Allogenic controls on the evolution of storm to tidal shelf sequences in the Early Proterozoic Uncompahgre Group, southwest Colorado, USA
Authors: HARRIS CHARLES W. , ERIKSSON KENNETH A. 
Published: June 2006

Dominantly coarse‐grained, shallow‐marine, metasedimentary rocks of the Early Proterozoic Uncompahgre Group (UG) record periods of shoaling and drowning on different temporal scales that are attributed to episodic long‐term oscillations in relative sea‐level with superimposed shorter duration excursions in relative sea‐level. Long‐term events are probably tectonic whereas short‐term events are eustatic.

The 2–5 km thick Uncompahgre Group consists of 250–600 m thick, dominantly coarse‐grained quartzite units (Q1–Q4) and 200–300 m thick mudstone/pelite units (P1–P5). Five depositional systems comprise the Uncompahgre Group. The outer shelf system (OSS) is composed of Bouma‐type beds and intercalated mudstones that are transitional vertically to parallel‐laminated to wave‐rippled sandstones and hummocky cross‐stratified sandstones of the inner shelf system (ISS). Trough cross‐stratified sandstones comprise the shoreface system (SHS). The tidal inner shelf/shoreface system (TIS/SHS) consists of a complex interlayering of cross‐bedded sandstones, thin‐bedded conglomerates, mudstones and rippled sandstones. Trough cross‐bedded pebbly sandstones and thin‐ to thick‐bedded conglomerates represent the alluvial system (ALLS).

Depositional systems in the UG are associated in transgressive and highstand‐systems tracts that make up four sequences (1 to 4). Sequence boundaries do not correspond with lithostratigraphic boundaries but are defined by subtle unconformities. The basal Q1–P1 unit (lower sequence 1) consists of ALLS to TIS/ SHS to ISS comprising a transgressive systems tract. A maximum marine incursion is reflected by deposition of OSS facies in stratigraphic units P1–P2. Shoaling in the transition from P2 to the uppermedial portion of Q2 (OSS—ISS—SHS to a thick TIS/SHS—ALLS) records the highstand systems tract of upper sequence 1. A subtle disconformity/paraconformity delineates a type 2 sequence boundary at the top of the highstand systems tract. The drowning to shoaling pattern is replicated in sequence 2 (upper Q2 to P3 to upper medial Q3); sequence 3 (upper Q3 to P4 to upper‐medial Q4); and an incomplete sequence 4 (upper Q4 through P5). Thinner shoaling intervals of OSS—ISS—SHS in P3 and in lower Q2, Q3 and Q4 represent parasequences.

Sequences of 107 years duration are attributed to periods of increasing and decreasing subsidence rates due to tectonism marginal to the sedimentary basin. Parasequences record shorter duration temporal controls of c. 104 to 105 years related to eustatic oscillations. As a consequence of shoaling and aggradation/ progradation in the highstand systems tract, TIS/SHS and ALLS overlie and are temporally separated from OSS to ISS to SHS. This transition records filling of the basin to sea‐level leading to a shelf geometry that was conducive to tidal amplification. A composite relative sea‐level curve integrating long‐term pulsatory subsidence and short‐term eustasy best explains the stratigraphic evolution of the Uncompahgre Group.