Organic rich source rocks buried deep in sedimentary basins charge petroleum systems. Mid-Cretaceous source rocks stand out because they have charged one-third to one-half of petroleum reservoirs worldwide. Some geologists believe that the reason for this lies deeper than sedimentary basins - in a mantle superplume that hit the southwest Pacific Ocean at 125 Ma (Early Cretaceous) and triggered a chain of oceanic and atmospheric changes favorable for deposition of black shales and marls. If true, this idea has far-reaching implications for the petroleum inventory of the Earth.
Source rock is crucial
What factors control the distribution of oil resources around the world? This seemingly simple question lies at the heart of the zillion-dollar global oil industry.
For petroleum to occur, all elements of a petroleum system should be present: Source rock, reservoir, cap rock, and trap. A sedimentary rock rich in organic matter and thermally matured is the trigger for the entire petroleum system, without which, no matter how porous the reservoir is or how impermeable the trap is, no petroleum can be generated in the first place.
Organic carbon in sediments is derived from life forms, and in this regard tiny phyotoplanktons and zooplanktons are "kings" of the petroleum realm. Therefore, the Phanerozoic eon, spanning the past 542 million years, and during which life forms dramatically evolved and expanded, is of utmost importance for petroleum.
Shale, limestone and coal are potential hydrocarbon source rocks. While these sedimentary materials have been deposited in various parts of the world during the Phanerozoic, several independent geological studies have noted that rich petroleum source rocks are abundantly found in several stratigraphic intervals, with the mid-Cretaceous being the major pulse. Of course, there is often an element of intelligent guess work in this type of research because we may not have analytical data tracing oil produced from a reservoir to a particular source rock in the basin.
Nonetheless, existing evidence for the unequal distribution of petroleum source rocks in the geologic timescale is too significant to be ignored. This unequal distribution does not appear to be an artifact of geological preservation of the source rocks because, for example, the Devonian source rocks are more prominent than the younger Triassic sediments. Moreover, when we examine a single region having a complete Phanerozoic history of sedimentation, rich source rocks are not distributed uniformly but confined to certain stratigraphic intervals. Of these source-rock pulses, the mid-Cretaceous tops the list, and to explain this phenomenon we review here a hypothesis based on a deep mantle plume proposed by Roger Larson in the early 1990s.
A Tale of the Deep Pacific
Larson devoted his life to studies of tectonic evolution of oceans, especially the Pacific, beginning with his graduate work on the Gulf of California at the Scripps Institution of Oceanography of the University of California at San Diego in the late 1960s, then as a research scientist in the Lamont-Doherty Geological Observatory at Colombia University during the 1970s, and finally as a professor of marine geophysics at the University of Rohde Island, where he led many Ocean Drilling Program expeditions and projects from 1980 until his untimely death in 2006, aged 63.
In 1972, Larson and Clement Chase published a paper in the Geological Society of America Bulletin in which they presented the results of their mapping of the Mesozoic (M-Series) magnetic lineations and fracture zones in the central and western Pacific and a tectonic model for the evolution of the Pacific during the Cretaceous. In another paper in the same journal, Larson and Walter Pitman correlated the Mesozoic ocean magnetic anomalies worldwide and noted that while Earth's magnetic poles have frequently flip-flopped in the geologic past, there was a long Mid-Cretaceous "quiet zone" during which the magnetic field retained its normal polarity and was not reversed. Moreover, they noted, this event coincided with extensive sea-floor spreading along mid-ocean ridges in the Pacific and Atlantic during 110-85 Ma.
Two decades later, Larson, now equipped with better mapping and age data, revisited this subject in two Geology papers. His new thinking followed shortly after the Ocean Drilling Program's 1989 expedition, led by Larson and Yves Lancelot, which discovered the world's oldest oceanic sediments and volcanic basement dating back to 165 Ma (Middle Jurassic) in the western Pacific Ocean. Larson first calculated the volume of oceanic crust production during the past 150 million years and found that there was 50% to 75% increase in rate of ocean crust formation at mid-ocean ridges, ocean basalt plateaus and seamounts between 125 and 80 Ma. Six major ocean plateaus that formed during this period are located close to one another in the southwest Pacific, indicating that it was the locus of a major upwelling of hot magma from deeper Earth. It was also during this period that the "Long Mid-Cretaceous Normal" anomaly (124-83 Ma; Aptian-Santonian), also called Superchron C34, took place. Since Earth's magnetic reversals are oscillations of the geomagnetic dynamo, itself believed to have generated by convection currents of boiling iron in the Earth's outer core, Larson related the 41-million-year-long mid-Cretaceous Normal anomaly to the upward removal of hot materials at the core-mantle boundary (a 100-200 km thick thermal boundary known as the D-layer), and thus rapid convection of the outer core to restore the large loss of heat.
In other words, Larson proposed that a "superplume" emerging from the core-mantle boundary hit the southwest Pacific at about 125 Ma (Early Cretaceous), and this was somehow responsible for the quiet magnetic Superchron C 34. Although a precise mechanism for magnetic reversals is yet to be proven, the inverse relation between frequency of magnetic reversals and volcanic production of oceanic crust on one hand, and the links between the Earth's outer core and magnetic field on the other hand, hold keys to resolving this mystery. Larson and Peter Olson (Earth and Planetary Science Letters, 1991) have described their thoughts on this issue.
Plumes Plumes Everywhere?
It was Jason Morgan who first proposed upwelling of abnormally hot rocks as a product of convection currents in the lower mantle (Nature, 1971; AAPG Bulletin, 1972). This concept opened a new way of looking into Earth's deep processes. As a plume approaches the Earth's crust it flattens into a large head which swells, stretches and collapses the crust (extensional tectonics); the plume head also undergoes decompressional melting, creating a "hot spot" on the Earth's surface and pouring a large volume of volcanic rocks that build ocean plateaus, seamounts and continental flood basalts. The Deccan Traps in west-central India, the Rajmahal Traps in eastern India, the Parana Basalts in Brazil, the Etendeka Basalts in west Africa, the Karoo basalts in South Africa, and the Ferrar Dolerite in Antarctica are well-known examples of continental flood basalts, each several km thick and several hundred kilometers long, and all can be traced to specific hotspots. Currently about 40 hotspots have been mapped, all of which are located in the oceans except for Iceland, Yellowstone in USA, and Afar in east Africa, which are continental.
Plumes are long-lived features, some lasting up to about 200 million years, as the plume tail rooted deep in the Earth's mantle continues to supply magma. Moreover, plumes have stationary locations relative to the moving lithospheric plates. (There is, however, evidence that not all plumes have absolute fixed positions; some plumes wander, albeit at a slower rate). As a tectonic plate passes across a mantle plume, a chain or trail of volcanoes erupts onto the plate, with older volcanoes located farther from the plume. The Hawaiian-Emperor chain of islands in the Pacific is a classic volcanic trail related to a hotspot under the island of Hawaii.
There are other lines of evidence for deep-rooted mantle plumes. First, the geochemical composition of hotspot basalts is different from those of mid-ocean ridges and subduction-related island arcs. Second, seismic tomography (density mapping based on teleseismic waves) of the mantle has provided maps of mantle plumes as hot plumes slow seismic waves compared to the surrounding cool mantle material.
Larson placed the root of the mid-Cretaceous superplume at the core-mantle boundary partly because its enormous surface expression in the southwest Pacific indicated a plume too large to be contained in the upper mantle. Some scientists, however, are not convinced that existing data prove a core-mantle boundary origin for plumes, and instead suggest the lower mantle-upper mantle boundary, about 670 km deep, for the origin of plumes. Irrespective of its depth, the mid-Cretaceous superplume appears to have left its mark on the topography of the Pacific Ocean floor. A close look at the map of the Pacific floor shows that while the eastern Pacific has a smooth, line-patterned morphology, typical of oceanic crust formed by sea-floor spreading, the western Pacific is characterized by randomly oriented plateaus and seamounts, probably formed by a superplume as Larson postulated.
What Has Plume Got to Do with Petroleum?
Larson also argued that the mid-Cretaceous superplume had important impacts on the deposition of rich petroleum source rocks via oceanic and climatic changes it probably caused. This model may be described in the following steps:
1. A superplume or more efficiently various plume heads emerging from a superplume pour large volumes of volcanic rocks and also probably play important roles in continental breakups and sea-floor spreading rates.
2. These volcanic activities emit large amounts of carbon dioxide into the atmosphere, causing a global warming by greenhouse effect.
3. Global warming extends warm climates to higher latitudes and raises sea levels worldwide resulting in extensive and rapid deposition of clay-rich sediments in marine environments.
4. Volcanic outpourings in the oceans introduce additional amounts of mantle carbon as well as sulfur, phosphorus and nitrogen.
5. High stand seas, increased marine continental shelf environments, warm climates, and ocean-water nutrients (sulfur, phosphorus and nitrogen) increase the plankton populations.
6. Marine transgressions, and especially high productivity of phytoplanktons in the world's oceans, result in oceanic anoxic (oxygen-deficient) events and thus formation of thick, widely-spread and organic rich black shales and marls on continental shelves.
Geologic evidence indicate that these events took place during mid-Cretaceous times in large parts of the world, which would explain why mid-Cretaceous rocks generated a large percentage of world's oil as they were subsequently buried to "oil window" temperature zones (60-120 ºC corresponding to 2-5 km burial depths) during the Cenozoic.
Plumes, Plates, and Petroleum Basins
While the superplume hypothesis offers an interesting perspective to explain the abundant mid-Cretaceous oil source rocks, it should be noted that processes leading to such rich source rocks are not direct evidence for the hypothesized superplume because these processes may as well be ascribed to the breakup of the Pangea (Gondwana-Laurentia) supercontinent and volcanic activities associated with sea-floor spreading and subduction during the Cretaceous. Indeed, some scientists, notably Don Anderson of California Institute of Technology, refute the superplume hypothesis and instead explain the observed features and evidence in light of normal plate tectonic processes. (See for example, Anderson's article, "Superplumes or supercontinents?" in Geology, January 1994).
Reading through the enormous plume literature produced by eminent scientists in both camps of the superplume-versus-supercontinent controversy, one feels that both sides highlight some aspects of reality about Earth's internal dynamism. Plate tectonics based on the process of divergence (rifting and sea-floor spreading) and convergence (subduction and collision) is already a known fact, but as unifying as it may seem, it is still confined to the outermost layers of the Earth - the solid lithospheric plates moving on the molten asthenosphere probably via convections currents. However, it seems unlikely that the lower mantle, which accounts for the bulk of the solid-Earth's volume, has absolutely no role in shaping global tectonic processes. When I was a doctorate student in the late 1980s, I remember attending a lecture by the renowned French pioneer of plate tectonics, Xavier Le Picheon, who opined that the next stage for plate tectonics is its integration with a deeper earth model. Perhaps the mid-Cretaceous events, which indicate both plate tectonic and plume tectonic activities taking place concurrently, provide a valuable case for an integrated geodynamic model. It is conceivable that the Pangea supercontinent assembled by the motion of tectonic movements was fragmented initially by the impingement of plume heads and then evolved into rift-drift events and sea-floor spreading.
Larson viewed the Earth as "a huge heat engine" where "the heat is dissipated mainly during the formation of oceanic crust," and the "Mid-Cretaceous Superplume Episode" (Scientific American, Feb. 1995) was the latest pulse of "an episodically pulsating Earth."
Living on the surface of the Earth and being more familiar with the Earth's crust and basins, we may find it hard to believe that mantle plumes probably influenced the geological concentration of petroleum resources. But consider this: The Earth's continental and ocean crusts are respectively 40 km and 6 km thick on average, which are insignificant values compared to the Earth's overall thickness (6370 km). If the planet owes its protective magnetic field to the processes in its core, it is conceivable that petroleum basins also owe some of their wealth to heat from deeper Earth.