Textural Analyses of the Gravels in the Linkou Tableland

林口臺地礫石層為臺地主要的岩層，分布於更新世的林口層和大南灣層中。過去利用沉積相分析得知礫石的沉積環境，大部分為扇洲的辮狀河環境，少部分為海灘環境。然而前人的組構研究中，缺乏比較不同沉積相的變化，加上測量方法不一，因此本研究重新測量林口臺地礫石層的組構，以了解各沉積相間的組構變化和古水力，得知礫石層的沉積作用。 本研究測量粒徑、覆瓦狀排列、外型與磨圓度，其中粒徑用最大5顆礫石平均計算古水力，覆瓦狀排列、外型和磨圓度則明確規範取樣數量、測量面積與粒徑範圍。將結果依扇頂、上扇、中扇、下扇和海灘的沉積相分類取平均和標準差，作為不同地點間比較之依據。 林口臺地礫石層的古剪應力分布顯示，上游有呈現數個連續性佳的主要河道，而至下游分支成扇狀的數條河道。而各沉積相的組構隨搬運距離增加，粒徑變小而淘選度變佳，覆瓦狀排列方向越集中。磨圓度在上游明顯變佳，下游變化不明顯。海灘與河流的組構比較，海灘的覆瓦狀排列方向較散亂，磨圓度較佳。外型則在各環境中無明顯變化。 林口扇洲的主要河道呈現固定位置，與其他現生沖積扇的河道特徵類似。從上游至下游的水力大小和流體沉積速率遞減，且流體的磨蝕作用集中在上游區域，並因沖積扇沉積的循環模式，導致上游區域沉積河道和後期填充的堆積物，使得粒徑和淘選度在上游呈現變異性大的情形。另外覆瓦狀排列受水力大小和流體沉積速率影響礫石排列方向的散亂情形。海灘的礫石則在磨圓度和覆瓦狀排列能明顯觀察出與辮狀河的差異。The gravels in the Linkou Tableland, distributes over Pliestocene Linkou Formation and Tanawan Formation. According to facies analysis, the depositional environments of the gravels are considered mostly braided river and a small part of the beach. However, previous texture studies lack comparison with different facies changes and strict measurement methods. Therefore, this study measured textures again to recognize changes for each facies and calculate paleohydrology. This study measures the grain size, imbrication, particle form and roundness. Grain size measures the 5 largest visible clasts for calculating paleohydrology. Imbrication, shape and roundness especially rule the sampling area, number and range of grain size. The results are separated from five facies to get the mean and standard deviation, including apex, upper fan-delta, middle fan-delta, lower fan-delta and beach. The paleohydrology distribution represents several continued main channels. % at upstream, and main channels branch into few smaller channels whose shape are like fans. With increasing transport distance, grain size decreases, sorting becomes better, and imbrication becomes more uniform. Roundness is better at upstream and no significant change at downstream. Compared with river, beach roundness is better and imbrication is more scattered. %The particle form is without significant change in each facies. The main channels of Linkou fan-delta presents a fixed location. This characteristics is similar to other alluvial fans. From the apex to the lower fan-delta, the flow competence and depositional rate decreases, and abrasion is concentrated in the upstream region. According to alluvial autogenic cycle, upstream region deposits channels and post-fill deposits, making the size and sorting has larger variability in upstream. Imbrication is influenced by flow competence and depositional rate, and gravel beach can observe a significant difference between braided river in roundness and imbrication.口試委員會審定書..................................... i 致謝............................................. ii 摘要............................................. iii Abstract........................................... iv 目錄............................................. v 圖目............................................. vii 表目............................................. ix 第一章 緒論....................................... 1 1.1 前言..................................... 1 1.2 地質背景.................................. 1 1.3 前人研究.................................. 3 1.4 研究目的.................................. 4 第二章 測量方法..................................... 5 2.1 外型與磨圓度 ............................... 5 2.2 覆瓦狀排列................................. 7 2.3 粒徑..................................... 9 2.4 與前人方法比較.............................. 9 2.5 野外工作流程 ............................... 9 第三章 粒徑....................................... 13 3.1 統計方法.................................. 13 3.2 最大10顆粒徑分布............................ 15 3.3 最大5顆粒徑分布............................. 18 3.4 不同沉積相之變化............................. 21 第四章 覆瓦狀排列................................... 23 4.1 取樣與測量................................. 23 4.2 統計方法.................................. 26 4.3 古水流方向................................. 27 4.4 不同沉積相之變化............................. 29 第五章 外型與磨圓度.................................. 33 5.1 外型及圓度指數與統計.......................... 33 5.2 不同沉積相之變化............................. 35 5.3 前人研究比較 ............................... 38 第六章 礫石層組構之地質意義............................ 41 6.1 林口臺地礫石層之古剪應力 ....................... 41 6.2 林口扇洲之沉積作用 ........................... 44 6.3 辮狀河礫石的搬運作用.......................... 47 第七章 結論....................................... 49 參考文獻.......................................... 51 附錄............................................. 57 附錄A 露頭位置..................................... 57 附錄B 粒徑....................................... 58 附錄C 覆瓦狀排列................................... 72 附錄D 外型與磨圓度.................................. 87 附錄E 剪應力與覆瓦狀排列傾角計算值...................... 10