Stratigraphic Analysis of the Distributary Fan in Holden NE Crater Kevin W. Lewis and Oded Aharonson, California Institute of Technology klewis@gps.caltech.edu, oa@gps.caltech.edu Introduction: We report the results of a detailed analysis of the fluvial distributary network in Holden NE crater, Mars. This remarkably well-preserved deposit shows possible evidence of persistent fluvial flow. Evidence comes in the form of preserved meandering channels, a multi-lobate form, and an intricately layered structure. To date, no feature has been found on Mars which has all of the notable characteristics found in the Holden NE structure. Using digital elevation models (DEMs) derived from high resolution stereo images taken by the Mars Orbiter Camera, we have determined the orientation of individual layers exposed along the distal end of the distributary network. This and other information is used to constrain both formation scenarios and the overall history of Holden NE crater. MOLA Topography: MOC images within Holden NE reveal the presence of fluvial channels in several places on the floor of the crater. These deposits are shown below in blue. Under the assumption that these deposits are roughly contemporaneous with the distributary fan, the present topography should be similar to the pre-depositional surface. Basin volume estimates can be calculated from the MOLA 128 pixel per degree DEM. Moore et al. (2003) estimated the mean discharge of the distributary system, using observed channel characteristics [2]. Their estimated flow rate was 700 m3/s. Using this figure, the modern basin would fill on a timescale of years, neglecting losses from infiltration and evaporation. Various timescales are shown as corresponding contours below. Flow would have to be episodic, or significantly reduced to maintain a base level at the elevation of the channel deposits. Technique: Stereo-derived DEMs were used in conjunction with the original MOC images to extract profiles along the edges of exposed layers. Dozens of individual profiles were acquired by hand, each ranging from a few hundred meters to over a kilometer in length. Data was only taken where a layer outcrop was clearly continuous; no attempts were made to connect or extend layers where they become indistinct in the images. For our analysis, we have used 40 such profiles along the margin of the layered deposits. Figure 2 (above left): Stereo derived Digital Elevation Model (DEM) of part of the fluvial deposits in Holden NE. The portion for which stereo data is available includes the layered exposures at the distal end of the fan. The complete stack of layers is approximately 100 meters high. Figure 3 (above): Layer profiles extracted from the stereo DEM (red). Blue vectors indicate the direction and magnitude of the measured dip for each layer. Results: Standard delta and alluvial fan formation mechanisms do not fit the observed evidence at Holden NE. In a standard Gilbert delta, exposed foreset beds at the distal end can have a dip of up to 30 degrees [8]. The slopes measured from our stereo-derived DEMs are much shallower, at about 4 degrees. Thus, a progradational delta seems unlikely in Holden NE. There is also little evidence that these channels once graded all the way to the floor of the crater. With the current topography, this would require the fan to have been more laterally extensive by a few kilometers, and there is little evidence to suggest it was. Rather, other channel deposits to the northeast, as shown in Figure 1, and the distance between lobes of the fan constrain the original extent of the channels. This implies that the terminal scarp of the fan is not simply an erosional feature. While the exact method of formation remains unclear, evidence indicates that this structure was formed mainly through aggradation. Our observations of the modern topography imply either episodic flow, or much reduced volumes than previously estimated. Figure 5: Times to fill Holden NE crater under previously estimated flow rates. The distributary network is shown by the blue channels on the left side of the image. Other channel deposits within the crater are also indicated. The background is a mosaic of THEMIS daytime IR images. Figure 1 (above): Mars Orbiter Camera mosaic of the distributary fan in Holden NE crater. Stereo data was available for the area indicated at the terminal end of the deposit. (Credit: M. C. Malin et al., “Distributary Fan Near Holden Crater” NASA’s Planetary Photojournal PIA04869, 11/13/03). Figure 4 (left): Histogram of layer slopes measured using stereo topography. The mean dip is 4 degrees, which is much shallower than would be expected for the foresets of a Gilbert-style delta. References [1] M. C. Malin and K. S. Edgett (2003) Science 302: 1931-1934. [2] J. M. Moore et al. (2003), GRL 30: 24, 2292. [3] M. C. Malin and K. S. Edgett (2001) JGR 106: 23, 429-23,570. [4] A. L. Albee et al. (2001) JGR., 106: 23, 291-23,316. [5] A. Ivanov (2003) LPS XXXIV, Abstract #2084. [6] A. Ivanov and J. Lorre (2002) LPS XXXIII, Abstract #1845. [7] R. Kirk et al. (2002) IAPRS XXXIV (B4), 200 (CD-ROM). [8] G. Nichols. (1999) Sedimentology and Stratigraphy, Blackwell Science, Oxford, UK.