turbidite facies and facies associations

PART IV TURBIDITE FACIES AND FACIES ASSOCIATIONS

Roger G. Walker and Emiliano Mutti McMaster University, Hamilton, Ontario, Canada, and Instituto di Geologia, Palazzo Carignano, 10123 Torino, Italy.

INTRODUCTION In these notes, we will be concerned with the CHARACTERIZATION of various types of turbidites (and associated deposits), and the DISPOSITION of the types within turbidite basins. We will begin by defining turbidite and turbidite basin.

A TURBIDITE is the deposit of a turbidity current, which is itself defined as a "density current in which the difference in density between the current and ambient fluid (commonly sea water) is due to the presence of dispersed sediment" (Middleton, 1969, p. GM-A-6). There is NO connota-tion of scale of flow, or depth of ambient fluid implied by this defini-tion, and turbidity currents occur in Lake Mead (Nevada-Arizona) and Lake Geneva (Switzerland) as well as in abyssal ocean depths.

The IDENTIFICATION of a turbidite depends upon the preservation of the distinctive characteristics imposed on the deposit by the turbidity current. These distinctive characteristics are fully discussed below, but embrace the assemblage of sole marks on the base of the bed, and the internal sedimentary structures of the bed. If a turbidity current depos-its a turbidite above storm wave base, there is a very high probability that the bed will be reworked,*and its turbidite characteristics destroyed. Consequently, the ONLY DEPTH CONNOTATION of turbidites is that they be deposited below storm wave base (which is commonly only tens of meters of water).

Although a TURBIDITE BASIN may be considered as any basin in which turbidites accumulate without significant reworking by tidal, storm, long-shore or semi-permanent ocean currents, thick turbidite accumulations take place only in the following basins, listed in approximate order of increas-ing scale and water depth:-

1 -- LAKES. Turbidity currents and turbidites have been described from the Swiss Lakes (Constance, Geneva, Zurich) by Forel (1887) and Kelts and Briegel (1971), and from Lake Mead (Arizona-Nevada) by Grover and Howard (1938).

2 -- CRATONIC BASINS. Turbidites are known in several cratonic basins, occurring essentially as deeper water pro-deltaic deposits. Examples include the Mississippian Borden Siltstone (Moore and Clarke, 1970)

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3 -- BLOCK FAULTED CONTINENTAL BORDERLANDS. Turbidites are important deposits in such basins, and the best example is the California borderland (Moore, 1969).

4 -- ELONGATE EXOGEOSYNCLINAL TROUGHS. Turbidity currents derived from deformed eugeosynclinal rocks may flow toward the craton, but be diverted to flow parallel to the craton margin in elongate exogeo-synclinal troughs of the Appalachian type. In North America, the best examples are the Ordovician Martinsburg Formation (Appalachians; McBride, 1962) and the Mississippian Stanley and Jackfork Groups (Ouachitas; Cline, 1970). In some basins of this type, there may also be some supply of material from the craton, for example, in the Ouachitas.

5 -- MAJOR OCEAN BASINS. Turbidites accumulate on modem continental rises, abyssal plains, ocean trenches, interarc and marginal basins. Volumetrically, the most important deposits occur on major submarine fans at the mouths of deep canyons (for example, the Monterey, Delgada and Astoria fans of northern California, Oregon and Washington).

We will be mainly concerned with the larger scale systems in these notes (3, 4 and 5 of the above list), although the general principles that we shall develop apply equally well to smaller scale systems. In basins 3, 4 and 5, the depth is measured in hundreds or thousands of meters, and the influence of reworking is minimal. Contour currents might be responsible for reworking very fine sediment on some continental rises.

For a historical review of the turbidity current concept, emphasiz-ing its importance in basin analysis, the reader is referred to the paper by Walker (1973).

SUMMARY OF TURBIDITE CHARACTERISTICS Instead of defining a "normal" turbidite (before considering the coarse grained abnormal types), it is best to describe briefly their characteristic features. COLLECTIVELY, a group of turbidites in outcrop will be very regularly bedded, and consist of alternating coarse and fine beds, Figs. 7 and 8. The coarse layers are normally sandstones, although bioclastic limestones may be present. The fine layers are shales, in places marly. The sand/shale ratio can range from almost infinity (all sandstone) to less than 0.1 (shales about 10 times as thick as sandstones). INDIVIDUALLY, each coarse layer has a sharp or erosive base with an assemblage of sole marks (scour marks, mainly flute marks; and tool marks, mainly groove and prod marks; for abundant photos and discussion see Dzulynski and Walton, 1965). Internally, the sandstone is commonly graded,