Abstract The Moroccan Turbidite System is unique in that individual gravity-flow deposits can be correlated across distances of several hundred kilometers, both within and between depositional basins. An extensive dataset of shallow sediment cores is analyzed here, in order to investigate the influence of gradient changes on individual siliciclastic gravity flows passing through this system in the last 160,000 years. The largest flows (deposit volumes > 100 km 3 ) are capable of travelling for more than 1000 km across slopes of less than 0.1°. The deposits of these flows display significant lateral heterogeneity as a consequence of changes in seafloor gradient. Increases in gradient can lead to sediment bypass and/or erosion, and unconfined flows may become channelized. Decreases in gradient can lead to significant changes in sand–mud ratio and the deposition of thick mud caps, while small-volume flow deposits may pinch out completely. One of the largest flows shows evidence for multiple transformations as it crossed the Agadir Basin, with the resulting deposits switching laterally from (1) a gravel lag and cut-and-fill scours (representing bypass and erosion across a slope of 0.05°), to (2) a thick linked turbidite–debrite bed containing a muddy sand debrite (in response to a decrease in slope to < 0.01°), to (3) a normally graded turbidite (following a subtle increase in slope to 0.02°). Although the changes in slope angle described here appear remarkably subtle, the relative changes in slope are significant, and clearly exert a major control on flow behavior. Such variations in slope would not be detectable in outcrop or subsurface sequences, yet will generate significant complexity in deep-water reservoirs.

Abstract Magnetostratigraphic dating of sedimentary strata is often the most precise technique available for temporally constraining the evolution of and controls upon sedimentary basins over I Ma in age. Uncertainties in the absolute dates derived by this technique are often difficult to assess quantitatively, despite the desirability of specifying their precision. An explicit discrimination should be made between correlations of the local magneto-polarity stratigraphy (MPS) to the global geomagnetic polarity time scale (GPTS) based on independent biostratigraphic or radiometric time control and those based on the smoothest derived sediment-accumulation rates. Situations in which there is a single, compelling correlation and those in which the correlation is the most reasonable of several possibilities should also be explicitly distinguished. In the latter case, alternative feasible correlations should be illustrated in order to permit a qualitative assessment of the uncertainties involved. Two classes of uncertainties are associated with the temporal calibration of magnetostratigraphic sections: those related to the creation of the local MPS and those related to the GPTS. Imprecision in measured stratal thicknesses and in the position of magnetozone boundaries can produce significant (up to 50 percent) uncertainties both in magnetozone patterns and in derived rates of sediment accumulation. Uncertainties in the GPTS result from uncertainties in the radiometric calibration of magnetic anomaly patterns. Comparison of available GPTS’s indicates uncertainties of (1) as much as 100 percent for sediment accumulation rate calculations involving intervals of less than 1–2 my and (2) up to 3 my in absolute ages. An example drawn from the Late Cretaceous to Eocene Axhandle thrust-top Basin of central Utah illustrates these uncertainties.