168
5 SEDIMENTARY STRUCTURES
recorded from continental shelves around the world, normally overlying a thick sedimentary succession (Hovland and Judd, 1988). Commonly pock marks overlie petroleum "kitchens" of thick organic-rich overpressured shale. Pock marks often, but not
invariably, connect at depth with fault systems that extend down to the "kitchen." Cold
seepages of brine and bubbles of carbon dioxide, hydrogen sulfide, methane, and other
petroleum gases emerge from modern pock marks. Thus it is often held that pock marks
result from the episodic explosive release of petroleum and petroleum-related fluids.
Lenses of gas-charged sand have been imaged on subsea seismic data beneath pock
marks, suggesting an analogy with the magma chambers beneath igneous eruptive centers (Brooke et al., 1995).
Investigations of a large field of pock marks in the Gulf of Patras, offshore Greece,
fortuitously coincided with an earthquake of magnitude 5.4 on the Richter scale. A rise
in seawater temperature was recorded prior to the quake, and gas bubbles were seen
emerging from the pock marks for several days thereafter (Hasiotis et al., 1996).
Pock marks are particularly well known from the petroliferous sedimentary basins of
the northwest European continental shelf, and from northwest Australia (e.g., Hovland
and Sommerville, 1985, and Hovland et al., 1994, respectively). In the Gulf of Mexico,
however, pock marks overlie gas-hydrate cemented sediments over mud diapirs and salt
domes. Here the explosive event may be linked to the destabilization of gas hydrate,
with a concomitant escape of methane gas through the water column to the atmosphere
(Bagirov and Lerche, 1997). This phenomenon has been advocated as an explanation of
unexplained maritime plane and ship losses, the so-called Bermuda Triangle effect (McIver, in Simmons and Jacobs, 1992).
Pock marks are also known on the floor of the Baltic Sea, where only a thin veneer
of modern sediment overlies fractured igneous and metamorphic basement. Here it is
more likely that the eruptions take place from mantle-derived gases (Gold, 1999).
5.4 PALEOCURRENT ANALYSIS
The preceding analysis of sedimentary structures shows that they can be used to determine depositional processes. Because depositional processes occur in several environments, few structures are immediately diagnostic of a specific environment; assemblages
of structures are most useful, as for example in a tempestite (Section 5.3.3.4), a turbidite (Section 4.2.2), or a point bar (Section 6.3.2.2.3).
There is, however, one further use for sedimentary structures. They can indicate the
direction of paleocurrent flow, paleoslope, paleogeography, and sand-body trend. Paleocurrent analysis, as this discipline is called, forms an integral part of facies analysis
both at outcrop and, using the dipmeter, in subsurface studies. There is an extensive literature on this topic. Potter and Pettijohn (1977) is the definitive text. The methodology, interpretation, and applications of paleocurrent analysis are now described in turn.
5.4.1 Collection of Paleocurrent Data
A wide range of sedimentary structures can be used in paleocurrent analysis. Some
structures yield only the sense of current flow, others yield both sense and direction. Examples of the first group include groove marks, channels, washouts and parting linea-
5 SEDIMENTARY STRUCTURES
recorded from continental shelves around the world, normally overlying a thick sedimentary succession (Hovland and Judd, 1988). Commonly pock marks overlie petroleum "kitchens" of thick organic-rich overpressured shale. Pock marks often, but not
invariably, connect at depth with fault systems that extend down to the "kitchen." Cold
seepages of brine and bubbles of carbon dioxide, hydrogen sulfide, methane, and other
petroleum gases emerge from modern pock marks. Thus it is often held that pock marks
result from the episodic explosive release of petroleum and petroleum-related fluids.
Lenses of gas-charged sand have been imaged on subsea seismic data beneath pock
marks, suggesting an analogy with the magma chambers beneath igneous eruptive centers (Brooke et al., 1995).
Investigations of a large field of pock marks in the Gulf of Patras, offshore Greece,
fortuitously coincided with an earthquake of magnitude 5.4 on the Richter scale. A rise
in seawater temperature was recorded prior to the quake, and gas bubbles were seen
emerging from the pock marks for several days thereafter (Hasiotis et al., 1996).
Pock marks are particularly well known from the petroliferous sedimentary basins of
the northwest European continental shelf, and from northwest Australia (e.g., Hovland
and Sommerville, 1985, and Hovland et al., 1994, respectively). In the Gulf of Mexico,
however, pock marks overlie gas-hydrate cemented sediments over mud diapirs and salt
domes. Here the explosive event may be linked to the destabilization of gas hydrate,
with a concomitant escape of methane gas through the water column to the atmosphere
(Bagirov and Lerche, 1997). This phenomenon has been advocated as an explanation of
unexplained maritime plane and ship losses, the so-called Bermuda Triangle effect (McIver, in Simmons and Jacobs, 1992).
Pock marks are also known on the floor of the Baltic Sea, where only a thin veneer
of modern sediment overlies fractured igneous and metamorphic basement. Here it is
more likely that the eruptions take place from mantle-derived gases (Gold, 1999).
5.4 PALEOCURRENT ANALYSIS
The preceding analysis of sedimentary structures shows that they can be used to determine depositional processes. Because depositional processes occur in several environments, few structures are immediately diagnostic of a specific environment; assemblages
of structures are most useful, as for example in a tempestite (Section 5.3.3.4), a turbidite (Section 4.2.2), or a point bar (Section 6.3.2.2.3).
There is, however, one further use for sedimentary structures. They can indicate the
direction of paleocurrent flow, paleoslope, paleogeography, and sand-body trend. Paleocurrent analysis, as this discipline is called, forms an integral part of facies analysis
both at outcrop and, using the dipmeter, in subsurface studies. There is an extensive literature on this topic. Potter and Pettijohn (1977) is the definitive text. The methodology, interpretation, and applications of paleocurrent analysis are now described in turn.
5.4.1 Collection of Paleocurrent Data
A wide range of sedimentary structures can be used in paleocurrent analysis. Some
structures yield only the sense of current flow, others yield both sense and direction. Examples of the first group include groove marks, channels, washouts and parting linea-