ABSTRACT The gently dipping (3-5° SW), complexly faulted, homoclinal structure of the Todhunters Lake Gas Field is deeply incised by a southwest trending tributary of the Markley Submarine Canyon. Formed during late Eocene, the canyon eroded Eocene Nortonville Shale through upper Cretaceous Starkey sands. Hemipelagic sediments infilled the canyon during early Oligocene creating an impervious barrier to hydrocarbon migration. Faulting is an equally important trapping mechanism, particularly in sediments not affected by the incisement of the Markley Canyon, i.e., lower Starkey and Winters sands. A series of subparallel, NW trending, normal faults traverse the field creating isolated dry gas reservoirs. With offsets usually less than 80 feet, faults are generally not recognizable on available seismic lines. Some controversy exists over the age of faulting, but subsurface data suggests that it predates canyon incisement. No discontinuities are seen in the structure of the base of the Markley Canyon fill and well data in the canyon indicates no offset. Since the discovery of the field in 1967, gas has been successfuly produced from the upper Cretaceous First, Second and Third Massive sands of the Mokelumne River Formation as well as the Starkey One through Five sands. Significant production has also been obtained in the upper Cretaceous Winters sands in the western half of the field. Cumulative production to 1982 is estimated by the Division of Oil and Gas to be 79,761 MMCF. Future development of the field will depend largely upon the identification of subtle traps controlled by faulting.

ABSTRACT During the early Tertiary, in what is now the Sacramento Valley, four submarine canyons successively fed sediments into a deep remnant of the Mesozoic trough that formed the western margin of the North American continent. The Markley Valley is the youngest of these “canyons” having formed after deposition of the Sidney Shale Member of the Eocene Markley Formation. Cross sections show truncation of rocks as old as Cretaceous in the northern reaches of the valley but show that erosion of Tertiary rocks predominates in the southern end of the valley. The valley fill ranges from about 1.2 miles (2 km.) wide at its outcrop near Wheatland in Yuba County to greater than 12 miles (20 km.) wide west and southwest of Sacramento. In its thickest axial portion the valley fill is greater than 2000 feet (600 m.) thick. The Markley Valley is 67 miles (110 km.) long. The Markley Valley trends generally S10°W but north of Rio Vista it abruptly turns westerly to its presumed outlet. Structure contour maps of the base of the valley fill show the feature to be surprisingly irregular with reentrants that suggest tributaries and occasional highs which appear as monadnocks. Along much of its length the filling of the Markley Submarine Valley was the last marine event and the fill is overlain by continental sediments of the late Tertiary Tehama Formation. The Markley Valley Fill is dominated by shales and as such is less prospective for gas exploration than areas outside the valley. However, the Green’s Lake Gas Field 3 miles (5 km.) southwest of Sacramento gained a small portion of its production from what is locally called the Markley Valley Sandstone, clearly within valley filling sediments. This sand and others that are similar are thin and discontinuous compared to reservoir rocks outside of the valley fill. The Markley Valley more importantly impacts gas exploration as its fine-grained fill material truncates and seals thicker, more permeable reservoirs outside the valley. In as many as twelve gas fields including the Catlett, Conway Ranch, Fremont Landing, Karnak, Liberty Cut, Liberty Island, Maine Praire, Millar, Rio Jesus, Sacramento Airport, Todhunters Lake, and Winchester Lake fields, a significant amount of the gas trapped is attributable to valley-filling sediments truncating and sealing older gas reservoirs.

Abstract As is well known, one of the great founders of British geology was the Cambridge professor, Adam Sedgwick (1785–1873). Aspects of his work have been intensively studied by Rudwick (1985) and Secord (1986), regarding his role in the establishment of the Devonian and Cambrian Systems respectively. Earlier, Sedgwick was the subject of a two-volume Life and Letters by Clark & Hughes (1890), and there is a short non-technical biography by Speakman (1982). Elsewhere, I have already written on his early work in the Lakes ( Oldroyd 1998 -1999 [2000]) and about his personal character and beliefs ( Oldroyd 2002 ). Some parts of the present chapter draw on my previous publications, but Sedgwick’s studies in the Lake District as a whole have not been analysed previously in any detail. Sedgwick came from Dent in the Yorkshire Dales, to the SE of the Lake District. His father was the local parson, and the family seems to have been reasonably well-off. It is interesting that parts of the floor of his father’s church of St Andrew’s are crammed with fossils, and one may wonder whether these curious enclosures may have turned the boy’s interests towards the rocks of the Earth’s crust and their origins and history. Adam was the third of seven children. He attended a nearby school of some reputation, Sedbergh, and went on to Trinity College, Cambridge. There he studied mathematics and theology with great diligence, and in 1808 he was placed fifth in the University for mathematics (5th Wrangler). This led

Abstract Graphical displays were used early in geophysics and crystallography, mineralogy, petrology and structural geology by the early 1800s, but nineteenth-century geology obstinately remained mainly descriptive. Charles Lyell's quantitative classification of the Tertiary Sub-Era in 1828 was a notable exception. Nevertheless, by 1920 the quantitative approach had become established. W. C. Krumbein, who introduced the computer into geology in 1958, encouraged use of probabilistic sampling and process-response models. Early work focused on databases, statistical data analysis and display. By the 1970s, stochastic simulation, deterministic modelling and spatial 'geostatistics' (pioneered by Matheron and his co-workers), were of growing importance. The introduction of the personal computer and the graphical user interface in the 1980s brought well-proven quantitative methods out of the research environment onto the workbench and into the field. Since the mid-1980s, the analysis, display and modelling of behaviour in three dimensions, underpinned by spatial statistics, computational fluid-flow, visualization technology, etc., has proved of economic benefit to mining, petroleum geology and hydrogeology. Other, computationally intensive, methods likely to be of importance in the Earth sciences are the application of 'robust' statistical methods, increasing use of Bayesian methods in uncertainty (risk) estimation (as a result of a renewed interest in statistical intervals and forecasting), and computational mineralogy.