“The last stand of the Pleistocene lake left a shoreline at an elevation of 543m…”

"... indicating a surface area of approximately 236 km3"

Physiographic history of the Afton basin, revisited.
Norman Meek, Department of Geography, University of California, Los Angeles, Ca 90024

Abstract
Surveying errors and interpretative differences between the first two reports (Ellsworth, 1932; Blackwelder and Ellsworth, 1936) on eastern Afton basin indicate that the physiographic history of the basin requires revision. New data indicating offset shorelines and new interpretation of the sediment wedge adjacent to the Manix fault suggest uplift of the Cady Mountains during the middle and late Pleistocene. Moreover, deposits attributed to a lake in the dissected basin may instead be due to local ponding.

Introduction
In 1932, Elmer Ellsworth, a doctoral student at Stanford University, completed an admirable dissertation entitled “Physiographic History of the Afton Basin (Ellsworth, 1932). Because of financial difficulties associated with the Great Depression, Ellsworth left academia to work in the oil business, and the task of further publication on the subject was left to his dissertation adviser, Professor Eliot Blackwelder. Although Blackwelder visited the basin for at least one day in 1931, and again in 1932, his responsibilities as chair of Geology at Stanford (1922-1945) and his fragile health apparently prevented him from independently investigating the basin in detail prior to writing the widely cited summary article “Pleistocene lakes of the Afton basin” (Blackwelder and Ellsworth, 1936).

In the fifty years since its publication, the Blackwelder and Ellsworth article has remained unchallenged. Partly because Ellsworth did not review the article prior to its publication, several important difference exist between the article and his dissertation. These differences, as well as the recent discovery of important elevational errors in the original study, lead to the conclusion that several parts of Blackwelder and Ellsworth (1936) are incorrect, and must now be revised. The purposes of this paper are to discuss the errors in the original study, to note the importance of factual and interpretative difference between the two works, and to provide the result of new surveys and observations of the same features studied by Ellsworth in order to revise the physiographic history of the eastern Afton basin.

“Lake Manix drained to the east, perhaps catastrophically, approximately 18,100 years ago, probably as a result of tectonic movement on the Manix fault or a major increase in river inflow that caused the lake to overflow and out its topographic dam.”

Important Difference Between Ellsworth (1932) and Blackwelder and Ellsworth (1936)
Three differences between the two earliest reports on eastern Afton basin involve critical aspects of basin history. First, Ellsworth (1932, p. 63) suggested that all lakes in Afton basin were probably correlative with substages of the Tioga glacial. Relying on the same evidence, Blackwelder apparently disagreed, correlating the two recognizable lacustrine periods with the Tahoe and Tioga glaciations (Blackwelder and Ellsworth, 1936, p. 463). The correlations in each report were based on qualitative assessments, such as the fresh appearance of the beach ridges and the time believed necessary to erode the basin. The age of the first lacustrine period is still not known even though absolute-age dating techniques have been available for decades. Consequently, previous correlations of lacustrine periods in eastern Afton basin are speculative.

Secondly, Blackwelder and Ellsworth (1936, p. 459) reported the maximum stage of the first lake to be 1795’ (547m). In his dissertation, Ellsworth (1932) did not provide data to support such a statement other than a dished line on one figure (p.23), and in fact, never specifically discussed that maximum stage of the first lacustrine period. Indirectly, he suggested a maximum stage of the first lake by stating that the second lake crested “about 20 feet higher” (Ellsworth, 1932, p. 490). Because Ellsworth provided data indicating that the second lake peaked at 1800’ (548.8m), one can infer that the first lake peaked about 1780 (542.7m). In any case, no evidence was provided to indicate the maximum stage of the first lake, probably because that uppermost shore facies of the first lake have been erosionally truncated in all know exposures.

Finally, Blackwelder and Ellsworth (1936) report a sheet of gravel atop the lacustrine clays “not less than 20’ thick near the middle of the basin and perhaps somewhat thicker toward the margin” (p. 459). In contrast, Ellsworth’s (1932) cross-sections show the same fanglomerate thinning to less than 20’ at distances far removed from the basin center (p. 26).

Surveying Errors in Ellsworth (1932)
Generally speaking, the descriptive data provided by Ellsworth (1932) are reliable, and serve as a simple, but adequate introduction to the stratigraphy of eastern Afton basin (Fig. 1). However, recent repetitions by the author of Ellsworth’s field surveys indicate that his measurements of beach-ridge elevations are erroneous.

Ellsworth reported a maximum elevation of 1800.1’ (548.8m) for north Afton beach ridge, and 1783.2’ (543.7m) for south Afton beach ridge. He also reported closing his survey circuits with net vertical closure error of 0.2’ (0.06m) and 1.2’ (0.37m) respectively (1932, 9. 73).

Recent resurvey of these features indicates that north Afton beach ridge tops out a 542.6m (1779.8’), and that south Afton beach ridge crests at 541.0m (1774.5’). The survey closure errors are 0.11m (0.36’) and 0.03m (0.10’) respectively. The accuracy of the north Afton beach ridge resurvey was independently verified at the beach ridge benchmark by the Los Angeles Department of Water and Power (1779.2’; 542.4m).

There is a possibility that the large surveying errors may not be entirely Ellsworth’s fault, as he mentions using a benchmark at Afton Station reported to be at an elevation of 1425.0’ (434.5m) by the Union Pacific Railroad (Ellsworth, 1932, p. 5). The modern benchmark at Afton Station is at an elevation of 1408’ (429m); NGS/USGS datum), and my casual observations suggest that Afton siding is probably at the same elevation today as in 1931. It is therefore possible that railroad surveyors could be responsible for a major error in the datum used by Ellsworth. However, at least one of Ellsworth’s circuits must have been inaccurate because the measurement errors are not constant.

Apparently failing to independently measure local shoreline features, numerous authors have reported 1800 +/- 5’ (549 +/- 1.5m) shorelines in the Manix basin. Weldon (1982, p. 80) even developed a tectonic hypotheses to explain the reported shoreline elevations. The fact that the maximum shoreline elevation is 543 m, and that the maximum stage of the first lake is unknown, indicates that Weldon’s hypothesis is based on erroneous information.

The reduction in maximum stage from 549 to 543 m also has enormous implications regarding Lake Manix hydrology. For example, lake surface area decreases from about 380 km2 to about 215 km2 (a 44% reduction), lake volume decreases from about 4.55 km3 to about 3.0 km3 (a 34% reduction), and the direction of flow over interfluves reverses in some lake-history reconstructions. Clearly, the original surveying errors have compounded over the years, creating a lake history based more on legend than fact.

Revision of the Physiographic History
The pre-lacustrine stratigraphic sections have not been investigated in detail, and so the physiographic history of the basin prior to the late Pleistocene lakes requires further study. However, preliminary investigations reveal that Ellsworth failed to describe several thick carbonate accumulations in the Brown Fanglomerate (Fig. 2), perhaps mistaking the indurated layers for mudflows (Ellsworth, 1932, pp. 15-16). The carbonate accumulations cement the top of fanglomerate members, and suggest that much of the basin history (early and middle Pleistocene, perhaps extending back into the Pliocene) was characterized by episodic deposition with long intervals of fan-surface stability and carbonate accumulation. As Ellsworth correctly noted (1932, p. 28-29), these fanglomerate deposits provide evidence of post-depositional tectonic folding in eastern Afton basin. There is some evidence east of Afton Station that suggests syn-depositional warping.

The thickness of the uppermost carbonate accumulation in the Brown Fanglomerate marks a lengthy period of surface stability throughout the area, and serves as a crude time-stratigraphic marker. Large boulders of Cave Mountain granite are found south of the Mojave River in the Brown Fanglomerate, indicating that the former basin depocenter was farther south than during the late Pleistocene. Cady Mountains volcanics are comparatively rare in the Brown Fanglomerate southwest of Afton Station. It is therefore unlikely that the Cady Mountain had significant relief in their present position during the deposition of the Brown Fanglomerate. However, increasing quantities of volcanics in carbonate-cemented strata to the east near Afton Canyon suggest that debris was locally shed to the north from the Cady Mountains.

The Gray Fanglomerate is a thick, uncemented, volcaniclastic deposit that overlies the Brown Fanglomerate south of Afton. Notably, the Gray Fanglomerate has only a thin corresponding depositional unit north of Afton Station (Fig. 2). A likely explanation for this difference is that renewed diastrophism has formed a secondary basin between the Mojave River and main trace of the Manix Fault 2 km to the south. In this interpretation, the Gray Fanglomerate is sedimentary fill in a basin formed adjacent to the Manix Fault during late, and perhaps middle Pleistocene time.

This secondary basin may represent a local depression at the surface of a south-dipping thrust fault which uplifted the Cady Mountains, or alternately, it may represent a down- dropped block adjacent to a predominantly strike-slip fault. In either case, continuous deposition is indicated by the relative lack of carbonates in the Gray Fanglomerate relative to the Brown Fanglomerate, and suggests that the Cady Mountains were rapidly uplifted south of Afton during the middle and late Pleistocene.

New shoreline elevation data support this hypothesis of basin development and suggest that tectonic deformation is continuing. Beach remnants south of Afton Station attain a maximum elevation of 541.0 m. Comparative topographic profiles and the relationship of the beach deposit crests to edge of undisturbed fan deposits just to the south suggest that the beach deposits are not highly eroded remnants of much larger feature, but rather, are locally intact and provide accurate lake elevations. The maximum shoreline elevation south of Afton Station is 1.6 m less than the maximum shoreline elevations north of Afton Station, which are all accordant. Because the south Afton beach deposits lie in the middle of the proposed secondary basin, it is likely that the area has dropped approximately 1.6 m in the past 14,000 years in much the same way as during the deposition of the Gray Fanglomerate. Tectonic instability is also suggested by a fault (described by Ellsworth, 1932) with apparent normal offset and a displacement of about 5 m that cuts lake strata in this area.

The lacustrine history of Afton basin is far more complex than Ellsworth (or I) imagined. Although several buried and/or truncated beach deposits have been discovered at various places and at many different elevations throughout Afton basin, none has yet permitted the maximum elevation of the first lake period in Manix basin to be determined. Only circumstantial evidence, and certainly not the ideal stratigraphic relations shown by Ellsworth (1932, p. 23, p. 26) and Blackwelder and Ellsworth (1936, p. 458), suggests that parts of the major beach ridges in Afton basin could be deposits from the first lacustrine period. To emplace the uniform interlacustrine fan unit on both sides of the beach ridges suggest that the ridges were probably eroded first. My observations of the cross-sectional exposure of the beach ridge north of Afton and stratigraphic sections south of Afton suggest that most of the beach deposits equivalent to the first lacustrine period were erosionally truncated during the interlacustrine interval. Consequently, no maximum elevation can be assigned to the first lake period, but it is probable that the lake attained about the same maximum stage as during the second lacustrine period.