Case Studies - PTTC Rockies

The Cretaceous Laramie Formation of the Denver Basin contains coal deposits that accumulated in coastal plain and alluvial environments. The coals are generally found in the lower 200 feet of the Laramie (800 to 1000 feet thick), which overlies the shoreline and shallow marine regressive sandstones of the Fox Hills Sandstone (60 to 350 feet thick).

The common swamp environments of the coal are channel margin, coastal, and abandoned channel-fill. Channel margin coal environments are of two general types: 1) restricted back-levee swamps that parallel channel trends, and also are the site of deposition of light-colored leached kaolinitic claystone; and, 2) more extensive flood basin swamps which are commonly associated with lacustrine deposits. Both types are interbedded with fresh water claystones, siltstones and sandstones.

Coastal swamps form landward from barrier islands and parallel shoreline trends. Thickest coal occurs during rising sea level because of greater accommodation space than during sea level fall. The coals may be associated with shales, siltstones and sandstones containing brackish to marine trace fossils.

Coals of the fresh water channel-fill environment are thin and aerially restricted. Other thin lenticular coals, derived from accumulation of transported organic material, may be found in both non-marine and shallow marine environments.

The critical environmental factors necessary for the formation of commercial thickness of coal are:

fresh clear water
accumulation of land organics only
balance between ground water table and depositional interface of swamp
climates
persistence of conditions through time during subsidence to give accommodation space

These conditions are most commonly found in ancient alluvial, deltaic plain and other coastal plain settings.

Penecontemporaneous (growth) faults may occur in the swamps and control locally increased thickness of coal. In the Upper Cretaceous deltaic sequence involving the Laramide and Fox Hills formations along the west margin of the Denver Basin, two types of growth faults are observed: 1) deep-seated to the basement; and 2) normal or reverse faults that die out at shallow depths. The first type is observed in the Golden - Leyden coal area and is related to early mountain flank deformation.

In the important Boulder-Weld County coal field, a target for coalbed methane, the second type of faulting with horst-graben patterns was a primary control on the number and rates of accumulation of coals. Coal beds with commercial thickness formed in the graben blocks where as many as 7 separate coal beds throughout the field were identified for mining. Individual coals range in thickness from a wedge edge to 14 feet. Usually only 2 to 4 coals are present at any one locality where an aggregate thickness may exceed 25 feet. Both the faulting and the unusual coal occurrence are unique to the Boulder-Weld County field in comparison with other parts of the Denver Basin, where coals are less abundant and thinner.

The origin of the fault system in this area is controversial with ideas ranging from all faults extending to the basement to shallow detached slide blocks with listric fault planes. The shallow faulting is believed by the author to be related to recurrent movement on the Ralston-Lafayette wrench fault system, and compares favorable with the divergent wrench fault style after Harding but with some modification along the fault zone trend.

The lateral continuity of the Laramie Formation containing the coal and the Fox Hills Sandstone is interrupted by as much as 300 feet of recurrent movement on some faults during the main phase of the Laramide Orogeny. This faulting in the Boulder-Weld County field could possibly have a negative impact on coalbed methane development.