林口臺地礫石層之組構研究

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。

關鍵字

林口臺地；礫石；組構；扇洲

並列摘要

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.

並列關鍵字

Linkou Tableland ； Gravel ； Texture ； Fan-delta

參考文獻

Blair, T. C. and Mcpherson, J. G. (1994). Alluvial fans and their natural distinction from rivers based on morphology, hydraulic processes, sedimentary processes, and facies as- semblages. Journal of Sedimentary Research, 64(3):450--489.

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Eriksson, P., Long, D., Bumby, A., Eriksson, K., Simpson, E., Catuneanu, O., Claassen, M., Mtimkulu, M., Mudziri, K., Brumer, J., and van der Neut, M. (2008). Palaeohy- drological data from the c. 2.0 to 1.8 Ga Waterberg Group, South Africa: discussion of a possibly unique Palaeoproterozoic fluvial style. South African Journal of Geology, 111(2-3):281--304.

Eriksson, P. G., Bumby, A. J., Brumer, J. J., and van der Neut, M. (2006). Precambrian fluvial deposits: Enigmatic palaeohydrological data from the c. 2-1.9 Ga Waterberg Group, South Africa. Sedimentary Geology, 190(1-4):25--46.

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林口臺地礫石層之組構研究

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