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Ancient turbidite depositional systems--not necessarily equivalent to deep-sea fans--can be defined as systems where channel-fill sediments are replaced in a downcurrent direction by nonchannelized sandstone lobes. Facies types and facies sequences that characterize channels and lobes have become sufficiently understood within the last 10 years to permit their relatively east recognition in both field and core analyses.
Channel-fill sequences may differ considerably from one system to another. However, two main types of channel-fill sequences, which may be intergradational, can be recognized in most ancient systems. The first type represents channel-levee complexes, and is made up of highly lenticular bodies of coarse-grained sediments originally deposited in channel-axis zones, and of thin-bedded mudstones and fine-grained sandstones produced by overbank processes both within the channels and in adjacent interchannel regions. The second type consists essentially of broadly lenticular bodies of thick-bedded, graded sandstones that are commonly characterized by an abundance of rip-up shale clasts and scoured surfaces. Most individual sandstone beds typically thin toward and onlap onto channel walls. B th types of channel-fill deposits develop facies sequences with an overall thinning- and fining-upward character. In addition, each sequence is typically bounded by a basal erosional surface.
Lobe sequences are characteristically expressed by an alternation of thick-bedded nonchannelized sandstone bodies, commonly between 3 and 15 m (10 and 50 ft) thick, and thinner bedded and finer grained deposits. Also in this case, facies types and facies associations may vary considerably from one system to another. The loci of maximum sand deposition of the lobe environment can either prograde basinward, particularly in small sand-rich systems, or shift laterally with time in systems which develop essentially through vertical aggradation processes. In both places, the resulting facies sequences do not show significant erosional boundaries and can be generally described as thickening upward. Minor thickening-upward sequences, commonly consisting of a limited number of sandstone beds, re virtually ubiquitous within lobe sandstone units and are thought to represent compensation features produced by the progressive smoothing out of the depositional relief during the process of lobe upbuilding.
The transition between channels and lobes is still very poorly understood in both modern and ancient turbidite depositional systems. However, evidence provided by ancient sequences suggests that, at least in some systems, the transition is characterized by an abundance of coarse-grained, cross-stratified, and megaripple-shaped sandstone beds that represent either the bypassing zone of turbidity currents still moving basinward or the most proximal depositional product of the same currents before they develop complete Bouma sequences.
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