162 SEQUENCE STRATIGRAPHY the resulting stratigraphic interpretation provides an improved understanding of stratigraphic geometry by demonstrating the connection of the thinner shallowmarine sands to thicker sand beds in a landwards direction Recognition of Sequences and Systems Tracts Figure Parasequence stacking patterns for (A) prograda tional, (B) retrogradational, and (C) aggradational parasequence sets Stacking patterns are indicated by the movement of facies in successive parasequences (numbered) in each diagram Rd, rate of deposition; Ra, rate of accommodation (Adapted from Van Wagoner JC, Posamentier HW, Mitchum RM Jr, et al (1988) An overview of the fundamentals of sequence stratigraphy and key definitions In: Wilgus CK, Hasting BS, Kendall CStCC, et al (eds.) Sea Level Changes: An Integrated Approach, pp 39 45 Special Publication 42 Tulsa: Society of Economic Paleontolo gists and Mineralogists.) A sequence is a succession of genetically related relatively conformable strata bounded by unconformities or their correlative conformities Based on parasequence-stacking patterns and facies trends, a number of distinct sequence components called systems tracts can be defined A systems tract is a linkage of contemporaneous depositional systems, with a depositional system defined as a three-dimensional assemblage of lithofacies The five systems tracts most commonly recognized are lowstand fan, lowstand wedge, shelfmargin wedge, transgressive, and highstand (Table and Figure 5) These systems tracts are separated by significant stratigraphic surfaces, the most important being the sequence boundary, transgressive surface, and maximum flooding surface Figure Comparison of (A) lithostratigraphic and (B) chronostratigraphic correlation styles for a prograding parasequence set The correlation datum for the lithostratigraphic section is the top of the sandstone; the datum for the chronostratigraphic section is the uppermost parasequence boundary PSB, parasequence boundary (Adapted from Van Wagoner JC, Mitchum RM, Campion KM, and Rahmanian VD (1990) Siliciclastic sequence stratigraphy in well logs, cores, and outcrops American Association of Petroleum Geologists Methods in Exploration Series 7: 55.)