Are Seismic/Depositional Sequences Time Units? Testing By Shads Cores And Graphic Correlation
Authors: Scott R.W. , Evetts M.J. , Stein J.A. 
Published: October 2008

Continuous core of the mid-Cretaceous section in the U.S. Western Interior cut by the Stratigraphic High--speed Advanced Drilling System (SHADS) enabled the accurate definition of sequence boundaries. The high-precision, quantitative stratigraphic technique of graphic correlation, based on detailed core and outcrop samples, tested the time relations of these sequences with those in the Gulf Coast. Sequence boundaries are diachronous surfaces that vary in age by 0.5 Ma or more. Erosional sequence boundaries in one basin correlate with transgressive contacts between subcycles in another basin suggesting that differential subsidence and sediment supply varied significantly between basins.

Introduction

The use of seismic record sections for the interpretation of stratigraphic relationships has revolutionized the science of stratigraphy. A key concept is the division of the sedimentary rock record into unconformity- bounded time units called sequences. A key assumption is that seismic reflectors are time lines. The synchroneity of the boundaries and reflectors is inferred but cannot be tested by the seismic data. Only the integration of biostratigraphic data by means of well samples, cores, well logs, and outcrops can test the time relations so that credible hypotheses can be formulated. Two technologies provide data for and constrain hypotheses of the synchroneity of sequence boundaries and seismic reflectors. Continuous, economical cores can be obtained in the exploration environment by SHADS - the Slim-Hole High-Speed Drilling System,1 also known as Stratigraphic High--speed Advanced Drilling System.2 Precise, high-resolution chronostratigraphy can be achieved by graphic correlation of fossil ranges determined in the core with those ranges in a global range data base, the composite standard.3,4The integration of these two technologies provide data to determine the age relations of middle Cretaceous strata in the Western interior Seaway cored in western Kansas and in the Gulf Coast outcrops in north Texas.

Concepts of Seismic/Depositional Sequences

The sequence is the basic stratigraphic unit in sequence stratigraphy, and particularly as it is practiced in the interpretation of seismic sections. A sequence is a relatively conformable succession of genetically related strata bounded by erosional unconformities or subaerial exposure surfaces that represent a significant hiatus. 5,6 As defined, sequences are allostratigraphic units, which are mappable stratiform bodies of sedimentary rock defined and identified by their bounding unconformities. 7 Sequences also form a chronostratigraphic framework of depositional cycles driven by changes in relative sea level.8 However, the boundaries are not synchronous because they are erosional surfaces of significant time duration. Therefore, sequences are not chronostratigraphic units bounded by synchronous surfaces, as defined by the Code,7 and cannot be time-equivalent rock packages9 because they are not isochronous, of equal age duration everywhere. The age span of sequence boundaries can only be determined by biostratigraphic, radiometric or paleomagnetic units tied to a time scale. This is the application of graphic correlation. Depositional sequences consist of facies sets that are bounded by erosional or nondepositional surfaces, and that record cycles of shoreline advance and retreat,10 the classic transgressive-regressive cycles.