Hummocks--Do They Grow?: ABSTRACT

Now that hummocky cross-stratification is being more widely recognized in the stratigraphic record, investigators are speculating on the hydraulic conditions under which it forms. An accurate knowledge of the geometry of hummocky lamination is necessary if we are going to determine how it is produced. I maintain that most if not all hummocky lamination is produced where sediment is draped over a scoured hummock-and-swale surface. As pointed out by several early workers, laminae thicken into the swales so that as sediment accumulates, hummocky laminae flatten out within a liminaset. Laminae tend to be parallel to the basal scoured surface. Although very low-angle tangential laminae are not uncommon, their relationship to the lower bounding surface is typically one of onlap. Several well-exposed hummocks in the Cape Sebastian Sandstone and Coaledo Formation, southwest Oregon, will be used to document my case. It is imperative to recognize that hummocky bedsets consist of several to innumerable laminasets bounded by low-angle truncations. In some places the lower bounding surface of a hummocky bedset is essentially flat (but scoured), and the basal laminae are nearly horizontal. Laminasets above this basal set may show more curvature, but I maintain that they are bounded by scoured surfaces and that the apparent progression upward from flat to hummocky lamination does not reflect growth of hummocks. Hamblin and Walker in 1979 suggested that basal flat lamination was produced by a density-flow mechanism before storm waves resculpted the sea floor. An alternative explanation may be that the scouring ability of a single storm (or several storms in succession) may have varied with time, in some places beginning with conditions that produced a relatively flat sea floor. Is there any convincing evidence that hummocks grow, i.e., that they develop by thickening of laminae beneath the crest? Thus far, only Hunter and Clifton in 1982 have convincingly illustrated, with a photograph, an example of laminae that thicken to form a hummock. Is this example a fluke? Can we find more? Until there is further documentation of "growing" hummocks, I suggest that we avoid theorizing about how hummocks grow. If, as I maintain, hummocky cross-stratification is essentially a scour-and-drape phenomenon, can we define its hydraulic significance? In the case of (smaller-scale) vertically climbing, current-ripple lamination, most workers believe that laminae form by fallout from suspension without traction. In the example of hummocky stratification, some authors have observed parting lineation separating hummocky laminae, which would argue for traction; others have noted the absence of parting lineation. Can we explain the geometry of a hummocky scoured surface by the nature of the waves about it, i.e., are there predictable hydraulic conditions under which hummocky cross-stratification forms? Can we treat hummocks as bed forms that grow and/or migrate? Do observations concerning vertically climbing ripples in an unidirectional flow apply to conditions beneath a storm-wave surface? How important is unidirectional flow during formation of hummocky cross-stratification? Careful observation and documentation of the geometry of hummocky cross-stratification are necessary in our search for the answer to these questions. End_of_Article - Last_Page 428------------