ABSTRACT

Following highstand deposition of the regional Waseca Formation, lowstand valleys were incised as relative sea level fell. Within one of these valleys, the reservoir succession at Pike's Peak accumulated. The exact nature of valley-fill accumulation in this and other valleys has been somewhat enigmatic. This investigation takes advantage of exceptional well control (160 wells/two sections) and presents a new slant on estuarine valley fill.

A generalized Waseca succession at Pike's Peak may be outlined as follows. A basal lithofacies consisting of planar tabular and trough crossbedded sandstones rests abruptly on regional highstand Waseca Formation or Sparky Formation lithologies. This unit grades upward or may be incised by an interbedded sandstone and shale succession that contains both flat lying and inclined attitude (IHS) bedding. This facies is in turn overlain and locally incised by a sideritic, ironstone-rich mudstone unit.

The entire succession accumulated during two primary events: 1) baymouth bar deposition, and 2) estuarine scour and fill. The basal sandstone lithology documents the initial event and has been interpreted as representing a composite baymouth bar complex of estuarine origin. The overlying interbedded sandstone and mudstone strata are the result of channel infill following subsequent incision into the baymouth bar as the estuarine complex prograded slightly seaward and collapsed. This incision/fill event can be documented with the identification and correlation of five time subunits (successive scour/fill episodes). Successive units display channel collapse upsection (sub-channel widths decrease), which corresponds with increased stagnation/clogging of the estuarine system. The uppermost sideritic-ironstone facies may correspond with sedimentation within the flocculation zone of the estuary, although it displays characteristics that typify channel abandonment and stagnation.

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