SLOSS, LAURENCE LOUIS

(b. Mountain View, California, 26 August 1913; d. Evanston, Illinois, 2 November 1996), stratigraphic geology.

The science of stratigraphy is focused on the layered sedimentary rocks of Earth’s crust, which provide the primary evidence for the physical history of the planet and for the history of life. Sloss was the preeminent American stratigrapher of the twentieth century. His work reorganized the discipline and gave rise to the concept of sequence stratigraphy.

Career Overview Laurence L. Sloss was a California native, the son of Joseph and Edith (Esberg) Sloss. He stayed in California long enough to attend Stanford University (AB, 1934) and then departed for the heartland, which was to be his permanent base of operations. He studied paleontology at the University of Chicago, completing a dissertation on Devonian corals under the direction of Professor Carey Croneis.

Sloss finished his PhD in 1937 and began teaching as an instructor at the Montana School of Mines, in Butte. He held a concurrent position as geologist with the State Bureau of Mines and Geology, which gave him access to stratigraphic data from across the huge area of the state of Montana. He began to compile information on the distribution of sedimentary rocks, utilizing drilling records from the petroleum industry to supplement the natural exposures in the mountain ranges. His summary paper on the Devonian System (with Wilson M. Laird) was published in 1947, in the Bulletin of the American Association of Petroleum Geologists, and won the AAPG President’s Award, as the best article of the year. That same year, Sloss moved to Northwestern University, where he remained for the rest of his life. At Northwestern he was part of a famous triumvirate (with William C. Krumbein and Edward C. Dapples) that modernized and quantified the study of sedimentary rocks. Krumbein and Sloss’s textbook, Stratigraphy and Sedimentation (1951) became thestandard classroom work for the next generation.

Sequence Stratigraphy Sloss’s primary accomplishment was the recognition that the rock record of the North American continent was naturally subdivided. Throughout much of the past 500 million years or so, the continent has lain below sea level; marine strata accumulated during these times. Sporadically there have been intervals of relative sea-level fall, each of which ended deposition and exposed the previously deposited rocks to erosion. The resulting erosion surface was covered by renewed deposition when relative sea level rose again. Such an ancient erosion surface is called an unconformity. Beginning in the 1940s in Montana, Sloss recognized the existence of several such unconformities that could be traced laterally, first on a regional basis and eventually all the way across the stable central nucleus (craton) of North America. The assemblage of strata lying between successive unconformities was

termed by Sloss to be a stratigraphic sequence. He named the sequences (see figure) using geographic terms for areas where the particular rocks were exposed; the names were derived from American Indian words, for terminological uniformity and to emphasize that the sequences derive from the study of North America, rather than Europe, the source of most previous stratigraphic concepts and terms. Sloss announced his basic concept and named the lower four sequences in 1949. In 1963 he published his most important paper, “Sequences in the Cratonic Interior of North America,” in which he added the final two sequences and documented the stratigraphic details for all six.

Significance The six cratonic sequences record the depositional and erosive history of the continent over a time span of half a billion years. Each sequence involves sedimentary accumulation lasting tens of millions to more than 100 million years (the sequences are not of equal duration). The intervals of emergence and erosion are shorter, a few million to a few tens of millions of years. The individual rock formations that make up the sequences record much shorter time intervals, and as a rule they are products of deposition in a local setting. Hence the formations cannot be traced far laterally, and the age relations between all the various formations in different regions are commonly obscure. That is, in one area the order of events is recoverable by studying the formation column, but in a second area, different events took place and the formations are not the same as in the first area. Matching the two orders of events (stratigraphic correlation) is very difficult. Sloss’s sequences, because they are based on (usually) unambiguous unconformity surfaces, surfaces evident even in geologic well cores and geophysical logs, allow geologists to trace them from one region to another. Each sequence is roughly equivalent in time. Thus the sequences provided a new method of correlation.

Although the data used by Sloss were largely based on subsurface records generated by the petroleum industry, Sloss’s concept was developed in academia, and for more than a decade his six sequences remained an academic curiosity. Some geologists adopted his terminology as a parallel system to the traditional geologic time scale terms (Cambrian, Ordovician, and so forth). The concept was not built upon, though, for two reasons: (1) the sequences were too thick to be useful in detailed stratigraphy in a small area; and (2) no one, including Sloss, knew what the cause of the sea-level fluctuations was. Sloss was determined, in his original paper and afterward, to show that the sequences were a function of dynamic movements of the craton, independent of movements in the mountain belts marginal to the continent (the Cordilleran and Appalachian chains). The craton-wide unconformities cannot be traced into the stratigraphy of the mountain belts; there, more localized upheavals generated a series of local unconformities, not synchronous with the breaks on the craton. In 1963, Sloss’s uncertainty as to cause was simply part of the larger uncertainty all geologists experienced in trying to explain structural movements of the earth (tectonics). In the ensuing decade, the new theory of plate tectonics revolutionized geologic thought. Mountains came to be understood in terms of plate margin collisions; the structures and uplifts of ancient mountain belts were shown to have detailed analogues in active plate settings in the early twenty-first century. However, vertical movements of the cratonic interiors of the continents proved more difficult to fit into the plate theory. Sloss’s ongoing assertion, which continued his entire life, that the sequences are fundamentally tectonic in origin was, and continued to be, regarded in 2007 as unproven.

Seismic Stratigraphy The sequence concept was eventually developed further by geologists in the petroleum industry, using subsurface information from the modern continental margins, especially seismic reflection profiling. Artificial seismic (sound) waves are capable of penetrating kilometers down into the thick piles of sediment that accumulate to form the continental shelves and slopes. The seismic waves reflect from subsurface structures back to the surface where they are recorded. The echoes reveal the buried patterns of stratification, especially the unconformities. Sloss’s former PhD student, Peter Vail at Exxon Production Research Laboratories, realized that these subsurface unconformities could be used to define sequences on the continental margins. These sequences were of short duration, orders of magnitude shorter than Sloss’s original sequences. By comparing the relative sea-level fluctuations on separate continental shelves, industry geologists were able to recognize a worldwide pattern of sea-level rise and fall extending back to Triassic time, more than 200 million years ago (“Vail curve”). Very short-term fluctuations on the sea-level curve (10⁴–10⁵ year duration) probably represent glaciers waxing and waning. Longer cycles (10⁶–10⁷ year) are probably plate tectonic in origin. In principle, the short sequences provide geologists with a global method of time correlation as precise as that based on fossils. Sloss’s concept of subdividing the rock record based on its internal breaks led to a new field in geology, sequence stratigraphy.

Honors Sloss’s new approach to stratigraphic geology made him one of the best-known geologists of his generation. He was called upon for service and given awards by the profession. After beginning as a lecturer at Northwestern in 1947, he progressed rapidly through the ranks, reaching professor status in 1954. In 1971 he was named William Deering Professor of Geological Sciences, the position he held until his retirement from teaching in 1981. Sloss was elected president of SEPM (Society for Sedimentary Geology) in 1961 and president of the American Geological Institute in 1968. In 1974 SEPM made him an honorary member, and in 1980 it conferred upon him its highest honor, the Twenhofel Medal. That same year he was elected president of the Geological Society of America. In 1986 he was the recipient of GSA’s top award, the Penrose Medal. Finally, he was memorialized in 1999, when the Sedimentary Geology Division of GSA instituted an annual medal, the Sloss Award.

Sloss was active during his retirement, editing the craton volume in the Geology of North America series and participating vigorously in the controversies over seismic stratigraphy and the sea-level curve. He continued to work until the very end of his life. In 1996 he attended the Geological Society of America meeting in late October. He died following surgery in Evanston a few days later.

BIBLIOGRAPHY

WORKS BY SLOSS

With Wilson M. Laird. “Devonian System in Central and Northwestern Montana.” American Association of Petroleum Geologists Bulletin 31 (1947): 1404–1430.

With William C. Krumbein and Edward C. Dapples.“Integrated Facies Analysis.” In Sedimentary Facies in Geologic History, Memoir 39, edited by Chester R. Longwell. New York: Geological Society of America, 1949.

With William C. Krumbein. Stratigraphy and Sedimentation. San Francisco: Freeman, 1951.

“Sequences in the Cratonic Interior of North America.” Geological Society of America Bulletin 74 (1963): 93–114.

“Areas and Volumes of Cratonic Sediments, Western North America and Eastern Europe.” Geology 4 (1976): 272–276.

“Tectonic Evolution of the Craton in Phanerozoic Time.” In Sedimentary Cover—North American Craton: U. S., edited by Laurence L. Sloss. Geology of North America, vol. D-2. Boulder, CO: Geological Society of America, 1988.

“Forty Years of Sequence Stratigraphy.” Geological Society of America Bulletin 100 (1988): 1661–1665.

OTHER SOURCE

“Memorial to Laurence L. Sloss, 1913–1996.” Geological Society of America Memorials 30 (1999): 79–82. This short summary includes Sloss’s acceptance remarks at his Twenhofel Medal ceremony. There is also a selected bibliography of his publications.

Charles W. Byers