ABSTRACT: The Zuni Megasequence of the Southern Rocky Mountains

Dag Nummedal

Sequence stratigraphic analysis of sedimentary strata has documented that Phanerozoic rocks can be packaged in unconformity-bounded sequences with a predictable internal architecture. These sequences span time-scales ranging from hundreds of millions of years for "Fischer sequences," tens of millions of years for "Sloss" or "megasequences," to 100,000 years or less for 4th and higher order sequences as recognized by Vail and associates.

A regional sequence stratigraphic study of the Zuni megasequence throughout the southern Rocky Mountains allows recognition of a series of successively smaller genetic sequences representing all these different time scales.

The Zuni megasequence extends from the base of the Middle Jurassic (base Aalenian, 177 Ma) to the top of the Cretaceous (uppermost Maastrichtian, 68 Ma). In the southern Rockies this encompasses rocks ranging from the Page and Entrada Sandstones at the base to the Price River Formation (Utah) and the Kirtland Shale (San Juan Basin) at the top. The Zuni megasequence consists of three supersequence sets: The Lower Zuni A is bracketed by the J2 and K1 unconformities and includes all strata between the Page/Carmel and the Morrison formations. The Lower Zuni B is bounded by the K1 and K2 unconformities and represented in this area only by the Lower Cretaceous Burro Canyon and Cedar Mountain formations. The Upper Zuni A is represented by a thick section of Upper Cretaceous marine, marginal arine, and fluvial strata.

These three supersequence sets consist of a total of 12 supersequences. One example of unconformity-bounded sequences of this rank is the Greenhorn cyclothem, extending from the base of the Cenomanian to the upper Turonian. In the Jurassic part of the section one supersequence is probably represented by Page/Carmel/Dewey Bridge formations. The smallest recognized, unconformity-bounded sequences are often referred to as 3rd order. There are 55 such sequences within the complete Zuni megasequence, but only about half of these are represented in the southern Rockies. Third-order sequences are recognized on a spatial scale equal to, or smaller than, most individual formations.

Higher orders of cyclicity are also commonly present, but these packages are normally not bounded by regional unconformities. Yet, 4th order sequences and parasequences can be recognized. Rhythmic beds of limestone and shale, which probably reflect deposition in response to climatic cycles operating on Milankovitch time scales, are observed in both Cretaceous and Jurassic marine intervals.

In summary, the lithostratigraphy of Jurassic and Cretaceous rocks in the southern Rocky Mountains is organized in a hierarchial pattern. The orders of organization correspond well to the "global" scheme of Vail and associates. That scheme, therefore, is an operationally useful approach for the ordering of stratigraphy even in a province subject to active tectonism, such as the late Mesozoic of the southern Rocky Mountains.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990