About This Item
Share This Item
Cyclical deposits within the Lower Member of the Honaker Trail Formation (Desmoinesian), between Durango and Silverton, Colorado, have been studied in detail and indicate tectonically controlled sedimentation along the western flank of the Uncompahgre uplift. These cycles were previously considered the result of eustatic sea level fluctuation. Fan delta deposits form a thick wedge of coarse clastics and are interbedded with thin carbonates and siliciclastic shelf-bar systems. Significant lateral variation in depositional style and stratigraphic succession occurs along the strike of this faulted basin margin.
Two major types of cyclic sequences are recognized. Clastics found in the northern and southern portion of the study area near Molas Lake and Engineer Mountain are dominantly thin (10 to 15 m, 33 to 50 ft), sheet-like, rapidly shifting fan delta complexes. Three subfacies within these fan deltas can be distinguished. (1) The bottomset beds, (2.5 to 8 m, 8 to 26 ft, thick) are parallel-laminated and rippled, moderately sorted, micaceous, fine (0.125 mm) to medium-fine (0.25 mm) sandstones. Large plant fragments, as well as macerated plant debris and bioturbation are common. (2) Forsets (2.5 to 4.5 m, 8 to 15 ft thick) are characterized by small to medium-scale trough cross strata and abundant soft sediment deformation in poorly sorted, arkosic, medium (0.25 mm) to coarse (0.71 mm) sand tones. (3) Topset beds (1.5 to 2.5 m, 5 to 8 ft thick) form a capping unit of very poorly sorted, arkosic, very coarse sandstones (2 mm) to conglomerates (4 mm +). Climbing units of topset beds are characterized by medium scale trough cross strata with occasional ripple stratification.
In association with these fan delta units are thin carbonates (0.5 to 2 m, 1.6 to 6 ft of wackestones/packstones) and nonmigrating shelf sands (0.25 to 2 m, 10 in. to 6 ft) with subparallel laminations and ripple stratification. The carbonates apparently do not cap the deltaic sequences but are more closely associated with the shelf sands. These fan deltas are fluvially-dominated with little or no evidence of reworking by marine processes.
The central region of the study area near Coal Bank Pass is characterized by marine-influenced high energy fan deltas (9 to 16 m, 30 to 52 ft) as evidenced by the abundance of hummocky cross strata at the base of the fan delta sequences and flanking shelf-bar systems. The topsets and forsets are similar in scale and in sequences of sedimentary structures and textures to those previously discussed. The bottomsets, however, are finer (0.17 mm), better sorted, and not as micaceous as those in lower energy areas.
Closely associated with these delta complexes are flat-bottomed, lense-shaped shelf sand-bars. These coarsening-upward, laterally migrating bars are the cleanest, most well-sorted, and finest (0.06 to 0.125 mm) sandstones. Oscillation ripple stratification subparallel laminations with both broad synforms and antiforms are the dominant sedimentary structures indicative of the marine reworking of fan delta sediments.
Variation in stratigraphic successions in adjacent areas and the areal distribution of fan delta types suggests major strike-slip motion along the bounding faults responsible for the development of upthrown and downthrown wedge-shaped blocks within the border fault zone.
End_of_Article - Last_Page 514------------