Sedimentary Geology, 67 (1990) 221-236 221 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands Coherence of surf zone and shelf current flow on the Texas (U.S.A.) coastal margin: implications for interpretation of paleo-current measurements in ancient coastal sequences John W. Snedden 1 and Dag Nummedal 2 1 Mobil Exploration Norway, Inc., Postboks 510, 4001 Stavanger (Norway) 2 Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803 (U.S.A.) Received July 24, 1989; revised version accepted March 2, 1990 ABSTRACT Snedden, J.W. and Nummedal, D., 1990. Coherence of surf zone and shelf current flow on the Texas (U.S.A.) coastal margin: implications for interpretation of paleo-current measurements in ancient coastal sequences. Sediment. Geol., 67: 221-236. Measurements from the Texas coastal margin, a microtidal, fetch-limited, wind-dominated shallow sea, reveal a strong correlation in timing and direction of surf zone and shelf flow during both fairweather and storm events. Coherence of current motion is statistically significant at many frequencies (cycle lengths) and is highest during storm events. The lack of a significant lag between surf zone and shelf flow indicates that both respond rapidly to changes in the magnitude and direction of the wind field. The alongshelf component of the wind stress is a major force in generating currents both on the shelf and in the surf zone, as exhibited by comparison of numerical predictions and observed velocities. During the 140-day study period, current flow was mainly directed alongshelf to the northeast and southwest, driven by southerly "fairweather" and northerly "storm" winds, respectively. However, the threshold for generation and migration of megaripple bedforms in upper shoreface bar-troughs was exceeded only during storm events when surf zone flow was oriented to the southwest, along the coast. This observation, coupled with the knowledge that storm-generated bedforms have the highest preservation potential, imply that multi-event traction bedding in upper shoreface bar-troughs of analogous ancient environments will be dominated by unimodal alongshore current orientations. Large-scale bedforms which respond to this time-averaged flow on the shelf are expected to display a similar unimodal alongshelf orientation with a low-angle deviation toward the offshore direction. Careful study of paleo-current measurements in a well-documented ancient shelf to shoreline sequence in the Cretaceous of New Mexico reflects this similarity in transport kinematics. Introduction Oceanographers and marine geologists have categorized fluid motion in the coastal ocean into a series of shore-parallel dynamic zones: surf, friction-dominated, transition, and geostrophic zones (Fig. 1). Under fairweather conditions and most storm events, these dynamic zones coincide with morphologic zones of the upper shoreface, lower shoreface, inner and outer shelf, respectively (Swift et al., 1985). Even though most workers 0037-0738/90/$03.50 © 1990 - Elsevier Science Publishers B.V. acknowledge that surf zone processes are dynami- caUy linked to the adjacent continental shelf, few studies have attempted to investigate the degree of coherence of flow within these domains. Work in the surf zone has tended to emphasize the role of wave-related processes (shoaling and breaking waves and wave-generated longshore flow). On the shelf, oceanographers and marine geologists have been more concerned with geostrophic currents, which result from interaction of wind-induced pressure gradients and Coriolis forces (Swift et al.,