La Manzanilla’s Basin Surface Morphology

1. La Manzanilla’s Basin Surface Morphology An exploration of the various processes responsible for the geometries and morphologies present

2. Overview of Morphology • Plan view of the area shows several intermittent streams terminating on the relatively small bajada on the eastern shore of Tenacatita Bay. • The bajada is the result of coalescing alluvial fans and rockfalls along the interface of bedrock exposure and the bajada. • There is a long, narrow beach with a back barrier mangrove lagoon with a filled tidal inlet at La Manzanilla.

3. Overview of Morphology

4. The Streams • Locals say that the streams only flow for a few weeks out of the year. • Data needs to be collected to confirm those assertions. • Regardless of the exact amount of time the streams flow, it is true that flow is violent, evidenced by boulder sized material in the stream channels.

5. Streams Continued • Although the streams have strong competency, they do not flow all the time. • This translates into short lived influxes of sediment into the bajada.

6. Beach Processes • Because of the geometry of Tenacatita Bay, waves enter from the west and refract along the edges of the bay, and they eventually end up breaking on the bajada. • Wave energy has created a beach on the bajada, leaving a low energy back barrier mangrove bay behind that.

7. Wave Refraction and Longshore Drift

8. Beach Continued • Due to the morphology of the beach and its spatial relationship to nearby intermittent streams, I suggest that longshore drift processes have transported sediment along shore from the northern terminus of the beach where a stream is present, to the south, creating a long narrow spit.

9. Longshore Drift and Spit

10. The Back Bay • The back bay is characterized by fringing mangrove stands along the channel. • I suggest that the geometry of the channel is induced in part by beach processes along the northern tier beach side of the channel and alluvial fan deposition to the south landward side of the channel around La Manzanilla. • The edges of the bajada are supplied with sediment from the adjacent hillsides during rock falls, not the streams.

11. The Two Regions of Morphologic Control

12. Additional Considerations • The morphology and geometry of the sand bodies in the La Manzanilla basin are the responses to sediment supply, wave energy and sea level. • Sediment supply is intermittent. • Wave energy is medium to low and constant. • Sea level is debateable.

13. Sediment Supply • The intermittent streams supply sediment periodically to the system. • The sediment supplied by the streams ranges from cobble to clay sized terrigenous clasts.

14. Wave Energy • Wave energy during the dry season is low, waves approached one meter during our January 2008 meeting. • Wave energy is inferred to be greater during the rainy season because of offshore and periodic nearby landfall of tropical cyclones. • Close proximity to an ocean-continent convergent zone exposes the area to occasional tsunamis.

15. Sea Level • During the past 20,000 years, world wide sea level has risen dramatically. • Tenacatita Bay lies in a graben, an extensional terrain feature that is a downthrown block of crust that results in a negative topographic feature. • The bay is bounded by two normal faults, so it is intuitive that fault slip would result in a local transgression. • However, regional volcanism is related to additional heat in the lithosphere and there may be uplift in the country rock, resulting in a local regression. • Basically, the tectonics are complicated and it is not known how sea level has fluctuated recently around Tenacatita Bay.