Jurassic Ergs, Colorado Plateau


Jurassic Regional Unconformities, the Key to Accurate Correlation and Interpretation of Jurassic Geologic History, Colorado Plateau; Ronald C. Blakey

Seven regional Jurassic unconformities are recognized from the base of the Wingate Sandstone to the base of the Morrison Formation across the Colorado Plateau. These unconformities occur within and define seven chiefly continental depositional accumulations. The unconformities lack appreciable local relief, occur within parallel strata (at the outcrop scale), and commonly separate similar strata above and below. Therefore the unconformities are recognized and defined based on regional lateral relations, especially regional low-angle discordances and on lapping relations of overlying strata. The J-0 unconformity (Pipiringos and O Sullivan, 1978) separates the Chinle Formation (Upper Triassic) from the Glen Canyon Group (Lower Jurassic) across the central and western Colorado Plateau. Paleogeology exposed on the surface demonstrates gentle eastward tilt of underlying strata with a sharp uplift immediately west of the region; this suggests a tectonic origin of the surface. Reworked silicified mudstone pebbles, polygonal cracks, and eolian granule ripples mark the unconformity at a number of locations. Only in the southern Four Corners area is the surface difficult to pick where similar red mudstone and sandstone occur both below and above the unconformity; however, even there, careful tracing of strata with binoculars displays slight angular discordance. The J-sk unconformity (Riggs and Blakey, 1993) separates the fluvial-eolian Wingate Sandstone and Dinosaur Canyon Member of the Moenave Formation below from the fluvial Kayenta Sandstone and Springdale Sandstone Member of the Moenave Formation above. It is the only extensive Jurassic unconformity to lack regional angular discordance; however erosional relief of up to 60 m and reworked clasts of Wingate Sandstone in the basal Kayenta Formation mark numerous locations. The contact is everywhere easy to pick on local outcrop although it is commonly inaccessible at the top of a sharp cliff. Sedimentological and stratigraphic relations suggest climate and possibly base level changes as the cause of the unconformity. The J-1 unconformity (Pipiringos and O Sullivan, 1978), separates the underlying Navajo Sandstone from the overlying Temple Cap Sandstone (Middle Jurassic). The unconformity is restricted by Jurassic and recent erosion to the SW corner of Utah and consequently yields little regional evidence to its original extent and origin. I interpret the unconformity to represent the first phase of the amalgamated overlying J-2 unconformity. The contact is easy to pick as everywhere red sandy mudstone overlies eolian sandstone. The J-2 unconformity (Pipiringos and O Sullivan, 1978; Absaroka/Zuni sequence boundary) marks a profound reorganization of the structural relations of the Colorado Plateau but surprisingly similar sedimentation above and below. Paleogeology demonstrates regional uplift to the east with up to several hundred meters of strata removed during the J-2 erosional event. The J-2 as currently recognized is an amalgamated surface of several lesser unconformities that onlap west to east the surface of erosion on the Navajo Sandstone; missing time regularly increases to the east across the unconformity. The J-2 surface is clearly of tectonic origin, probably related to an eastward- migrating forebulge representing the early stages of the Nevadan orogeny. Overlying strata of the San Rafael Group onlap the J-2 surface from west to east. The J-2 is easy to pick at many locations, however, similar strata are commonly juxtaposed across the surface making reliance on regional patterns extremely critical. The J-2 separates strata within the San Rafael Group to the west and Precambrian rocks from the San Rafael Group to the east. The J-sup unconformity (Havholm and others, 1993; Riggs and Blakey, 1993) marks a sharp tectonic pulse of uplift in east-central Utah and amalgamates with the underlying J-2 surface. The surface separates strata within the Page Sandstone or the Page Sandstone from the overlying upper part of the Carmel Formation. At least 30 m of erosion in 15 km distance is documented. The surface formed during abrupt forebulge adjustment after sedimentation had been renewed on the J-2 surface. The J-3 unconformity (Pipiringos and O Sullivan, 1978) separates younger strata of the San Rafael Group from the underlying Entrada Sandstone. The unconformity is restricted by Jurassic and recent erosion to south-central Utah and north-central Arizona. The underlying Entrada is locally folded (compaction and evaporite removal) but strata above the J-3 are never effected. The J-3 is typically difficult to pick on the local outcrop as similar strata commonly occur both above and below. Regional trends and locally continuous sharp bedding planes help identify the surface. Sedimentation patterns and apparent lack of significant regional angularity suggest that the J-3 is a eustatic sequence boundary. The J-5 unconformity (Pipiringos and O Sullivan, 1978), like the J-2, represents significant tectonic readjustment of the Colorado Plateau during forebulge migration caused by the Nevadan orogeny. Late Jurassic strata of the Morrison Formation overlie varying strata of the San Rafael Group. The Morrison represents the first large-scale infusion of sedimentary detritus from the Cordilleran arc onto the Colorado Plateau. Although locally very difficult to identify on local outcrop due to juxtaposition of similar strata above and below, the J-5 shows strongly within regional stratigraphic patterns. These patterns document that many tens to perhaps hundreds of meters of underlying strata were removed locally by pre-Morrison erosion. When coupled with the intervening sequences of accumulation, the seven regional Jurassic unconformities provide powerful information regarding the correlation of strata and the structural and sedimentologic history of the Colorado Plateau and vicinity. They especially demonstrate the regional tilting caused by the migration of forebulges associated with the developing Nevadan orogeny. (Abstract from: Blakey,1996, in press, Museum of Northern Arizona, Jurassic Symposium)

Jurassic Erg graphics

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