The San Benito Gravels: Fluvial Depositional Systems, Paleocurrents, and Provenance

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1 ES120 FIELD TRIP #2 10 April 2010 NAME: The San Benito Gravels: Fluvial Depositional Systems, Paleocurrents, and Provenance Introduction The San Benito Gravels (SBG) consists of a 500 m thick section of Plio-Pleistocene sands and gravels. These deposits are exposed in the Paicines syncline in southern Santa Clara Valley, in San Benito County. They cover an area of approximately 500 km 2. These sediments accumulated in response to the uplift of both the Diablo Range to the east and the Gabilan range to the west (see map). The SBG lie unconformably upon marine sediments in several areas within the valley. Geologic evidence indicates that the SBG were deposited in a terrestrial setting. The coarse nature of the deposits and the apparent lack of silt and clay within the deposits suggest that deposition occurred within a braided fluvial system (but you should ascertain this for yourself!). The SBG are contemporaneous with gravels found in a roughly north-south trend through central California, including the Livermore gravels and the Paso Robles gravels. The deposits are evidence of the change from marine to terrestrial conditions experienced by large areas of the Great Valley geologic province during late Pliocene and early Pleistocene time. In addition, the successive appearance of various rock types and the paleocurrent directions indicated by structures within the SBG can indicate source areas and uplift processes associated with the deposition of the SBG. Within outcrops of the SBG vertical sequences are not always easily observable. However, some coarsening-upward sequences are apparent in the sandy strata. Those sequences exhibit planar crossstratification overlain by horizontally stratified pebbly sands. The thickness of these sequences may reflect variation in the periodicity of flood conditions associated with the ancient depositional system. Relatively thicker sections of planar cross-stratification may represent extended periods of fair weather conditions. Within the gravel and cobble deposits, detailed observation may reveal vertical sequences of planar to trough cross-stratified gravels overlain by massive to horizontally stratified gravels; these deposits presumably formed in a manner analogous to the sand deposits, but under higher flow conditions. The origin of the large vertical variations between gravel and sand deposits is unclear. These variations may result from tectonic changes that caused local uplifts, leading to deposition of coarser material (i.e., gravels). If the gravels do show vertical sequences like the sand deposits, then a tectonic origin for the large-scale variations may be likely. Table 1. Facies Characteristics of braided streams. Facies Lithofacies Sedimentary Structures Interpretation Identifier Gm Gravel, massive or crudely bedded; Ripple marks, cross-strata in sand Longitudinal Bars, channel-lag deposits minor sand, silt, or clay lenses units, gravel imbrication Gt Gravel, stratified Broad, shallow trough cross-strata, Minor channel fills imbrication Gp Gravel, stratified Planar cross strata Linguoid (transverse) bars or deltaic growths from older bar remnants St Sand, medium to very coarse; may Solitary or grouped cross-strata Dunes (lower-flow regime) be pebbly Sp Sand, medium to very coarse; may be pebbly Solitary or grouped planar crossstrata Linguoid (transverse) bars, sand waves (upper- & lower-flow regimes) Sr Sand, very fine to very coarse Ripple marks of all types, including Ripples (lower-flow regime) climbing ripples Sh Sand, very fine to very coarse; may be pebbly Horizontal lamination, parting or streaming lineation Planar bed flow (upper- & lower-flow regimes) Ss Sand, fine to coarse; may be pebbly Broad shallow scours (including Minor channels or scour hollows cross stratification) Fl Sand (very fine), silt, mud, interbedded Ripple marks, undulatory bedding, bioturbation, plant rootlets, caliche Deposits of waning floods, overbank deposits Fm Mud, silt Rootlets, desiccation cracks Drape deposits formed in pools of standing water

2 ES120 FIELD TRIP #2 San Benito Gravels - p.2 Braided Stream Depositional Systems The following information is intended to give you a general picture of a braided stream depositional environment, much as it was presented in lecture. While you will not be required to make any detailed environmental interpretations concerning the SBG, you will need to have a working, flexible, general knowledge of braided river depositional environments when you are working in the SBG deposits. Therefore, study this information BEFORE we arrive at the field sites today, to maximize your learning experience in the field (and follow all directions!). Elements of a Braided River System: River bed: the major course of a river; is covered during bankfull flow River channel: the topographic low where flow is concentrated during normal flow levels Longitudinal (braid) bars: topographic highs which separate channels during normal flow Levees: topographic highs that separate the channel from the floodplain Floodplains: low lying areas adjacent to a river bed; exposed except during extreme flood events Depositional Model for Braided Stream Systems Braided stream deposits result from two distinct depositional episode styles: aggradation during fair weather conditions (normal flow), and aggradation during floods (high water flow). During normal flow conditions, flow is confined to channels and sediment is transported in them via lower flow regime bedforms (Froude number< 1) of ripples, dunes, and minor sand bars. If aggradation occurs during normal flow conditions the resulting deposit will be dominated by trough or planar cross- stratification, depending upon the dominant bedform that is preserved. During large flood events, flow is unrestricted, covering bars and spilling over levees onto the flood plain. In these conditions, bars migrate downstream, covering previously deposited channel sediments. Upper flow region conditions (Froude number> 1) dominate during flooding, and sediments deposited during such conditions will show planar to low-angle stratification. A period of normal flow condition followed by flood conditions should produce a couplet of trough or planar cross-stratified sediments, overlain by planar to low-angle stratification. This couplet may coarsen upward, reflecting the increased flow velocity that occurs during floods. Erosional contacts may be present between the normal flow (fair weather) and flood deposits, depending upon the amount of erosion that takes place during the initial stages of flooding.

3 ES120 FIELD TRIP #2 - San Benito Gravels - p.3 Field Trip Assignment Today you will: 1. consider the vertical lithologic sequence, identify, and measure stratigraphic units, 2. measure crossbeds and imbricated clasts for paleocurrent information, 3. identify clast compositions, and measure clast sizes, for lithologic and provenance information. As always, take careful notes: you never know what information you will need for interpretations once you are away from the field area. Next week in lab you will learn how to plot your paleocurrent measurements on stereonets, correct your data for tectonic tilting of the outcrops, and ultimately integrate your paleocurrent results with your provenance data and the locations of the known source areas. The observations and measurements to be made today are listed below. Field methods will be demonstrated during the first hour at stop 2. (Note that three dimensions of a bed or clast are needed to get an accurate reading of the strike & dip.) You will work in teams of two in order to accumulate a meaningful number of measurements. Each team member should keep accurate notes of the team s findings, because each student is required to write a summary of his/her findings, to be turned in as an extended abstract. You will be using your own data in lab next week, so be sure to take careful measurements. Also, be careful not to destroy the structures at the outcrops, so that your colleagues also can collect data. Field Trip Stops: Stop 1. ~3.8 miles east of Hwy 25 on Panoche Rd.; Tres Piños River. (1) Mark the location on your topo map and record the elevation. (2) Take note of the distribution of bar deposits. Sketch a rough diagram below. (3) Dig a shallow hole in a sand bar and examine the sedimentary structures and grain sizes, shapes, orientations, and compositions. (4) Would you classify this as a braided or meandering river?

4 ES120 FIELD TRIP #2 -- San Benito Gravels - p.4 Stop miles east of Hwy 25 on Panoche Rd; roadcut on both sides of the road. Collect the following data: (1) Outcrop location: a. Locate the field site on the Cherry Peak quadrangle map on page 8. Mark the location and record the elevation. You will need the full map to identify the lat/long coordinates. Record these in your field book. (2) Lithologies & Stratigraphic Column: a. First roughly sketch the southern outcrop, showing the sedimentary facies that are present. You will then use the Jacob Staff to measure the thicknesses of each unit (for your Stratigraphic Column), and record the position of individual samples. As you (later) make your measurements of paleocurrents, mark their locations on your sketch as well. b. Locate areas to measure clast compositions and paleocurrents. (3) Paleocurrents a. Measure the strike & dip of the bedding. b. Measure the orientation of up to 15 imbricated clasts (if you can find them). c. Measure at least 15 crossbed sets for the orientation of the cross laminations. Note whether the sets are planar-tabular or trough crossbeds. If trough cross-stratified, measure the trend and plunge of the trough axis; the dip direction of the crossbeds on the flanks of the trough will not indicate the paleocurrent direction correctly. (4) Clasts a. Identify at least 20 clasts chosen at RANDOM (method explained in field lecture) picked from the conglomerate horizons. For the purposes of this exercise, identify the clasts using the following general categories: plutonic rocks high-grade metamorphic rocks (schist, gneiss, marble, blueschist) low-grade metamorphic rocks (greenstone, graywackes) volcanics chert (plus classify by color) unmetamorphosed sedimentary rocks (id more specifically if possible) ultramafics (e.g., serpentine) b. Measure the long axis of the clasts; note each length & composition (5) Lithologies - Identifying a Cycle a. Draw a typical lithologic cycle in a stratigraphic (vertical) sequence, (with noted scale). Also be sure to label all lithologies and sedimentary structures on your outcrop sketch. A sketch of the outcrop may also show significant lateral variations, such as a channel, or the presence of lenticular bedding. You can also make other sketches of different scale sedimentary features, but remember your time limitations. This information will help you with constructing a stratigraphic column and interpretation of field data later.

5 ES120 FIELD TRIP #2 -- San Benito Gravels - p.5 Summary The purpose of this exercise is to provide you with first hand experience in applying sedimentologic and stratigraphic field techniques. The primary scientific goal, however, is to determine the nature of the depositional facies at Stop 2. This includes the general orientation of the beds and depositional environment (and processes). Paleocurrent analysis and diagrams will be explained and constructed next week in lab. You will produce (in and out of lab) stereonet projections and rose diagrams of your paleocurrent data, and pie diagrams and histograms of clasts types and sizes. Abstract Assignment Your abstract will provide an interpretation of the outcrop (stop#2) data in light of your understanding of fluvial processes and the geologic history of the region. More specifically, your abstract must explain the nature of the field site (e.g. location, rock types, age, tectonic setting, geomorphology), the purpose of your work, results of data collection and analysis, and interpretation of the depositional facies including river current, direction, and clast provenance. For this assignment, you may submit an extended abstract, with a word limit of 400 words. Include the total word count at the end of your abstract. Along with the abstract, you will turn in for each site: (1) A stratigraphic column showing beds, sedimentary textures/structures, and general lithology. The column should follow the guidelines set forth in the handout provided in lab. (2) Pie charts of your clast lithologies, (3) Rose diagrams representing paleocurrent directions (4) Histograms of clast sizes (These diagrams are mean to support the assessments and interpretations presented in your abstract.) (5) Although you do not include reference citations within an abstract, you must turn in a separate page listing the references that you consulted for this work. (6) Don t forget to list your field partner. Abstracts are due by 2:00 PM Thursday, April 22

6 ES120 FIELD TRIP #2 -- San Benito Gravels - p.6 Conglomerate Clast Lithologies from the San Benito Gravels Quien Sabe Volcanics: rhyolite, dacite Salinian block: marble, quartzite (vein?), granite Franciscan: sandstone, chert, chert conglomerate, graywacke, shale, serpentine, basalt, blueschist Gabilans: rhyolite, marble Undifferentiated Tertiary rocks: siliceous mudstone, conglomeratic sandstone, arkosic sandstone

7 ES120 FIELD TRIP #2-- San Benito Gravels - p.7 Geologic Map

8 ES120 FIELD TRIP #2 -- San Benito Gravels - p.8 Topographic Map

9 Road Map ES120 FIELD TRIP #2 -- San Benito Gravels p.9

10 ES120 FIELD TRIP #2 -- San Benito Gravels p.10 Additional Resources on Braided River Deposits: Figure above provides an example of how you might identify and subdivide units based on crossbed orientations and bedding planes. Figure to the right shows the typical lithofacies and vertical profiles from a shallow, gravel-bed braided river (A) and a sheetflood distal braided river (B). (From Boggs, 2001). These represent typical profiles, but are not meant to be models for all braided river systems.