Ancient sand-rich submarine fans: depositional systems,
models, identification, and analysis
r Geologische Wissenschaften, Freie Universita
t Berlin, Malteserstra
e 74-100, D-12249 Berlin, Germany
Received 12 December 2003; accepted 8 December 2004
Sand-rich submarine fans are radial or curved in plan view depending on the slope of the basin floor. They occur isolated or
in coalescing systems. The fans’ average lateral extent measures close to 25 km and their thickness usually less than 300 m. The
thickness of outer fan sequences averages around 120 m and that of middle fan successions around 160 m. Rarely reported inner
fan sequences have a maximum thickness of 80 m.
The formation of sand-rich fans is closely related to tectonic activity. Their sediment is coarse-grained and compositionally
immature as indicated by significant feldspar content due to close provenance and rapid transport by short rivers with a steep
gradient controlled by tectonism. Tectonic activity also provides for narrow shelves making the fans relatively insensitive to
sealevel changes. Formation of sand-rich fans typically occurs in restricted continental basins. The tectonic settings are highly
variable. Sand-rich fans typically receive their sediment through submarine canyons which intercept sand from longshore drift
and/or are fed more or less directly by regional rivers.
The type of ancient fan system (radial, curved, isolated, coalescing) may be identified through paleocurrent map plots, facies
map sketches, recognition of lateral thickness variations and sediment influx centers, as well as lateral bed correlations defining
the minimum fan extent.
Important in distinguishing different environments of ancient fans are detailed measured sections, their comparison and
correlation. Channelized inner fan and middle fan deposits may be distinguished from the unchannelized outer fan successions
through bed correlation tests which reflect their different stratigraphic architectures and bedding patterns. Bedding in outer fan
deposits (lobes) is relatively simple, parallel, and regular. The lateral bed continuity is relatively high. Channel fills, especially
those of middle fan distributary channels, display a complicated bedding pattern with vertical and lateral random distribution of
channel fills, axial erosion, and bed convergence towards the channel margins. Channel fills exhibit only linear bed continuity.
Thus, the probability in carrying out local to regional scale lateral bed correlations is almost exclusively limited to outer fan
The measured sections will help further distinguish fan environments by revealing: (1) different facies associations in outer
fan sequences (mainly B, C and D) and middle fan successions (mainly A, B, C, D, and channel margin facies); (2) greater
average bed and layer thicknesses in middle fan as opposed to outer fan successions (bbedQ and blayerQ as used herein); (3) more
frequent amalgamation surfaces in channel fills than in unchannelized outer fan deposits; (4) more frequent tabular
0012-8252/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
E-mail address: firstname.lastname@example.org.
Earth-Science Reviews 70 (2005) 167 – 202