depth relates to the continental slope length, growth faults can achieve regional scale and the initiation mechanisms may be masked due to large offset.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas